Low breakdown field and high ionization index in ReSe_2 avalanche field-effect transistors

Domain: chemistry
Published: 2026-02-26
Editorial outcome: major_revision

Abstract
Avalanche field-effect transistors (AFETs) based on two-dimensional (2D) materials have attracted
growing interest in optoelectronics due to their enhanced performance via carrier multiplication and their
potential applications in nanoelectronics. However, most AFETs employing 2D materials face challenges
with high breakdown fields and low ionization indexes, which limit their applications in optoelectronics.
Here, we report a ReSe -based AFET that achieves a breakdown electric field down to 2.55 kV/cm and
2
an ionization index up to 38.79. This performance is attributed to using anisotropic ReSe as the channel
2
material, which reduces unnecessary carrier collisions. Moreover, the incorporation of HfZrO as the
2
dielectric enhances gate modulation, which further mitigates scattering effects. The underlying
mechanism is validated through calculations of electron effective masses along both in- and out-of-plane
directions. Moreover, scattering probability within ReSe based on simulation model and experimental
2
data further corroborates the proposed mechanism. As a demonstration, ReSe avalanche phototransistors
2
with a high responsivity of 1.71×104 A/W and a high gain of 173 are realized based on this platform. By
incorporating anisotropic 2D materials and high-k dielectric with less carrier scattering, this AFET design
provides a promising pathway for developing high-performance avalanche photodetectors.

Introduction
The phenomenon of avalanche multiplication has been extensively studied in optoelectronics due to its
potential to enhance optical performance through carrier multiplication1,2. Avalanche photodetectors,
which leverages this phenomenon, have attracted significant research interest in various fields including
optical communications3, biomedical imaging4, and sensing5. For these applications, it is critical for
avalanche devices to operate at low bias voltages and room temperature. However, conventional
avalanche devices made from bulk materials including Si, Ge and III-V compound often face limitations,
such as high threshold breakdown field (E ) and low ionization index (n)6,7 due to huge waste of energy
b
induced by scattering.

To overcome these challenges, two-dimensional (2D) materials8-10 have emerged as promising candidates
for low-bias avalanche devices due to their atomically thin structures11,12. Among these materials,
transition metal dichalcogenides (TMDs) have shown great potential for high-performance devices.
TMDs can be classified based on their crystal symmetry13, including highly symmetrical hexagonal TMDs
such as MoX and WX (with X =S, Se)14,15, as well as anisotropic TMD with low-symmetry crystal
2 2
structures such as MX (M = Re, Pd; X = S, Se)16. For symmetric TMDs, tailored device architectures
2
have been utilized to enhance electric-field efficiency, thereby enabling a lower avalanche threshold
voltage17. Complementarily, minimizing energy losses from carrier scattering provides an additional and
equally important pathway to reduce the avalanche breakdown voltage.

The anisotropic TMDs show less scattering due to much weaker interlayer interactions compared to
symmetrical TMDs, which results from their distorted 1T diamond-chain structures18,19. This property
reduces scattering probability by restricting charge transport along the out-of-plane direction in the
channel20. Consequently, the carriers are apt to gain sufficient kinetic energy to trigger impact ionization
under low electric fields with minimal energy losses from scattering19. Therefore, the unique
characteristics of anisotropic TMDs make them ideal for designing avalanche devices with low E and
b
high n. Among anisotropic TMDs, ReSe is particularly promising for optoelectronic applications due to
2
its favorable optical properties21,22. However, despite its potential, the integration of ReSe in avalanche
2
devices has not yet been demonstrated.

On the other hand, the E can be modulated by gate voltage, making the choice of the dielectric layer
b
which affects gate modulation, a key factor in determining the E of avalanche devices. Pioneering works
b
utilized low-k materials like silicon dioxide (SiO )19,23 or boron nitride24 as dielectrics, which enabled
2
modulation but at the expense of high E .
b

To address these challenges, we propose an avalanche field-effect transistor (AFET) design utilizing
ReSe as the channel material and high-k HfZrO (HZO) as the dielectric. This design achieves both one
2 2
of the lowest E and one of the highest n among other reported AFETs based on 2D materials. The superior
b
performance arises from minimizing scattering energy losses due to the unique anisotropic carrier
transport of ReSe , combined with effective gate modulation due to high-k HZO dielectric. We further
2
explore the underlying mechanism behind the high breakdown performance by calculating the electron
effective mass (m*) along different crystallographic directions and the scattering probability from
experimental and theoretical aspects within ReSe . Furthermore, we demonstrate a ReSe -based avalanche
2 2
phototransistor (APT) with highly competitive characteristics built on this AFET structure. This work
underscores significance and potential of anisotropic 2D materials and high-k dielectrics in advancing
avalanche optoelectronics.

