Emerging avalanche field-effect transistors based on two-dimensional semiconductor materials and their sensory applications

This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15767 Cite this: J. Mater. Chem. C, 2025, 13, 15767 Emerging avalanche field-effect transistors based on two-dimensional semiconductor materials and their sensory applications Ehsan Elahi, †* a Muhammad Farooq Khan, † b Jamal Aziz, c Umer Ahsan, a Payal Chauhan, a Mohammed A. Assiri, d Kalyan Jyoti Sarkar, a Umer Asgher ef and Zdenek Sofer * a Recently, two-dimensional (2D) layered semiconductors have been the subject of promising research work due to their intriguing physical and chemical characteristics. In electronic nano-devices, impact ionization is a viable condition to investigate or probe the level of sensitivity upon the application of external stimuli. However, avalanche field-effect transistors (FETs) have emerged as promising candidates for a wide range of sophisticated applications, especially for sensing traits. In this review, we explore the incorporation of 2D materials into avalanche FETs, highlighting their auspicious properties such as high carrier mobility, variable band gaps, and atomic thickness, which provide significant advantages over typical materials. 2D materials significantly improve the sensitivity, speed, and power efficiency of avalanche FETs. This study also encompasses the advances in photo-, bioand gas-sensing technologies, emphasizing their implications in contemporary applications such as optoelectronics, imaging, and environmental monitoring. Thus, our review provides a thorough investigation of material attributes, device architecture, and prospective applications by establishing avalanche FETs with 2D materials as the keystone in power and rectifying applications. 1. Introduction After the discovery of graphene, 1 the scientific community has explored two-dimensional (2D) layered semiconductor materials that have been recognized as promising contenders for a variety of applications due to their exceptional characteristics, which include a dangling-bond-free surface, rich physical properties, and flexibility at the atomic level. 2–6 Due to high contact resistance, large leakage current, and short channel effects, the conventional solid-state technology invokes to meet the need for miniaturization in electronic devices. 7,8 The potential for the revolutionary role of 2D materials in nanoelectronics and optoelectronics is essentially admired in the modern era. 9–11 This motivates the investigation of 2D semiconductors, such as TMDCs, 3,12–17 group III chalcogenides (e.g., InSe, GaSe, In 2 Se 3 , etc.), 18–21 and mono-elemental materials (e.g., phosphorene, tellurene, etc.). 22–24 Therefore, 2D semiconductors are extremely useful for developing electronic, 25,26 photonic, memory, 27 sensing 28 and neuromorphic devices 29–32 due to their exclusive electronic band structures and atomically thin physical architecture. 12,33,34 These materials can play a substantial role in accelerating the advancement of both electronics 35–37 and optoelectronics 38–40 due to their ease of fabrication and integration (without the need for strict lattice matching or epitaxy), high carrier mobility, 18,19 strong light–matter interactions, 41,42 and strong electrostatic control. 43 2D semiconductor technology is now advanced enough to be compatible with Si-based electronics, allowing diversity in fabrication of nano-devices and circuit-level configurations on the wafer scale. 44–47 1.1. Avalanche breakdown phenomenon In recent years, a few efforts have been made towards impact ionization in avalanche transistors made using 2D materials. Avalanche FETs 48 have been gaining gigantic attention in a Department of Inorganic Chemistry, University of Chemistry and Technology, Prague, Technicka ´5, Prague 616628, Czech Republic. E-mail: [email protected], [email protected] b Department of AI Convergence Electronic Engineering, Sejong University, Seoul, Republic of Korea c Chair of Smart Sensor Systems, University of Wuppertal, Wuppertal, Germany d Department of Chemistry, College of Science, King Khalid University, P.O. Box 9004, Abha, 61413, Saudi Arabia e Laboratory of Human Factors and Automation in Aviation, Department of Air Transport, Faculty of Transportation Sciences, Czech Technical University in Prague (CTU), 128 00 Prague, Czechia f School of Interdisciplinary Engineering and Sciences (SINES), National University of Sciences and Technology (NUST), Islamabad, 44000, Pakistan †These two authors contributed equally. Received 21st May 2025, Accepted 8th July 2025 DOI: 10.1039/d5tc02021b rsc.li/materials-c Journal of Materials Chemistry C REVIEW Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue 15768 | J. Mater. Chem. C, 2025, 13, 15767–15795 This journal is © The Royal Society of Chemistry 2025 electronics due to their ability to perform photo-, gasand biosensing (Fig. 1a). The avalanche breakdown is a phenomenon in semiconductor devices where a sudden increase in current occurs due to the ionization of carriers (holes and electrons) within the material. This happens when an electric field (E-field) across the p–n junction is sufficiently strong, leading to rapid multiplication of charge carriers. The process of avalanche multiplication occurs when an extremely fast charge collides with atoms in a substance, creating electron–hole pairs that are assisted by free valence electrons. The threshold of E-field at which avalanche breakdown originates and a material transits from a higher-resistance state to a lower-resistance state is used to interpret the electrical breakdown voltage (V EB ); the avalanche breakdown region is shown in Fig. 1(b). It is a crucial factor to figure out the highest voltage that an electronic device can tolerate before suffering permanent damage of the crystalline structure of materials. The relationship between V EB and the dimensions of the channel material, such as channel length and thickness, profoundly influences the device configuration, functionalities and fabrication. The electrical breakdown observed in 2D-FETs under a high E-field originated from impact ionization within the 2D channel, commonly referred to as avalanche multiplication. 49 The critical electric field (E CR ) and impact ionization rate (a) are the key Ehsan Elahi Dr Ehsan Elahi currently works as a postdoctoral researcher in the Sofer group at the Department of Chemistry, University of Chemistry and Technology (UCT), Prague, Czech Republic. He obtained his MSc in Electronics from Quaid-iAzam University Islamabad, Pakistan and MPhil from Riphah International University Lahore Campus, Punjab, Pakistan. He attained a doctorate degree (PhD) from the Department of Physics, Sejong University, Seoul, South Korea in 2024. Ehsan’s research interests include 2D materials (TMDCs), 2D ferromagnetic materials, electronics, optoelectronics, spintronics, and spin-caloritronics. Muhammad Farooq Khan Dr Muhammad Farooq Khan is an assistant professor at the Department of AI Convergence Electronic Engineering, Sejong University, Republic of Korea. He earned his PhD (Physics) from Sejong University in 2018. After that, he was a postdoc fellow at Yonsei University in 2019. His research primarily focused on nanofabrication of electronic, memory, and energy harvesting devices manifested by two-dimensional layered semiconductor materials. His contributions extend beyond academia; he successfully won two research grants from the National Research Foundation (NRF) of South Korea and has authored a book chapter on ‘‘2D Material Photonics and Optoelectronics.’’ In addition, he is serving as a referee for various scientific journals ensuring the dissemination of high-quality research. Umer Ahsan Mr Umer Ahsan holds a Master of Science in Physics from COMSATS University Islamabad, Lahore Campus, completed in 2020. He is pursuing a PhD in Chemical Technology at the University of Chemistry and Technology, Prague, where his doctoral research focuses on synthesis, characterization, and device integration of advanced two-dimensional transition metal dichalcogenides and their novel alloys. His work explores applications in nextgeneration nanoelectronic and optoelectronic devices—including field-effect transistors, photodetectors, and chemical sensors—by investigating structure–property relationships to enhance performance. He has authored several peer-reviewed articles advancing sustainable, high-performance electronic systems. Payal Chauhan Dr Payal Chauhan is a Marie Skłodowska-Curie Actions postdoctoral ERA Fellow in the Sofer Group at the University of Chemistry and Technology (UCT), Prague, Czech Republic. She received her PhD in Physics from the Department of Physics, Sardar Patel University, Gujarat, India, in 2022. Prior to joining UCT, she held a postdoctoral position at CHARUSAT University, Gujarat, India. Her current research focuses on two-dimensional composite materials for electrocatalytic applications, including water splitting, supercapacitors, and anion exchange membrane electrolyzers for seawater electrolysis, aiming to advance sustainable energy and hydrogen production technologies. Review Journal of Materials Chemistry C Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15769 parameters for quantifying the avalanche breakdown mechanism. The E CR is referred to as the lowest E-field required for the avalanche multiplication and a(cm 1 ) is referred to as the number of electron–hole pairs produced per unit distance travelled by the hot carrier. Here, E CR =V EB /L,whereV EB is the value of source–drain voltage (V DS ) at which the breakdown begins. When the device operates under the breakdown zone, the carrier multiplication gain (G) becomes limited, making it suitable for photon energy discernment, and the device output current is proportional to the incoming photon power. Conversely, when the device functions above the breakdown voltage, known as Geiger mode, an incident photon can initiate avalanche breakdown, resulting in substantial carrier multiplication. This multiplication can be unlimited and may instigate a self-sufficient avalanche development, thereby assisting single-photon detection. 50 The breakdown behavior of some 2D materials, such as MoS 2 , 51 WSe 2 and graphene, 52 has been the subject of recent investigations. Along with various other device applications, the 2D materials including graphene, TMDCs and black phosphorus (BP) have provided new opportunities for improving the performance of avalanche FETs. 51,53,54 An avalanche photodiode (APD) is an enormously sensitive photodetector that can transform light into current/voltage. Basically, the APD operates at high reverse bias voltages of tens or even hundreds of volts. 