Results
Design of ReSe AFET.
2
Figure 1a schematically illustrates the ReSe FET. The back-gated ReSe FETs were fabricated on a
2 2
heavily doped Si substrate, with an 8-nm HZO dielectric layer, a mechanically exfoliated ReSe flake as
2
channel material, and 25-nm nickel (Ni) as the source/drain electrodes. Detailed information regarding
the device fabrication process is provided in the Methods section and Supplementary Fig. 1. The optical
image of the device and the ReSe layer are shown in Fig. 1b, with a zoomed-in view in the inset. Fig. 1c
2
presents an atomic force microscopy (AFM) image of a ReSe flake, revealing a thickness approximately
2
3.4 nm, equivalent to about 5 layers. Fig. 1d shows the Raman spectrum of ReSe films with varying
2
numbers of layers. The Raman spectrum of ReSe exhibits multiple peaks due to the reduced crystal
2
symmetry in the 1T’ distorted structure, which splits the Raman peaks25. Peaks at 124 cm-1 and 176 cm-1
correspond to in-plane E -like mode and out-of-plane A -like mode, respectively22. Moreover, the Raman
g g
signal of ReSe remains consistent across different thicknesses, indicating weak interlayer coupling.
2

AFET characteristics.
Before characterizing the AFET, the transfer and output characteristics of the ReSe FET are measured at
2
room temperature in a dark environment, are illustrated in Supplementary Fig. 2. This device operates in
depletion mode, utilizing all five ReSe layers functioning as the primary transport channels
2
(Supplementary Fig. 2c). The threshold voltage values, extracted from the transfer curves at different
drain voltages (V ) are consistently 0.2 V, indicating an absence of drain-indicated barrier lowering
ds
(DIBL) in this ReSe FET26. The output curves in the linear region, plotted in Supplementary Fig. 2d,
2
confirm good contact between the Ni source/drain electrodes and the ReSe channel24. While the finite
2
contact barrier still exists at ReSe /Ni interface, as analysed in Supplementary Fig. 2e and f. To assess the
2
electrical breakdown behavior of the ReSe FETs, the drain-source current (I ) is measured across
2 ds
different gate voltages (V ) on a logarithmic scale, as shown in Figure 2a. A drastic increase in I is
gs ds
observed when V exceeds the threshold breakdown voltage (V ), corresponding to an avalanche current
ds b
rise rate of 250 mV/dec at V of -2.5 V. This rate outperforms that of commercial InGaAs devices, which
gs
exhibit a slower current climb rate of nearly 400 mV/dec17. In addition to avalanche multiplication, other
potential causes of electrical breakdown such as DIBL, thermal breakdown, and ferroelectric polarization,
are considered. Supplementary Section 3 provides a detailed analysis that rules out alternative
mechanisms. Furthermore, the temperature-dependent I-V characteristics and the extracted breakdown
voltage (V ) are presented in Fig. 2b and c. As the temperature decreases, lattice vibrations are
b
progressively suppressed, leading to a reduction in electron–phonon scattering. Under these conditions,
charge carriers can more readily accumulate excess kinetic energy under an applied electric field, thereby
facilitating the onset of impact ionization. Consequently, the breakdown voltage decreases with
decreasing temperature, exhibiting a positive temperature coefficient of approximately 1.80×10-3 ℃-1
above 160 K. This behaviour is a hallmark of avalanche breakdown processes27. Taken together, these
results unambiguously confirm that the observed current breakdown originates from avalanche
multiplication rather than alternative conduction mechanisms. Additional details on the temperature-
dependent dark current and V evolution are provided in Supplementary Section 3. Furthermore, the
b
increase in dark current with decreasing temperature prior to avalanche, as well as the relationship
between the device avalanche threshold voltage and the ReSe₂ bandgap at low temperatures, will be
investigated in future work. Avalanche multiplication, a carrier multiplication process, occurs when high-
energy charge carrier impacts the lattice under a high electrical field23. This process generates a large
number of electron-hole pairs, thereby leading to an abrupt increase in I . The highest V applied in this
ds ds
work was 5 V, which is nearly 90% of the total breakdown voltage (5.4 V, Supplementary Fig. 3a), and
did not cause any permanent damage to the device. A 180-cycle V sweep at a fixed V (shown in
ds gs
Supplementary Fig. 3b) demonstrates the robustness of the breakdown behavior in this design. Moreover,
the device exhibits asymmetric avalanche characteristics under bidirectional drain bias (Figure 2a) due to
the finite contact barrier shown in Supplementary Fig. 2e and f. The corresponding electric-field
distribution across the ReSe channel, obtained by TCAD simulations, is provided in Supplementary
2
Section 4. Fig. 2d illustrates the dependence of V on V , and the V extraction is described in detailed in
b gs b
Supplementary Section 5. The V is extracted from the linear-scale I–V plot, while the corresponding I–V
b
characteristics are presented on a logarithmic scale to enable wide-range visualization. From Fig. 2d, V
b
decreases as the V decreases from 0 V to -2.5 V, attributed to reduced carrier scattering due to less
gs
electrons in depletion region. The detailed mechanism will be discussed in subsequent paragraphs. This
reduction in scattering minimizes energy loss, leading to a lower V required to trigger avalanche
b
breakdown17. Notably, the V of this device is significantly lower than that of a ReSe₂ AFET with a SiO₂
b
dielectric layer (see Supplementary Fig. 6). This highlights the crucial role of the high-k HZO dielectric
in lowering V by enhancing gate control and minimizing carrier scattering, as discussed in a later
b
paragraph. Fig. 2e shows the E at a V of -2.5 V, with extraction details provided in the Methods section.
b gs
The obtained E is as low as 2.55 kV/cm. The avalanche behavior of the device under positive V is
b gs
illustrated in Supplementary Fig. 7. In this regime, the device sequentially operates in the linear, saturation,
and eventually avalanche breakdown regimes. As a result, the avalanche breakdown voltage is higher than
that observed under negative gate bias. The I -V curves for 59 devices are presented in Supplementary
ds ds
Fig. 8a, demonstrating the reproducible avalanche breakdown behavior of this design.

To investigate the characteristics of the impact ionization process in ReSe AFETs, the multiplication
2
factor (M) is defined as23: M= I (V )/I (V ). The relationship between M and the V at various V is
ds ds ds b ds gs
shown in Supplementary Fig. 8b. By theoretical model of the avalanche effect, 1-1/M can be expressed
using equation (1) as follows28,29:

n
1-
1
= (
𝑉𝑑𝑠)
. (1)
𝑀 𝑉𝑏

Here, the index n represents the rate of charge carrier multiplication during avalanche multiplication,
reflecting how rapidly the carrier density grows in response to an increasing electric field29,30. Fig. 2f
displays a plot of 1-1/M as a function of V /V on a logarithmic scale for various V values. The linear
ds b gs
region observed in this plot confirms the occurrence of carrier multiplication, further validating that the
observed electrical breakdown is due to avalanche multiplication. The values of n at different V are
gs
shown in Fig. 2g, with details on its extraction through linear fitting provided in Supplementary Fig. 9.
Notably, the n increases as V decreases, reaching a maximum of 38.79 when the V is -2.5 V. This
gs gs
significant n is quantitatively justified by random path length model (Supplementary Section 9) to show
its reasonability. This increased n as V is attributed to gate modulation reducing the scattering probability
gs
in the channel, as analyzed in the subsequent mechanism section. Along with the low E , the high n makes
b
this device stand out compared to other state-of-the-art 2D-material AFETs23,24,30-33, as shown in Fig. 2h.