55 In this phase, the E-field accelerates the photogenerated electron–hole pairs, allowing them to impact ionize and produce additional carriers. The APD can therefore be employed as an incredibly very sensitive detector that requires minimal electrical intensification. Furthermore, due to the atomically thin nature, 2D materials may be able to start the impact ionization in a short active region (o10 nm) with relatively low electrical bias, resulting in carrier multiplication with a high Gand better noise performance owing to its nanoscale active zone. Fig. 1(c and d) depicts the heterojunction band alignments at various bias voltages to evaluate the intrinsic phenomenon of the avalanche photodiode. 56 The pink area shows the depletion zone in the p-type/n-type heterojunction. When the FET is under bias, the applied voltage aligns with the built-in E-field. At low voltages, as demonstrated in Fig. 1(c), the external E-field is insufficient to cause avalanche breakdown, limiting the performance of the device to a typical photodetector with limited gain. Fig. 1(d) depicts the device’s band alignment at high bias voltage, resulting in carrier multiplication via the avalanche process. 56 Increasing bias voltage at the p–n junction accelerates photogenerated carriers in the E-field, resulting in more energy. Carrier multiplication generates more electron–hole pairs, significantly increasing photocurrent. 57,58 The key metrics parameters of several types of APDs based on 2D materials and their heterostructures are compiled in Table 1. 55,59 EQE is defined as the ratio of the number of collected electrons to the number of injecting photons, which is equivalent to Rhc el%¼Iph P hc el%. 60,61 Here, I ph is the change in photocurrent, lis the wavelength of the incident light, his Planck’s constant, cis the speed of light and Pis the incident optical power. Nevertheless, 2D photodetector devices use impact-ionized carrier multiplication, and all incident photons are not absorbed to generate free electron– hole pairs to contribute to a photocurrent; their EQE is less than one. However, due to the high Schottky barrier among the 2D channel and metal contacts, the majority of 2D APDs require a significantly high bias in order to commence impact ionization. In 2D APDs, the V EB can be decreased by substituting 2D vdW heterojunctions for metal/semiconductor Schottky junctions. Thus, further developments in 2D-APDs that operate at room temperature with low bias and high gain (G) are required. Responsivity (R), 62,63 detectivity (D*) 64,65 and Kalyan Jyoti Sarkar Dr Kalyan Jyoti Sarkar received his PhD degree from the Indian Institute of Technology Kharagpur (IIT Kharagpur), India in 2020. Currently he is working as a postdoctoral researcher at Sofer Group, University of Chemistry and Technology Prague (UCT Prague). Prior to joining the Sofer Group in January 2022, he was a SERB National Postdoctoral Fellow at the Indian Institute of Science Bangalore (IISc Bangalore). He has also worked as a research associate-I in the QuEST-DST project for six months at the Indian Institute of Science Education and Research, Thiruvananthapuram (IISER TVM). His research focus is on fabrication and characterization of 2D material-based devices for optoelectronics, memory applications, and quantum transport studies. Zdenek Sofer Dr Zdene ˇk Sofer has been a full professor at the University of Chemistry and Technology Prague, Czech Republic, since 2019. He also received his PhD from the University of Chemistry and Technology Prague in 2008. During his PhD, he spent one year at Forschungszentrum Julich (Peter Gru ¨nberg Institute, Germany), followed by postdoctoral experience at the University of Duisburg-Essen, Germany. His research interests include semiconductors and 2D materials, their synthesis, crystal growth, chemical modifications and functionalization. He is particularly focused on various applications of two-dimensional materials including energy storage and conversion. Journal of Materials Chemistry C Review Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 15770 | J. Mater. Chem. C, 2025, 13, 15767–15795 This journal is © The Royal Society of Chemistry 2025 EQE 66,67 are the important parameters for the photodetection phenomenon. 67 Ris defined as the ratio of the photocurrent to the incoming light power: R¼Iph PA, 68,69 here Ais the effective area and Pis the incident power density. D* is a term that typically refers to the ability of a sensor to detect a signal or stimulus D¼Rffiffiffiffi A p ffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2eIdark p ! . 70 On he other hand, for outstanding performance of the sensor, and human–computer communication applications, piezotronic and piezo-phototronic devices have been constructed. 2,71,72 Examples of these devices Table 1 The summarized characteristics of avalanche devices Materials Avalanche device type Responsivity (R)(AW 1 ) Detectivity (D*) (Jones) Rise/fall time (s) Gain (G)l(nm) EQE Ref. MoS 2 Phototransistor 3.4 10 7 4.3 10 16 27/1.2 s — 520 E8.1 10 9 %97 Bi 2 O 2 Se Photodiode 3 10 3 4.6 10 14 2.5 ms 400 515 — 222 BP Photodetector 160 — — 7 520 382 10 2 %59 WSe 2 APD Phototransistor 5910 5.3 10 12 8/8.2 ms 500 532 — 109 WSe 2 /WS 2 FET 135 1.3 10 12 131.8/146.5 ms — 400– 1100 — 116 BP/InSe Photodiode 80 — — 10 4 4mm 24.8 10 2 % 165 MoS 2 Photodetector 10 4 210 12 — 24 532 — 100 WSe 2 /MoS 2 Photodiode 88 m——E1300 532 — 117 InSe APD Photodetector — — 87 ms 152 543 866% 141 ZnO Photodetector 1.7 10 4 3.2 10 12 20 ns/98.9 ns 294 367 — 138 p-Ge/n-MoS 2 Photodiode 170 — 357/365 ms 320 532 — 244 MoTe 2 /WSe 2 / MoTe 2 Photodiode 6.02 7.24 10 9 475 ms 587 400–700 1406% 245 Gr/epitaxial silicon Photodetector 0.38 6.63 10 12 1.4 ms 1123 300– 1100 60% 218 InSe APD Photodetector 1 10 5 7.3 10 12 1 ms 500 405–785 — 223 MoS 2 -WSe 2 Bionic 7.6 10 4 — 108/268 ms 1.5 10 4 635 10 7 % 241 WS 2 Photodetector 74 1.45 10 13 — — 532 — 54 Monolayer MoS 2 Phototransistor 8.84 10 8 1.65 10 13 2 ms — 450–650 — 246 Pt/WSe 2 /Ni APD Photodetector E0.28 — 45/50 ms510 5 520 60% 158 Fig. 1 (a) Schematic of an avalanche FET/avalanche photosensor: which can be further used for gas and biomolecule sensing. (b) Schematic representing the avalanche breakdown region in the form of the (I–V) curve. The band configuration of the p–n heterojunction: (c) under low bias (VoV EB ). (d) High bias V4V EB . Review Journal of Materials Chemistry C Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15771 include piezoelectric FETs, 73 nanogenerators, 73 solar cells, 74 phototronic photocells 73 and piezotronic strain sensors. 75 Monolayer MoS 2 was used to produce piezotronic transistors. 76 The piezotronic GaN tunnelling junction responded quickly to external mechanical stimuli, taking only a response time of 4.38 ms. 76 GaN vertical nanowires have a temporal response of less than 5 ms. 77 In general, the piezoelectric field and potential govern the built-in field in the p–n junction, as well as the height of the barrier at the metal–semiconductor interface. 78,79 The piezotronic devices built with multijunctions have a gauge factor above 10 4 and may be used as ultra-high sensitivity strain sensors, due to integrating piezoelectric control with bipolar transistor amplification. 80 However, there has always been a difference between advanced applications and basic research. The absence of a consistent and practical method for characterizing their characteristics, which ought to be well-suited with the conventional photodetector performance assessment scheme, has been one of the primary causes of this disparity. Determining the level of interoperability between laboratory prototypes and industry technology is crucial. We present broader principles for the evolution of the figures of merit for 2D-based devices and looked at frequent instances in which the particular D*, R,I dark , and speed might be misjudged. This review article explains the latest advancements in avalanche FETs that use the distinctive characteristics of 2D materials. In addition, we also explore the potential applications of avalanche FETs in sensing technology, which is becoming more important for recent applications of drug screening, healthcare, and cybersecurity systems. To understand the use of 2D semiconductors in electronics, optoelectronics, and sensing technology as avalanche FETs, it is important to explore fundamental concepts and recent breakthroughs. Finally, we discuss the challenges and potential of 2D semiconductorbased devices for practical applications and their scalability, which should be addressed by both academic and industrial researchers. Our perspective can assist academia for a better understanding of 2D semiconducting materials and their fundamental concepts with deep insights. 2. Avalanche breakdown phenomena in 2D and TMDC materials 2.1. Avalanche effect in MoS 2 -based FETs TMDC nanosheets have gained popularity due to the lack of dangling bonds on their surface, their exceptional mechanical flexibility, and their high surface-to-volume ratio. 81–83 Exploration has focused on layered MoS 2 for its exceptional electrical properties and thickness-dependent band configuration, which shifts from an indirect bandgap of 1.2 eV to a direct bandgap of 1.8 eV with fewer layers. 84–86 MoS 2 has the potential to revolutionize electronics, including ultrathin transparent FETs, 87,88 logic circuits, 89,90 and sensor applications. 91–93 To employ MoS 2 in spatial integrated electrical proposals and applications, FET channel lengths needed to be reduced to sub-micrometers. As a result, exposure to strong lateral E-fields may cause electrical breakdown. Furthermore, the electronic band configuration in semiconductors has an impact on the electrical breakdown. As MoS 2 -FETs show the quantum confinement effect, the thickness of MoS 2 determines the electrical breakdown in the device. Under strong E-fields, the electrical characterization of MoS 2 is rare due to thermal breakdown as the channel layer’s MoS 2 offers poor heat dissipation capability. 94,95 The significant heat resistance of insulating materials like SiO 2 intensifies this phenomenon. 96 For these motives, to avoid thermal breakdown due to Joule heating, most of electrical tests in MoS 2 FETs have been carried out in the linear domain with smaller V DS . Here, we review the investigation of avalanche multiplicationrelated electrical breakdown processes in MoS 2 FETs with various channel lengths and thicknesses. 51 By varying the number of stacking layers, one can control the avalanche multiplication by modifying band configuration in MoS 2 due to the phenomenon’s quantum confinement impact. Fig. 2(a) displays the optical image of the device based on MoS 2 exfoliated flakes with Ti/Au metal contacts. The impact of high E-field on the electrical breakdown was investigated with different channel lengths (1.49, 1.95, 3.42, and 4.97 mm) with uniform thickness of the MoS 2 layer (B2.4 nm). The electrical characteristics of the devices were checked at room temperature. The adsorbed air molecules on MoS 2 reduce the out-ofplane phonon vibration as observed, thus reducing energy dissipation by electron–phonon scattering. 51 As a result, electrical breakdown in the air can happen at lower V EB . Analyzing the normalized I DS –V DS curves reveals that as the channel length is raised, the early voltage V EB is accompanied by a sudden rise in channel current and switched to the positive V DS as represented in Fig. 2(b). The impact of temperature on avalanche multiplication has also been examined with a MoS 2 flake of 17 nm thickness. The measurements were conducted at a fixed V GS of 0 V while varying the temperature from 80 to 300 K. Fig. 2(c) demonstrates that the normalized I DS experienced a rapid increase in the low E-field region until it reached its saturation value. This occurred as the thermal energy supplied to the electron surpasses the Schottky barrier at the junction between the metal electrode and MoS 2 layer, as indicated by the low E-field regime. Under conditions of high E-field, the electrons had enough energy to traverse the Schottky barrier and initiate interactions with optical phonons. Consequently, the normalized I DS declined as temperature increased owing to the dissipation of energy resulting from the interaction with optical phonons, as seen with the high E-field. The E CR values at the onset of avalanche multiplication were similarly influenced by temperature since a greater E-field was necessary to counterbalance the energy dissipation caused by electron–phonon scattering with increasing temperature. These findings provide significant evidence for the impact of temperature-dependent electron–phonon scattering on the generation of electron–hole pairs. This behaviour may be explained by two compensatory factors: firstly, a greater temperature enhances electron–phonon scattering, which has a negative influence on impact ionization. Secondly, it decreases the bandgap energy of MoS 2 , leading to enhanced charge carrier concentration, which has a significant influence on Journal of Materials Chemistry C Review Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 15772 | J. Mater. Chem. C, 2025, 13, 15767–15795 This journal is © The Royal Society of Chemistry 2025 impact ionization. The effects of carrier concentration and E-fields on electrical breakdown imply that the electrical breakdown in MoS 2 FETs arises from avalanche multiplication. The occurrence of avalanche multiplication was contingent upon the thickness of MoS 2 . The impact of electron–phonon scattering and the power law analysis of the correlation between E CR and bandgap energy demonstrated that the avalanche multiplication characteristics in MoS 2 agreed with those seen in 3D semiconductors. This research aims to enhance comprehension of the electrical breakdown events in MoS 2 when exposed to strong E-fields. Additionally, it will offer valuable understanding for the future accomplishment of controllable avalanche multiplication properties, which are now only achievable in thickness-dependent 2D layered MoS 2 and other TMDCs. For instance, the reliance of ‘‘E CR ’’ and ‘‘a’’ on the thickness of the MoS 2 layer was significantly associated with the quantum confinement effect in a 2D layer as observed in this study. On the other hand, the authors demonstrated an ultrasensitive avalanche device that utilizes chemical vapor deposition (CVD) to produce monolayer MoS 2 , 97 The optical picture of CVD-grown MoS 2 flakes is shown in Fig. 2(d). Fig. 2(e) displays an optical image of the fabricated device based on the MoS 2 FET. ‘‘Au’’ is utilized for metal electrodes, while a highly p-doped Si with a resistance of about 5 10 3 Ocm was employed as the back gate. A certain portion of the MoS 2 flakes was chosen to be utilized as a FET channel, and it was placed onto an h-BN flake. The device illustrates the electrical breakdown phenomenon of MoS 2 FETs under high V DS at different V GS levels. 97 The observed breakdown at various V GS levels is shown in Fig. 2(f). At V GS values of 10, 20, 30, 40, and 50 V, there is a noticeable sharp rise in both I DS and V DS . Typically, drain-induced barrier lowering (DIBL), thermal breakdown, junction punch through, and avalanche carrier multiplication are the four major factors that might result in a rapid increase in I DS of FETs. DIBL represents a potential short-channel effect in MoS 2 FETs. Monolayer MoS 2 ’s electrostatic properties result in low DIBL even in ultra-short channel devices. 51,98 MoS 2 FETs fabricated on a SiO 2 substrate show little temperature rise. Given that h-BN has a much higher thermal conductivity (B420 W m 1 K 1 ) than SiO 2 (B1.40 W m 1 K 1 ), this device architecture would be less affected by the thermal impact of Joule heating than MoS 2 FETs on a SiO 2 substrate. Accordingly, thermal breakdown is also not a likely cause for an unexpected spike in I DS . When the depletion zones between the bulk and n + – drain contact and the p-bulk and n + – source contact overlap, the junction punch-through effect occurs, enabling I DS to pass through the overlapped depletion regions. 99 The previous work 51 suggests that other possible causes of the observed breakdown mechanism such as DIBL or thermal effects can be similarly neglected, and the breakdown phenomenon can be attributed to avalanche multiplication. It’s interesting to note that the breakdown is closely correlated with the value of V GS at which V EB and DI DS /DV DS were measured. V EB and DI DS /DV DS values dropped when V GS went from 50 V to 20 V. However, it was not possible to notice a significant rise in I DS at V GS =10 V (Fig. 2(f)). This can be attributed to an excessively high channel current (I DS ) caused by the gate-field-induced carriers prior to the avalanche Fig. 2 (a) Optical image of an MoS 2 -based device with varying channel lengths. (b) Normalized I DS of the MoS 2 FET explored at fixed V GS = 0 V and different channel lengths; the breakdown voltages with E-fields are indicated by coloured dashed lines. (c) Normalized I DS associated with E-field recorded at V GS = 0 V at different temperatures between 80 and 300 K. Figures (a)–(c) are reproduced with permission from ref. 51, ACS Nano (2018). (d) Optical image of the CVD-grown MoS 2 . (e) Optical image of the fabricated device based on the structure h-BN/MoS 2 . (f) I DS –V DS characteristic curve at different V GS values. Figures (d)–(f) are reproduced from ref. 97, Copyright, Advanced Science (2021). Review Journal of Materials Chemistry C Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15773 breakdown. It is evident that the compliance current was attained at a V DS value of 48.5 V, even lower than V EB (49.5 V) for V GS =20 V, while the I DS was measured at V GS =10 V. Overall, the modification of V GS may be used to explain the observed dependency of V EB on V GS by altering the height of contact barrier between the channel and metal electrodes. Since greater E-fields in the channel are felt by electrons inoculated from the source to MoS 2 , when V DS increases, the value of DI DS /DV DS increases with an increase in V EB , resulting in a drop in V GS . The fundamental features of breakdown processes in MoS 2 FETs might also be examined using the electrical characteristics that were acquired. The formula 51 for calculating the multiplication factor (M), or the amount of channel current generated by the electrical breakdown, is M(V DS )=I DS (V DS )/I DS (V DS =V EB )=I DS (V DS )/I sat (1) where I sat is the saturation current, it is observed that the ‘‘M’’ depends on V DS . Next, at various V GS levels, it is shown that ‘‘11 M’’ is a function of V DS /V EB . Empirically, the following equation signifies the link between ‘‘11 M’’ and V DS /V EB . 11 M¼VDS VEB  m (2) Here, the fitting equation may be used to obtain ‘‘m’’. 100 As in other studies, the fitting was carried out in this case close to the ln (V DS /V EB ) value of 0.05, that is, shortly after the breakdown began. 59,100 Overall, it was shown that the interaction between carrier injection via the contact barrier and carrier multiplication by avalanche breakdown is crucial. This work offers a prevailing approach to enhance the performance of avalanche devices and provides a thorough knowledge of thin avalanche FETs, which are uninvestigated areas of investigation in electronics and optoelectronics. 2.2. Absorption-multiplication avalanche WSe 2 -FETs It has been established that avalanche devices based on 2D semiconductors have achieved improved performance and efficient G. Here, we reviewed an avalanche device based on the Au/WSe 2 /Ge structure, where infrared absorption and avalanche zones are observed for the Au/WSe 2 Schottky-based junction utilizing the germanium (Ge) substrate, correspondingly. Fig. 3(a) presents the schematic illustration of the Schottky heterojunction (S-HJ) based separate absorption multiplication (SAM) device with its circuitry representing the grounded source terminal Ge node and the applied negative drain bias; the inset displays the equivalent circuit of the device, wherein the resistance of the WSe 2 channel material is distributed into R 1 and R 2 , and the Schottky contacts of WSe 2 / Au are symmetrical to the Schottky diodes S 1 (drain node) and S 2 (source node) as demonstrated in the inset of Fig. 3(a). Furthermore, the WSe 2 /Ge junction forms the diode (D 1 ), which is what gives the device its infrared detecting capability. The device’s fundamental electrical properties are displayed in Fig. 3(b), where the curves representing the drain current and Fig. 3 (a) Schematic illustration of the S-HJ SAM device with the electrical structure: inset displays the equivalent circuit. (b) I–Vcharacteristic curves of the drain side (blue) and bulk Ge side (pink dashed line) of the S-HJ-SAM. Temperature-dependent I–Vcurves of the device. (c) Temperature-dependent breakdown voltage. Figures (a)–(c) are reused with permission from ref. 109, Copyright, ACS Photonics (2023). (d) Optical image of the device with scale bar 5 mm: inset shows the schematic diagram of the avalanche device and height profile of the channel material (WSe 2 ). (e) Output curves of the device: p-type mode, (f) n-type unveiling ambipolar transport features. These figures (d)–(f) are reproduced with permission from ref. 105, Copyright, ACS nano (2022). Journal of Materials Chemistry C Review Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 15774 | J. Mater. Chem. C, 2025, 13, 15767–15795 This journal is © The Royal Society of Chemistry 2025 Ge-node current vs. drain voltage are represented by the black solid line and the red dashed line, respectively. The drain current and Ge-side current are 2 10 11 A and 3 10 12 A, correspondingly, when the bias voltage is 1 V. Both the D 1 and S 1 Schottky diodes are reverse biased when the V DS is negative, and the reverse leakage current gradually rises as the V DS rises. The significant rise in drain current seen at 25.5 V is ascribed to the device’s critical breakdown. It is noteworthy that only the drain current had a sudden rise upon reaching the breakdown, whilst the bulk Ge side current did not exhibit a sharp increase. This suggests that D 1 diodes are not susceptible to breakdown under high reverse voltages. As an outcome, the breakdown in the S-HJ-SAM device happens in the WSe 2 channel around the Au/WSe 2 Schottky diode (interface S1). The device temperaturedependent I–Vcharacteristics are shown in Fig. 3(c). When the temperature drops, the breakdown voltage (V EB ) decreases as well. This characteristic demonstrates that the avalanche effect dominates the breakdown at high bias voltages. At higher temperatures, carriers lose more energy owing to the lattice scattering events, leading to a positive temperature coefficient for avalanche breakdown voltage compared to lower temperatures. At greater temperatures, carriers require a higher bias voltage to achieve adequate energy for impact ionization. 