Mechanism analyses.
The mechanism underlying the excellent gate-modulated impact ionization behavior of the ReSe AFET
2
is analyzed through carrier scattering in both in- and out-of-plane directions, as shown in Figure 3. Fig.
3a presents the electron effective mass (m*) obtained from density functional theory (DFT) calculations
(Methods) for the bottom conduction band at the Z point (Supplementary Fig. 10a) along different
crystallographic axes. The calculated m* values are consistent with previously reported results20. The axes
correspond to the Re-Re chain (a-axis), the orthogonal in-plane direction (b-axis), and the out-of-plane
direction (c-axis) of ReSe 22, as illustrated in the inset of Fig. 3a. The electron m*along the c-axis (2.43m )
2 e
is the highest among the three axes in ReSe , attributed to weak interlayer coupling (Supplementary Fig.
2
10b). This limits carrier transport in the out-of-plane direction while demonstrating avalanche
characteristics that are independent of ReSe thickness (Supplementary Fig. 11), effectively minimizing
2
scattering7. Additionally, the electron m* values for ReSe are larger than those for symmetric TMDs
2
along their corresponding axes34. For comparison, the calculated m* values (a-axis: 0.63m ; b-axis: 0.63m ;
e e
c-axis: 0.60m ) for MoS , a representative symmetrical TMDs, are provided in Supplementary Fig. 12.
e 2
This difference explains the higher n of ReSe₂ AFETs compared to MoS₂ AFETs24, establishing it as the
top-performing device. Gate modulation further influences scattering behaviour. As the V decreases
gs
from 0 to -2.5 V, electron concentration is reduced, leading to lower scattering probability in the in-plane
direction. Moreover, a more negative V generates additional holes that occupy trap sites within the
gs
channel, as schematically shown in Fig. 3b. This is corroborated by the hysteresis characteristics of the
ReSe
2
FET, as plotted in Supplementary Fig. 13.

To further explain the scattering mitigation via gate modulation, the scattering probability coefficient (β)
is extracted from avalanche characteristics using the Shockley model12 and is illustrated in Fig. 3c. β is
proportional to the scattering probability during avalanche multiplication35, with the extraction process
described in Supplementary Fig. 14. In region I of Fig. 3c, β decreases as V becomes more negative,
gs
consistent with the earlier analyses of reduced scattering due to gate modulation. However, as V
gs
continues to decrease from -2.5 V to -3.5 V (region II of Fig. 3c), out-of-plane scattering intensifies due
to the stronger vertical field, which increases the vertical velocity component, leading to more collisions36
and increasing the overall scattering probability, as presented in region II of Fig. 3c. Consequently, region
I is dominated by reduced scattering from gate-induced hole filling in trap sites, while region II
experiences increased out-of-plane collisions. As a result, the V and n show the lowest values at V of -
b gs
2.5 V, as shown in Fig. 3d. Furthermore, Supplementary Fig. 14c shows the carrier mobility extracted
from transfer characteristics. It increases as V rises from 0 to 2.5 V and then falls from 2.5V to 3.5V.
gs
The mobility trend exhibits symmetry across positive and negative V regimes, arising from the consistent
gs
dominance of the same scattering mechanisms in the 2D material under both bias polarities37. This trend
aligns with the scattering behavior analyzed above, where scattering first diminishes and then intensifies
with increasing gate electric field. To link the experimental observations with theoretical analyses, we
developed a model (Supplementary Section 13). Two dominated scattering mechanisms for 2D FETs
including ionization impurity scattering and photon scattering are considered in this model38. The carrier
scattering rate obtained from this model matches with the experimental data well, yielding a coefficient
of determination (R2) of 0.91. Furthermore, the mobility under different temperatures confirms the
accuracy of the scattering mechanisms analysed above. In summary, the anisotropic carrier transport
properties of ReSe , combined with effective gate modulation, result in reduced carrier scattering in both
2
in-plane and out-of-plane directions. The reduction in scattering contributes to the low threshold E and
b
enhances the n, positioning ReSe
2
AFETs as superior to other recently reported 2D-material AFETs.