101 High Gfor an APD based on the WSe 2 /Ge heterojunction remains problematic due to tunnelling effects. However, the S-HJ SAM device offers a novel approach of designing avalanche breakdown devices. 2.3. Avalanche multiplication in channel-length-modulated ambipolar WSe 2 FETs Avalanche multiplication has gathered significant attention in 2D material-based FETs. 102 Prior research has mostly employed a unipolar material as the active channel, with an emphasis on making highly efficient devices. Developing efficient ambipolar electronic devices and novel structures for avalanche breakdown is still a challenge. While the easy carrier-type tuning of ambipolar 2D materials may be achieved by electrostatic gating. When an ambipolar material is utilized as the active channel, the high V DS needed to start avalanche multiplication inverts the gating effect close to the drain electrode, allowing both carriers to go through the channel simultaneously in an ambipolar manner. 103,104 It is possible to separate the two opposing phenomena by using channel length modulation, and it is feasible to analyze the properties of avalanche multiplication in ambipolar WSe 2 FETs by focusing on the fact that avalanche multiplication is controlled by an E-field while ambipolar transport is controlled by voltage. Conventional MOSFETs usually reach saturation mode when a sufficiently high V DS is applied, as the pinch-off occurs near the drain electrode. Nevertheless, with the additional rise in voltage, a multitude of physical phenomena may manifest inside the semiconductor channel. An optical microscopic view of the long-channel FET device is illustrated in Fig. 3(d): the inset denotes the schematic of the device. Multiple short-channel devices were constructed on a single WSe 2 flake. It is important to observe that for the device having a shorter channel, current density was reduced due to additional constraints including thickness, contact, and interface conditions. Although the transfer curve of this device displays ambipolar behaviour consistent with the long-channel device as shown in Fig. 3(d), the output curves at high voltages vary significantly, suggesting that the fundamentalphysicalprocessin the short-channel transistor is distinct. Fig. 3(e) and (f) demonstrate the electrical characteristics of the device under high V DS in n-type and p-type modes, correspondingly. The output curves highlight the triode as well as the saturation region at smaller V DS .WhenV DS is beyond a certain threshold, the current increases beyond its saturation level. This supports ambipolar transport behaviour and has been recognized in FETs with diverse channel materials. 105,106 The device functions with majority carriers until reaching the saturation threshold, after a subsequent increase in current results from the buildup of opposing charge carriers (i.e., minority carriers) at the drain electrode (Fig. 3f). Upon increasing the V DS beyond the saturation zone, there was a subsequent increase in current until the compliance limit was applied to avoid device failure caused by thermal breakdown. Unlike ambipolar transport observed in longer-channel FETs, the increase in the current did not initiate at distinct voltage levels. In contrast, the current increased at a constant rate from its saturation level, independent of the gate voltage, as given by M=I/I sat . Two possible reasons for the rise in current after saturation are Joule heating at the contact and the current crowding effect. 107,108 Nevertheless, as the current increased at a low level and there was minimal reliance on the gate voltage, this phenomenon can be attributed to avalanche multiplication. The contrary forms of output curves at higher voltage between longand short-channel devices arise from two separate physical events happening inside the channel. Long-channel FETs are characterized by ambipolar transport, shown by a parabolically growing drain current upon saturation. A critical voltage (V cr ) is defined as the threshold for the commencement of ambipolar transport, as illustrated by the highlighted arrows in Fig. 3(e) and (f). Short-channel FETs exhibit features of avalanche multiplication, with the V EB referred to as the threshold at which avalanche breakdown occurs. The V cr values shown by blue open circles of long-channel devices demonstrated a linear correlation with the tuning of gate voltage for both nand p-types. Furthermore, the slopes of the fitted lines were almost equal to ‘‘1’’ for all devices, suggesting ambipolar transport. The value of V cr does not depend on the channel length but is contingent upon the threshold voltages of any polarity. Consequently, the threshold voltage is affected by the dielectric constant of the insulator and the circumstances at the interface. Overall, this study aims to comprehend the avalanche multiplication properties in atomically thin materials and make a valuable contribution to the advancement of efficient and emergent device designs by selectively combining ambipolar transport and avalanche multiplication. 3. Avalanche FETs based on TMDC heterostructures 3.1. Avalanche FETs based on the WSe 2 /MoS 2 heterostructure Fig. 4(a) depicts an optical microscopic view of the 2D heterostructure-based APD. The photodiode is protected by h-BN Review Journal of Materials Chemistry C Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15775 flakes, and WSe 2 and MoS 2 monolayers are linked to the Au (source/drain) electrodes. The electrical characteristics of the diode were explored, and the device represented rectification behaviour.Asthegatevoltageincreases,theMoS 2 area gets extensively doped, leading to reduced contact resistance. In contrast, WSe 2 and the heterostructure regions become depleted and partially n-type doped, correspondingly (Fig. 4(b)). This feature canbedemonstratedbyann + –n–i junction. The strongly doped MoS 2 area (marked as red dots in Fig. 4(b)) has a high number of electrons, resulting in a high current under forward bias. 110,111 The I–Vcharacteristics of the measured avalanche device at the reverse bias varying from 0 to 10 V and under vacuum conditions are plotted in Fig. 4(c). The current grows linearly with the reverse bias o6.5 V. The dark current begins to improve rapidly when the reverse bias is raised over 6.5 V, signifying the avalanche multiplication to occur 59 and highlighted by the blue line. The red dashed line shows the linear fitting of the dark current prior to the avalanche effect. The relation M=I dark /I s couldbeusedtogetthe multiplication factor (M). 105 I s is the saturation current and referred to as I dark at V=V cr . The breakdown voltage in FETs can be tuned by the gate voltage (V gs ). 51,97,105 But to achieve low V EB ,a large value of V gs is required and hence the power consumed is still high. Alternatively, the minimal V EB can be attained by the TMDC P–N junction because the voltage reduction that takes place across the space charge region is prominent. 112 Nevertheless, definite doping and energy band modification of TMDCs are essential in the heterojunction. 112,113 Furthermore, the avalanche phenomenon becomes difficult to detect, as the presence of the tunnelling effect will cause the breakdown before the accomplishment of the avalanche breakdown. 114,115 There is another approach that we have reviewed here, a lowvoltage avalanche device based and out-of-plane WSe 2 /WS 2 p–n heterojunction FET referred to as HJ-FET. There is a noticeable reduction in ‘‘V b ’’ detected in the HJ-FET as comparable with a single-channel material-based WSe 2 -FET. 116 The reduction in V EB in the HJ-FET is initiated by the E-field rearrangement in the channel after the creation of the out-of-plane P–N junction. Fig. 4(d) shows the two device architectures, a WSe 2 FET and another flake of WSe 2 placed on top of WSe 2 (WSe 2 –WSe 2 flakes) and HJ-FET rooted with ah-BNspacerlayer(WSe 2 –hBN–WSe 2 FET). So, the avalanche breakdown characteristics of the WSe 2 FET practically remained the same after placing another WSe 2 flake. The associated V b values were 41 V and 40 V congruently. In the WSe 2 –hBN–WSe 2 FET, the V b values of the WSe 2 –hBN FET and the WSe 2 –hBN–WS 2 FET are 35 and 36.4 V, correspondingly, as revealed in Fig. 4(e). All these values are partially greater than those of WSe 2 -based devices. The utmost V EB is attained in the WSe 2 –hBN–WS 2 -based device. This feature may be identified as enhanced defect-induced scattering brought about by the mechanical stress applied during the process of depositing a WS 2 flake on the WSe 2 –hBN and defect-induced scattering at the h-BN/WSe 2 junction. The implanted h-BN isolation layer can stop the creation of heterojunctions among the WSe 2 and WS 2 in the WSe 2 –hBN–WS 2 FET. Consequently, in the HJ-FET the development of heterojunctions offers a vital role in dropping the value of V b . In the FET, the drain-source E-field is perpendicular to the built-in electric field produced by the vertical WSe 2 /WS 2 p–n heterojunction and carriers are only increased and stimulated by the drain–source E-field. The deficiency of the carrier is enhanced Fig. 4 (a) Optical image of the heterostructure based on MoS 2 /WSe 2 with h-BN at the top. (b) The energy band diagram of the diode at V g = 50 V. (c) I–V curve of the diode at V g = 50 V as reverse bias is applied, representing the avalanche phenomenon at greater reverse bias beyond V cr . The figures (a)–(c) are reproduced by ref. 117, copyright, Nano Letters, ACS (2022). (d) Schematic structure of the HJ-FET. (e) I–Vcharacteristics of the WSe 2 FET without and with an h-BN isolation layer, for the arrangement WSe 2 –hBN–WSe 2 FET; the inset shows the I–Vcurves as biasing is nearly equal to V b . (f) V b vs. the V gs of the WSe 2 -FET, HJ FET, and WSe 2 –hBN–WS 2 FET. Figures (d)–(f) are reproduced from ref. 116, Copyright, Nano Research (2023). Journal of Materials Chemistry C Review Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 15782 | J. Mater. Chem. C, 2025, 13, 15767–15795 This journal is © The Royal Society of Chemistry 2025 and holes undergo multiplication in MoS 2 and WSe 2 monolayers, correspondingly, by the avalanche multiplication phenomenon. Fig. 7(e) illustrates the energy band configuration, representing TPA-based optical absorption and carrier multiplication during transit. Because the energy of the excitation photon is less than the bandgap energies of both MoS 2 and WSe 2 ,holes(electrons)in the conduction band (valence band) must absorb two photons to be elevated to the valence band (conduction band). 180 In WSe 2 , photoexcited holes are amplified during transit by the avalanche effect, but in MoS 2 , photoexcited electrons undergo multiplication.Fig.7(f)displaystheI–V characteristics of the TPA-based APD under both dark conditions and under 1030 nm light. The TPA process is fundamentally weak, resulting in a negligible photocurrent even at a substantial optical power density of 31.8 kW cm 2 when the reverse bias is less than 10 V, as seen in the inset image of Fig. 7(f). The enhanced avalanche effect at high reverse biases enhances the Rby about three orders. This study demonstrates unprecedented R, attributable to both a high two-photon absorption coefficient and a substantial avalanche G in monolayer MoS 2 and WSe 2 . The rapid charge transfer mechanism of photogenerated charge carriers in the heterostructure area may enhance R.Herewehavereviewedanotherpower efficient WSe 2 avalanche photodetector designed using an in-plane WSe 2 FET and an out-of-plane WSe 2 /WS 2 P–N junction. The decreases in breakdown voltage (V b ) in the HJ-FET are seen and the greater ‘‘R’’ in the HJ-FET is explored under light exposure. Fig. 7(g) presents the I–Vcurves of the WSe 2 FET and HJ FET under dark and light modes: inset illustrates the schematics of the APD. Both kinds of devices represent an enhanced current and decreased breakdown voltage under 532 nm laser light and the highest Ris 135 A W 1 .Fig.7(h)illustratestheGin bothkindsofsamples.ItisclearlyshownthattheHJFEThas greatergainwithrespecttothesameV DS when the bias is in the range of 5to16.5V.Furthermore,themaximumGover 60 is observed in both types of devices. Fig. 7(i) shows the band structure of the WSe 2 /WS 2 -based structure, which denotes the movement of charges after illumination. These outcomes indicate that the use of the WSe 2 /WS 2 heterostructure is a substantial way to improve the efficiency of APDs with low power utilization. 116 6. 2D-hybrid heterostructure based on avalanche FETs 6.1. Plasmonic waveguide (APD) Silicon photonics 181 has emerged as a viable platform for several significant applications including on-chip optical sensors, 182 optical telecommunications 183 and nonlinear photonics. 184 We reviewed here a high-speed and very sensitive device based on Si/BP hybrid plasmonic waveguide (HPWG) APDs effective at the wavelength of 1.55/1.95 mm. The HPWG on a thin silicon-oninsulator (SOI) platform is specifically designed to augment light absorption by the BP while concurrently facilitating brief carrier transit times, which is crucial for attaining high speed and high ‘‘R’’ in the photodetector. Fig. 8(a) illustrates the graphic architecture of the current Si-BP HPWG-based APD combined with various passive components, with the two-channel (1.55/ 1.95 mm) wavelength-division (de)multiplexers and the grating couplers at the input/output terminals. These passive parts are implemented to effectively characterize the current devices operating throughout a wide wavelength range, considering that a grating coupler is typically wavelength-sensitive and has a restricted operational bandwidth of around several tens of nanometers. In the active area, a horizontal Si-HPWG is introduced, including double nano-slots flanking a central silicon core and two metal strips on each side. 185,186 The horizontal HPWG is specifically enveloped by a multilayer BP sheet, as seen in Fig. 8(a) and (b), therefore augmenting light absorption in the BP layer. The two metal strips function as source (S) and drain (D) electrodes, therefore minimizing the transit distance between them to enhance reaction speed. The ‘‘Si’’ core and the BP layer are physically separated by an ultrathin Al 2 O 3 insulating layer. Instead of using the usual Si with a thickness of 220 nm, a thin Si-core was employed to increase light absorption by the BP. 187 The exfoliated BP film is successfully floated above the nanoslots within the electrodes and the Si-core due to the exceptional mechanical characteristics of the BP flake. 185 The study indicates that raising the thickness of the BP layer enhances its optical absorption. 188 Conversely, a thinner BP layer is favoured to get reduced mode-mismatching loss and enhanced mobility, 189 which is crucial for obtaining a rapid response. A modest BP thickness (about 30 nm) is used for the existing photodetector to optimize the balance between Rand speed. Furthermore, due to the anisotropic nature of BP, its orientation significantly influences the electrical and optical characteristics of the photodetectors. 190 The biasing is established to remain below 2 V to prevent thermal breakdown. Fig. 8(c) illustrates the recorded photocurrent of the current BP photodetector functioning at a 1.95 mm wavelength. The optical power ranges from 1.4 to 186 mW. The photocurrent has a behavior similar to that of the dark current, demonstrating a superliner growth at elevated voltages. The photocurrent exhibits a quick increase at low optical power levels (e.g.,o15 mW) and then rises more gradually as the optical power ascends to 186 mW. Rsignificantly rises, attributed to the space charge effect, a phenomenon extensively reported in fundamental APDs. 191 Increased photocarrier densities resulting from greater optical power diminish the E-field strength inside the multiplication area. In the coming times, electrical bandwidth may be improved by reducing the gap between both electrodes, while the dark current can be minimized by properly regulating the height of barrier. In comparison to previously reported photodetectors, the current Si/BP HPWG based APD is among the most superior devices, offering a potential alternative for future optical applications. 6.2. Broadband fiber-integrated APD of WS 2 /Bi 2 O 2 Se Layered bismuth oxychalcogenide (Bi 2 O 2 Se), a recently identified 2D material, exhibits exceptional mobility (B20 000 cm 2 V 1 s 1 ), a suitably slight band gap of 0.8 eV, and outstanding air stability, thus positioning it as an outstanding contender for optoelectronic devices. 192,193 WS 2 has garnered significant scientific attention due Review Journal of Materials Chemistry C Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15783 to its exceptional optical characteristics, particularly in the domains of sensors and photonic devices. 194,195 The inadequate efficiency of Bi 2 O 2 Se-based photodetectors is attributed to elevated dark current and a diminished ON/OFF ratio while restricting light absorption due to the poor responsivity of pure WS 2 . 196,197 Assembling vdW heterojunctions has been described as an active techniquetoresolvetheissueofhighdarkcurrentandtoadjust interlayer transition energy. The bandgap of Bi 2 O 2 Se is 0.8 eV, whereas WS 2 has an indirect bandgap of 1.38 eV, enabling the formation of a type-II vdW heterostructure between 2D WS 2 and Bi 2 O 2 Se. 192,198,199 We review a self-powered high-speed fibreintegrated photodetector (FIP) based on the WS 2 /Bi 2 O 2 Se heterostructure on the end face of the optical fibre. This FIP operates in the avalanche zone, exhibiting an EQE of 141% and a ‘‘G’’ of 44. Notably, this FIP has strong performance in wrist bending measurement, indicating significant potential motion recognition application alongside traditional photodetection. The conduction band energies of WS 2 and Bi 2 O 2 Se are 4.5 eV and 4.25 eV, correspondingly. 200,201 The Bi 2 O 2 Se electrons will migrate to the WS 2 side due to different Fermi levels. Consequently, a type II band configuration is established at the junction of the overlapping WS 2 /Bi 2 O 2 Se as seen in Fig. 8(d), accompanied by the emergence of a built-in E-field in the depletion region. The limited band gaps of WS 2 and Bi 2 O 2 Se facilitate the absorption of a greater number of incoming photons. Fig. 8(e) illustrates the device’s photo-response measured under dark and light at a lof 650 nm with varying power of incident light. Even when the applied bias surpasses 80 V, the dark current remains significantly low. Conversely, with an applied bias below 39 V (region I), the photocurrent rises gradually; however, it escalates significantly when the bias voltage exceeds 39 V owing to the avalanche effect. In Fig. 8(e), the critical device parameters are presented at a bias of 68 V (area II), and the R,EQEandGare calculated to be 0.74 A W 1 ,141%, and 44 under a power intensity of 56.7 mW. Moreover, at a bias of Fig. 8 (a) Schematic arrangement of the device-based Si/BP. (b) SEM image of the fabricated device. (c) The examined photocurrent of the APD as the bias varies. Figures (a)–(c) were reproduced with permission from ref. 216, copyright ACS Photonics (2022). (d) Schematic representation of the energy band configuration of WS 2 /Bi 2 O 2 Se under light exposure. (e) The photocurrent of the FIP with various intensities (650 nm) may be categorized into three distinct zones: the normal reverse bias effective zone (I), the avalanche breakdown zone (II), and the reverse bias breakdown zone (III). The red circles indicate the avalanche ‘‘G’’ achieved at a light intensity of 56.7 mW. (f) The photoresponse of the device at 1550 nm, with intensity (0 mW to 7.0 mW) is revealed, with the red circles indicating the avalanche ‘‘G’’ of the device under 1550 nm light at 7.0 mW. Figures (d)–(f) were reproduced with permission from ref. 217, copyright, Optics Communications (2023). (g) Working phenomena and avalanche multiplication behaviour of nMAG/Si APD are illustrated schematically. (h) Dark and photocurrent inspection in the avalanche region. (i) ‘‘G’’ at different power intensities of radiant light as a function of reverse biasing. Figures (g)–(i) are reproduced with permission from ref. 218, copyright, Advanced Optical Materials (2024). Journal of Materials Chemistry C Review Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 15784 | J. Mater. Chem. C, 2025, 13, 15767–15795 This journal is © The Royal Society of Chemistry 2025 80 V (region III), the EQE attains 642% with an improved Rof 3.36 A W 1 atincidentlightwiththepowerof5.2mW. The elevated EQE above 100% is ascribed to collisional-ionized carrier multiplication. Additionally, the distinguishing feature of APDs is investigated, which shows a linear relationship between the incident power of light and photocurrent response at different biases. The APD of FIP is wideband; meanwhile, the avalanche effect is distinct at 1550 nm of incident light (Fig. 8(f)). The avalanche effect was seen under a bias voltage above 32 V. Upon increasing the biasing voltage up to 46 V, reverse bias breakdown commenced. The amount of photocurrent was minimal at bias voltages below 32 V at a wavelength of 1550 nm. The photocurrent rose quickly as the bias voltage increased. The photocurrent reached 148 mAwitha Gof 12.88 when the illumination intensity was 7.0 mW and the bias voltage was 60 V. This ‘‘FIP’’ has excellent capabilities in measuring bending deformation due to its great sensitivity to variations in light intensity. A reliable method to produce rapid, broadband fibre-integrated photodetectorsisprovidedbythis work, which has potential uses in fibre-integrated multifunction systems. 6.3. Self-quenched avalanche photodetectors There have been many recent reports on APDs based on III–V compound semiconductors, silicon (Si), and germanium (Ge). 202–204 Because of their small bandgap characteristics, APDs constructed from Ge and III–V semiconductor materials have been extensively used for sensing in the near-infrared (NIR) band. 102 Nevertheless, these APDs’ hole–electron dissociation ratio (k-value) is almost ‘‘1’’, which leads to a significant amount of extra noise that makes it challenging to improve the device’s efficiency even further. 205,206 Si electronic components are perfect for achieving high carrier multiplication because of their very low k-value (o0.1), low dark current, and superior multiplication capabilities. 207 Consequently, enhanced Sibased APDs that allow for sensitive NIR spectrum detection are needed, especially in the communication band. The most appropriate method for achieving this is to develop a heterojunction APD by combining a 2D material with Si, which can increase the operational bandwidth that goes beyond the communication band. Graphene has special properties including durability, small band gap, and greater carrier mobility and acts as a significant 2D material. 208 High-performance photodetectors may benefit from macro-assembled graphene nanofilms (nMAG) because of their large-area, high crystallinity, and precisely controlled thickness. 209 We reviewed here a vertical heterostructure photodetector made of nMAG and epitaxial silicon (epi-Si) with an Rof 0.38 A W 1 and a response time of 1.4 ms. Additionally, the APD shows a very low noise level and a significant avalanche Gof 1123. In addition to enabling selfquenching by switching from light to dark via avalanche multiplication, it can operate with relatively smaller avalanche turn-on voltages and transfer data at a real-time rate of 38 Mbps over data networks for near-infrared light communication. The suggested structure makes it possible to use complementary metal-oxide-semiconductor (CMOS) compatible methods to fabricate high-performance APDs in the infrared spectrum. The schematic diagram of an APD is presented in Fig. 8(g); when the bias voltage is raised, the nMAG layer turns as the absorption layer for the NIR longwave spectrum in the nMAG/ epi-SI device, as presented in Fig. 8(g). The mildly doped epi-Si layer, conversely, functions as a multiplication layer. Electrons are driven into charge multiplication zones in epi-Si by the provided reverse bias, which also separates photon-generated electron–hole pairs. Photogenerated electrons gain significant kinetic energy under an extensive internal electric field in a relatively large depletion region, and avalanche multiplication can be accomplished through impact ionization with valence electrons in the lattice, resulting in free electrons that grow exponentially, causing the rapidly increasing photocurrent. In epi-Si, phonon concentration rises with temperature, leading to rapid energy loss through electron–phonon collisions. Therefore, carriers can only acquire the energy needed for impact ionization and avalanche multiplication in greater electric fields. The I–V graphs in Fig. 8(h) show that when the voltage reaches a particular threshold, the current rises quickly. As the power density varies from 0.11 to 1.14 W mm 2 (see the inset of Fig. 8h), the breakdown voltage (V br ) gradually drops from 13.5 to 11.9 V, which is much lower than the dark current (23.7 V). Since the high-power density increases the number of nMAG-excited photogenerated carriers, which enables early impact ionization of electrons, the change in threshold voltage enhances the avalanche multiplication effect. Additionally, an unequal resistance distribution brought about by the high-power density increases the influence of ionization gain and the electric field on nMAG. 210 The self-quenching action of the device is very beneficial in protecting APDs from damage caused by high current densities. The illuminationdependent V br allows the nMAG/epi-Si photodetector to perform avalanche self-quenching. It was established that the device’s avalanche voltage is greater when it is not lighted than when it is, according to the results shown in Fig. 8(h). The avalanche breakdown will begin at 1550 nm with 0.11 W mm 2 illumination if the operation voltage is adjusted to 13.5 V. However, when there is no light present, the device’s avalanche voltage rises to 23.7 V. When the device is not lighted and the breakdown voltage exceeds the operating voltage, the avalanche breakdown within the device will automatically quench, causing the current to drop rapidly. The self-quenching mechanism used in the nMAG/epi-Si photodetector protects against operational failures, increasing its longevity. The avalanche multiplication mechanism in this device is activated only by incident light, but only if the operating voltage is improved to a critical point. These findings show that avalanche multiplication dominates the internal benefit. The gain of 1123 (Fig. 8(i)) is calculated employing the equation M=I ph /I ph0 at a low reverse bias, where I ph and I ph0 are photocurrents with and without multiplication, correspondingly. Because of the avalanche effect, the nMAG/epi-Si photodetector displays outstanding performance when equated with the previous research results, including a low dark current, high responsivity, and detectivity, as well as a fast response time. 211–215 Additionally, the photodetector has been successfully employed to record pictures and Review Journal of Materials Chemistry C Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15785 may be used for dual-colour detection. This work offers recommendations for the design of NIR image sensors and optical communication devices based on 2D/Si heterostructures. 7. Avalanche FET application in bio-sensing phenomena 7.1. Avalanche regulation in a bionic transistor Visual adaptive devices provide the capacity to streamline circuits and processes in machine vision structures, enabling adaptation and perception of photos under diverse brightness conditions; nevertheless, this capability is constrained by a slow adaptation process. Here, we review avalanche modification as feedforward inhibition in bionic 2D transistors to achieve high-frequency visual adaptation with perceptual accuracy at the microsecond level, attaining a response speed that is over 10 000 times more rapid than that of current bionic sensors and the human retina. To enable ultra-fast scotopic and photonic adaptation processes of 108 ms and 268 ms, as well, the bionic transistor autonomously transitions between avalanche and photoconductive effects in response to changes in light intensity. An adaptive machine vision system was developed by combining convolutional neural network technology with avalanche-based bionic transistors. It demonstrates remarkable microsecond-level speed adaptation and reliable image recognition with 98% accuracy under both low and intense light conditions. Fig. 9(a) presents the graphical illustration of the reviewed device structure. This junction-FET (J-FET) consists of a MoS 2 transport channel and a WSe 2 gate. Fig. 9 (a) Graphical illustration of the device. (b) I–Vof the device under illumination with V gs =3 V. (c) Schematic of the bionic neuronic system; red circles denote the circuit themes. Figures (a)–(c) are reproduced with permission from ref. 241, copyright, Nature Communications 2024. (d) Schematic of the MoS 2x O x -based gas sensing device. (e) V b and ‘‘a’’ with V DS =35VatV GS = 20 V revealed in the form of balls and squares, correspondingly. Figures (d) and (e) are reproduced from ref. 242, (2024). (f) Energy band diagram. (g) The carrier transport phenomenon inside the device. (h) The arithmetic simulation calculated the current-reverse bias with various functional strains; the inset shows the schematic illustration of a piezotronic GaN avalanche device. (i) The (I DS –V DS ) curves of the reverse breakdown in an FET in the presence of piezoelectric charges. Figures (f)–(h) are reproduced from ref. 243, copyright, Nano Energy. Journal of Materials Chemistry C Review Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 15786 | J. Mater. Chem. C, 2025, 13, 15767–15795 This journal is © The Royal Society of Chemistry 2025 Fig. 9(b) shows how the incident light stimulus may greatly alter the output current and avalanche mechanism. As incident light intensifies, it causes an increase in the photocurrent, exhibiting positive photoconductivity (PPC) in both the saturation and linear zones; but, in the ionization region, the PPC progressively shifts to negative photoconductivity (NPC). The photocurrent first rises to 5.1 mA and thereafter declines to 2.2 mA as light strength increases, resembling spontaneous visual adaptation that mitigates the output of overstimulation information. The avalanche ‘‘G’’ decreases from 1.5 10 4 to 8 as light intensity rises, indicating a shift from the avalanche to the photoconductivity effect as the major photo-sensing mechanism. The Rin the ionization zone undergoes substantial variations in both value and polarity, ranging from 7.6  10 4 to 110 3 AW 1 , but in the saturation zone, it fluctuates little from 158 A W 1 to 5 A W 1 . The sensitivity progression in the ionization area parallels that in the retina, hence affirming the device model’s credibility. 219 The I DS exceeds the leakage current by over 10 3 , validating the avalanche effect and demonstrating the device’s great steadiness. In the human retina, transitioning from dim to bright environments results in the alternating dominance of photoreceptors, specifically rod cells (high sensitivity) and cone cells (low sensitivity), in perceptual function. The transformation from rod to cone cells in the retina is analogous to the switch between avalanche and photoconductivity effects in a device. In this order, the J-FET has been given visual behaviour by the avalanche tuning under different light illumination circumstances. Because of this, the retina’s sensitivity progressively alters over time throughout a protracted process of visual adaptation. Feedback inhibition and regeneration/bleaching of the photopigment govern the photoreceptor cell’s transition. 220,221 In contrast, the device sensitivity development in the avalanche ionization area is light-adaptive and real-time, guaranteeing the user quick detection of environmental changes and preventing any possible injury from the human retina’s protracted scotopic and photopic adaptation process. The changeover of the photo-sensing mechanism is reported to be accompanied by sign reversal and magnitude variations with more than five orders of Rand avalanche G. In comparison to the retina and the reported bionic device with visual adaptation, a significant distinction in sensitivity at weak and strong light stimuli may aid in imagining contrast improvement. Upon associating the Rand Gwith previously tested APDs, this device exhibits exceptional APD characteristics, featuring a Gof 1.5 10 4 and Rreaching 7.6  0 4 AW 1 , indicating significant potential for weak-light detection and enhanced visualization in low-light environments, functioning as a bionic visual sensor. 59,100,116,117,141,222,223 As seen in Fig. 9(c), this device, in contrast to other 2D bionic devices, could improve optical adaptation beyond the retina by providing an effective bionic neural network. The avalanche effect’s photosensitivity is four orders of magnitude more than that of the photoconductivity effect, which means that it may mimic the roles of rod and cone cells in this network. The photogenerated voltage opposes the direction of the built-in E-field at the MoS 2 /WSe 2 junction, functioning as an inhibitory cell that modulates the avalanche effect. Light (stimulation), ‘‘Photo-generated voltage’’ (Inhibition cell), and ‘‘avalanche’’ (Rod) may establish a feedforward inhibitory circuit in which the ‘‘avalanche’’ gets both stimulatory and inhibitory signals to prevent excessive output under photopic adaptation circumstances. Under intense light irradiation, the avalanche effect is suppressed, transitioning the photo-sensing process to ‘‘photoconductivity’’ (Cone). The transition between ‘‘photoconductivity’’ and ‘‘avalanche’’ during both photopic and scotopic adaptation occurs far more rapidly than the chemical reaction-based switching between cones and rods in the retina. The feedforward excitation circuit including the ‘‘Output current,’’ ‘‘avalanche,’’ and ‘‘photoconductivity’’ demonstrates multiplexed modulation properties and significantly enhances the ‘‘SNR’’. This device offers significant benefits in photographic adaptation over the human retina and previously described bionic devices by using a feedforward circuit as a rapid-switching phenomenon. By using a more predictive and quick feedforward inhibition circuit, the avalanche tuning-based bio-inspired visual device can avoid prolonged visual adaptation. This has great potential for wide-ranging machine vision applications, promoting concepts and schemes for bioinspired optical systems while reducing reliance on intricate systems and computation. 8. Avalanche FET application for gas sensing 8.1. NO 2 -gas sensing in doped-MoS 2 Recently, avalanche multiplication has been seen in TMDCs such as MoS 2 and various other 2D-TMDC materials, attributed to robust Coulomb interaction-induced quantum confinement effects. 117,224 Operating TMDC-based FETs in an avalanche multiplication mode via gate-voltage tuning allows for the amplification of weak signals generated by light or molecular adsorption. An enhanced signal via avalanche multiplication demonstrates reduced background noise and extreme sensitivity, allowing detectors to attain outstanding performance. 97,101 In order to achieve outstanding sensing efficiency, there is a growing push to improve and make use of carrier multiplication phenomena for gas sensing. Nearest-neighbour hopping (NNH) is a prevalent occurrence in materials with higher defect concentrations, facilitating the production of greater carrier concentrations in defective substances. 225–227 In order to tackle this challenge, the research looks into avalanche multiplication in MoS 2 with high defect concentrations. The impact of NNH on the effectiveness of MoS 2 avalanche multiplication is clarified in this work. By substituting oxygen (O) atoms for sulphur (S) atoms in a monolayer of MoS 2 , the defective configuration is generated as referred to as MoS 2x O x , with xmeticulously regulated between 0 and 0.51. When xsurpasses 0.44, significantly doped O defects allow MoS 2x O x to demonstrate NNH. While maintaining MoS 2 -like carrier mobility, the presence of O defects in MoS 2x O x increases the possibility of carrier collisions compared to pure MoS 2 . Additionally, a gas sensor was constructed that exploits the avalanche multiplication Review Journal of Materials Chemistry C Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15787 properties of MoS 2x O x . This sensor (MoS 20.51 O 0.51 ) possesses a limit of detection (LOD) of almost 1.4 10 4 ppb and demonstrates an outstanding gas response to 50 ppb NO 2 at ambient temperature, achieving an impressive signal Rof 5.8  10 3 %, exceeding conventional resistance-type gas sensors utilizing TMDCs by two times. The MoS 2x O x gas-sensor FETs function using avalanche multiplication features, in contrast to traditional detectors that use two parallel electrodes for gas ionization and breakdown voltage assessment. 228–231 The monolayer MoS 2 utilized in this work is produced by the CVD method. Following treatment, O atoms replace S vacancies, resulting in the O doping of the irradiation film designated as MoS 2x O x , as seen in Fig. 9(d): the inset shows the optical image of the device. Upon exposure to NO 2 gas, the sensor continues to demonstrate avalanche multiplication behaviours, with the breakdown voltage (V b ) rising from 13.8 V to 21.8 V as the gas amount escalates from 50 ppb to 200 ppb, while the ‘‘a’’ decreases from 954.5 cm 1 to 669.8 cm 1 (Fig. 9(e)). Simultaneously, the I DS of the MoS 20.51 O 0.51 based FET reduces by two orders of magnitude in the avalanche zone (V DS 4V b ). In comparison to the MoS 2 FET, the MoS 20.51 O 0.51 FET shows a substantial sensitivity to NO 2 gas, as shown by the much greater I DS change. The O-substituted configuration is linked to the anticipated transfer of charge of the NO 2 molecule at the adsorption location. During adsorption, NO 2 removes electrons from the MoS 20.51 O 0.51 surface like that of adsorption on the MoS 2 layer. MoS 20.51 O 0.51 presents n-type semiconducting behaviour; hence its carriers are exclusively electrons. The adsorption of NO 2 on the MoS 20.51 O 0.51 surface diminishes the carrier density of MoS 20.51 O 0.51 . This sensor is adaptable and appropriate for detecting gas concentrations, especially at low levels, since it does not need precise gas pressures. This work highlights avalanche multiplication as a successful physical mechanism for the development of ultrasensitive gas sensors and shows how to modify its characteristics using hopping transfer. 9. Piezotronic and piezophototronic avalanche devices The piezotronic effect may be exploited to fabricate robust devices based on avalanche multiplication. Avalanche devices, such as single photon avalanche diodes (SPADs), employ nonlinear current amplification to detect single photons. 80,232 SPADs respond in 10 12 s, whereas standard photomultiplier devices respond in 10 9 s. 233 The maximal Gfactor and V EB of SPADs are critical factors that may be improved by altering the device’s architecture, 234 building heterojunctions by switching materials 235–237 and improving interface polarization using dopants. The vertical conductive configuration of the p–n diode, which had a critical breakdown E-field of 3.5 MV cm 1 and a breakdown voltage of 2600 V, served as the basis for GaN avalanche devices. 238 The GaN avalanche device was theoretically investigated by means of the finite element approach, as we reviewed here. The strain-induced polarisation controls the avalanche process. This refers to the impact of piezotronic and piezophototronic processes on single-photon avalanche diodes that are extremely quick and sensitive. Because strain-induced polarisation can efficiently regulate the carrier’s transit at the interface, piezotronic strain sensors offer extremely high sensitivity. Both centrosymmetric materials with non-uniform strain and non-centrosymmetric materials, such as wurtzitestructured semiconductors like ZnO, GaN, etc., can be used to create piezotronic devices. Under reverse bias, the piezotronicsMOSFET breakdown model was constructed. Under various strains, the current–voltage characteristics were computed. Polarization brought about by strain amplifies the current in the depletion layer. It is evident that both the gauge factor and the breakdown time can be improved. It is possible to greatly enhance the efficiency of piezotronic and piezophototronic avalanche diodes. The perfect p–n junction design is applied to the avalanche diode in accordance with Shockley’s theory. Carriers smash through the barrier area at high reverse bias voltages. In a sufficiently large space charge region, the covalent bond’s electrons are energized to produce a free electron– hole pair. Fig. 9(f) displays the E-field and charge; ionization will result from the acceleration of the free carriers in the depletion area and their collision with the lattice atoms under the strong E-field. The impact ionization process creates free electron–hole pairs. When the reverse current rises quickly as an effect of the multiplication, breakdown occurs. At the time of the avalanche collapse, the depletion layer becomes wider. The characteristics of the depletion layer determine the avalanche breakdown conditions. 239 Fig. 9(g) shows the GaN piezotronic avalanche device schematic construction and carrier transport. In earlier studies, the strain was used to efficiently control the piezoelectric polarization charges at intersection. 240 The strain direction determines the direction of charges; piezoelectric charges have a width of Wpiezo. The piezotronic device has been investigated using a 2D strained model that incorporates the p-type area and n-type zone with piezoelectric charges in order to get a detailed understanding of the piezotronic and piezophototronic impacts on the avalanche development. The electric field scenarios are described by ideal Ohmic connections and Dirichlet boundary conditions. The inset of Fig. 9(h) displays the piezotronics avalanche diode schematic. Within the width Wpiezo, the piezoelectric charges are dispersed at the n-type zone of wurtzite GaN. The reverse voltage is fixed at V= 4 V and the uniform strain varies from 1 to 1%; the current voltage characteristics are displayed in Fig. 9(h). The current increases gradually at first when the reverse voltage rises, and the speed clearly improves as the voltage rises. The current progresses with the externally functional strain for a given reverse voltage, particularly when it is close to the breakdown voltage. The piezotronic polarization field gives the avalanche process an alternative starting point. Positive and negative piezoelectric charges are produced by the applied strain in distinct directions. Eventually, the device breaks down due to the rising drain–source voltage. Fig. 9(i) displays the current–voltage characteristics for I DS and V DS at various stresses. The applied strain causes the current to rise at Journal of Materials Chemistry C Review Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 15788 | J. Mater. Chem. C, 2025, 13, 15767–15795 This journal is © The Royal Society of Chemistry 2025 a constant voltage. It is clear that the MOSFET at 0.02% strain generates carriers more quickly than the other one. It is described how the avalanche mechanism controls the piezoelectric electric field. The gauge factor can reach up to 10 6 –10 7 , and the device has extremely high sensitivity. Previously, the voltage was employed to regulate the critical state of the classic avalanche device; today, the external strain is controlled. The link between carrier transport and piezoelectricity in the junction zone also clearly reduces the reaction time. For ultrafast and ultra-high sensitivity piezotronic and piezophototronic strain sensors, which have enormous potential applications in biosensing and human–computer interfaces, the study has considerable guiding importance. It is also evident that the connection between carrier transport and piezoelectricity in the junction zone shortens the response time. The discovery has significant implications for ultrafast and ultra-high sensitivity piezotronic and piezophototronic strain sensors, which have vast potential applications in biosensing and human–computer interfaces. 10. Challenges, solutions and future perspectives The avalanche transistors are typically operated in a certain phase where they deliberately endure an avalanche breakdown. This distinct mode experiences high voltages and switches rapidly, but it also faces a set of challenges. However, the fundamental problems for the avalanche breakdown phenomenon are the fabrication strategy and integration of high-quality 2D materials onto FET architectures. 247 So far, sustaining the crystalline integrity, consistency, and scalability of these materials has remained an issue for circuit integration and limited power. The quality of the interface between 2D materials and substrates is also critical for device performance. The defects, residue, or incompatible interfaces may result in suboptimal electrical performance or device instability. 248 The avalanche FETs inherently depend on regulated avalanche breakdown; attaining an equilibrium between substantial gain and low noise without compromising the device is technically demanding. 249 Therefore, the quantum engineering of materials and device configurations at the nanoscale level are essential to mitigate the random behaviour of the avalanche process. This would help to control the heat dissipation effect during avalanche multiplication in the channel materials. 250,251 The 2D materials have good conductivity, but still, the voltage overshoot, optical crosstalk and reliability under high stress can cause the breakdown voltage drift, which can potentially damage the devices. In addition, avalanche FETs should sustain a high ‘‘SNR’’ for practical sensing applications. Reducing noise and maintaining uniformity may be technically indispensable for advanced real-world applications. 252,253 Long-term stability and performance reliability are essential for commercial sensing applications. Moreover, ensuring the reliable and stable operation of devices over a long period under diverse environmental conditions (temperature and humidity) is a major challenge for 2D-based avalanche FETs. 254,255 The 2D materials are susceptible to environmental deterioration, such as oxidation in the atmosphere, which may impair their performance. 256,257 Despite all, the promising electrical transport features of 2D materials have significant potential in sensing applications. 258,259 Owing to their extensive surface area and electrical characteristics, 2D materials can achieve high sensitivity and selectivity for various analytes (e.g., gases and biomolecules). 260,261 Further to enhance the performance of the devices it is essential to develop ultrahigh-efficiency avalanche devices for various applications in optical communications, biomedical imaging, and green energy. Protection solutions, including encapsulation, should be devised without impairing device performance. Ongoing investigation for innovative 2D materials and their heterostructures may provide unique advantages for avalanche FETs. The ongoing investigation of various materials beyond graphene, such as h-BN, BP or hybrid organic-2D systems, may provide distinct benefits for bandgap engineering, breakdown voltages, and electrical characteristics. Employing novel manufacturing processes, including CVD, MOCVD, 262 atomic layer deposition (ALD), and molecular beam epitaxy (MBE), can enhance the quality and scalability of 2D materials, which may pave the way for the future of avalanched FETs. Also, the advancements in transfer systems and the incorporation of 2D materials with current silicon-based technologies will be crucial for the progression of avalanche FETs. Avalanche FETs using 2D materials may become essential for next-generation sensors in IoT applications, environmental monitoring, and healthcare diagnostics. The distinctive characteristics of 2D materials render them ideal for the detection of gases and biomolecules with unparalleled sensitivity and reaction times. Avalanche transistors demonstrate a groundbreaking and robust approach to the development of multifunctional devices, making them a promising frontier for modern and fast electronics of next-generation. Recently, 2D materials have become an ideal contender for future soft electronics, facilitating diversity in high-frequency and high-power applications. 263–265 The avalanche effect by impact ionization can be exploited to establish novel technologies in optical, chemical and bio-sensory applications. Such sensors could be connected with healthcare, realtime diagnostics, and environmental monitoring. Also, the optimization of APD performance can lead to devices with enhanced performance as compared to traditional avalanche designs by selecting 2D materials with promising band alignments and structures that allow for the use of Schottky junctions to reduce dark currents and increase operational wavelengths under light. In the future, the machine learning and integration of antifouling technology may be linked with avalanche FET-based sensors to augment data processing and in vitro and in vivo diagnostics. With the advancement of 2D material-based avalanche FETs, industries such as electronics, healthcare, environmental monitoring, and defence could see a significant shift in sensor technology. High-performance, lowpower, and miniaturized sensing devices could open new possibilities in industrial automation, smart cities, and more. Review Journal of Materials Chemistry C Open Access Article. Published on 11 July 2025. Downloaded on 10/8/2025 7:19:31 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 15767–15795 | 15789 By mitigating the current issues in both material and device designs, the future of avalanche FETs using 2D materials seems auspicious for advanced sensing applications. 11. Conclusion In this review, we thoroughly discussed the mechanism and recent progress of avalanche transistors especially manifested by 2D materials. In addition, we explored the emerging developments in photo-, bioand gas sensing applications employing avalanche photodetectors/photodiodes (APDs). Also, it is concluded that the 2D materials offer novel approaches for the advancement of APDs due to effective carrier multiplication at the nanoscale, which potentially facilitates their scope in soft wearable sensors, organ-on-a-chip applications and cell-based sensors for drug screening. Such integration of 2D materials into avalanche FETs marked significant progress in advanced detection technologies of the contemporary era. Finally, we highlighted the challenges of avalanche transistors and their possible solutions to compete in the race of advanced sensing technology. Furthermore, we provided our future perspectives on 2D material-based avalanche FETs regarding material selection and device configuration. However, many challenges and principles are elusive, so we expect that the incorporation of 2D materials in superjunction MOSFETs also paves the way for high-frequency driving photocathodes of fast-gating image intensifiers. Author contributions E. E. and MFK: writing – original draft, reviewing and editing; J. A. and U. A.: validation; P. C. and M. A. A.: visualization; K. J. S. and U. A.: proofread the manuscript; and Z. S.: supervision and funding acquisition. Conflicts of interest There are no conflicts to declare. Data availability No primary research results, software or code have been included and no new data were generated or analyzed as part of this review. Acknowledgements Zdenek Sofer was supported by the ERC-CZ program (project LL2101) from the Ministry of Education Youth and Sports (MEYS). The authors acknowledge the assistance provided by the Advanced Multiscale Materials for Key Enabling Technologies project, supported by the Ministry of Education, Youth, and Sports of the Czech Republic. Project No. CZ.02.01.01/00/ 22_008/0004558, Co-funded by the European Union. This work was supported from the grant of Specific university research – grant No A1_FCHT_2025_013. K. J. S. was supported by the Johannes Amos Comenius Programme, European Structural and Investment Funds, project CHEMFELLS VI (no. CZ.02.01.01/00/22_010/0008122). 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