APT characteristics.
Building on the superior avalanche characteristics of the ReSe FETs, we illuminate the FET channel with
2
a 658-nm laser to evaluate the performance of this APT. Figure 4a shows the I -V curves measured in
ds ds
both dark and under illumination with varying effective powers (P ). As shown in Fig. 4a, increasing P
eff eff
from 0 to 285 pW leads to a rise in photocurrent (I ) and a significant decrease in the V . This phenomenon
ph b
is attributed to enhanced impact ionization, which generates more electron-hole pairs as laser power
increases. To characterize the ReSe₂ APT, we calculated photoresponsivity (R) and avalanche gain (G)
using the following equations (2)-(4)30,31,39:

R =
𝐼𝑝ℎ−𝐼𝑑𝑎𝑟𝑘
(2)
𝑃𝑒𝑓𝑓

G =
𝐼𝑝ℎ−𝐼𝑑𝑎𝑟𝑘
(3)
𝐼𝑝ℎ0−𝐼𝑑𝑎𝑟𝑘0

Here, I is the dark current, I and I are photocurrent and dark current at V =V , respectively. Fig.
dark ph0 dark0 b
4b shows the calculated values of photoresponsivity as a function of V under a P of 57 pW. The
ds eff
relationship between avalanche gain and V at different power levels is illustrated in Fig. 4c. Notably, at
ds
V > V , the gain increases dramatically due to avalanche multiplication. The APT detectivity calculation
ds b
is shown in Supplementary Section 16. The maximum photoresponsivity and avalanche gain achieved by
this design are 1.71×104 A W-1 and 173, respectively. These results position the ReSe APT as highly
2
competitive compared to other recent reports on APTs17,19,23,24,30-33,40-42, as summarized in Supplementary
Table 2. In addition, Supplementary Table 3 provides a benchmarking comparison between ReSe₂ and
commercial devices, underscoring the key performance metrics of the ReSe₂ avalanche device. Fig. 4d
shows the extracted avalanche gain as V
gs
varies from 0 to -2.5 V. The gain increases with decreasing V
gs
,
attributed to reduced scattering, as discussed in Fig. 3b and Fig. 3c. Moreover, the response time of the
ReSe APT at V of -2.5 V and the P of 285 pW is measured and shown in Fig. 4e. The response time
2 gs eff
is defined as the time required for the photocurrent to reach a steady-state value following illumination
by a light pulse43. From Fig. 4e, the decay time is extracted to be approximately 40 μs, corresponding to
the interval over which the photocurrent decreases from 90% to 10% of its peak value44. The decay time
also varies with the gate voltage, exhibiting a descending trend as V becomes more negative (Fig. 4f).
gs
This behavior is attributed to a reduced number of defect sites that could capture photoinduced holes, as
analyzed in Fig. 3b. Furthermore, the fast photo-response of this design originates from both a high n and
good contact between the source/drain electrodes and the channel19, as evidenced in Supplementary Fig.
2d. Furthermore, this finding proves that the higher gain obtained at V of -2.5 V is due to enhanced
gs
ionization from hole-filling defect states45 rather than a prolonged lifetime of photo-generated carriers due
to the photogating effect44. This finding is consistent with the above mechanism analyses. Supplementary
Fig. 17 plots the decay time as a function of P , showing a decreasing trend with increasing irradiation
eff
power. This behavior is attributed to trap-state saturation caused by the higher density of photogenerated
carriers under strong optical excitation, which enhances the photo-response dynamics.

Methods
Device fabrication.
The fabrication process of the AFET began with the atomic layer deposition of an 8-nm HZO layer onto
a heavily doped Si substrate at a temperature of 250 ˚C. Following this, ReSe flakes were mechanically
2
exfoliated from bulk ReSe crystals (obtained from HQ Graphene Supplies) and transferred onto the
2
substrate using a poly (methyl methacrylate) (PMMA)-assisted method, as depicted in Figure S1.
Subsequently, a 25-nm Ni layer was then deposited via electron-beam evaporation at a controlled low rate
of 0.1 Å/s to ensure good contact with the thin ReSe layer, and subsequently patterned through a lift-off
2
process to serve as the source/drain electrodes.

Material and device characterization.
The thickness of the ReSe materials was measured using an atomic force microscopy (NX 20 AFM, Park
2
Systems). Raman spectra were collected with a Raman microscope system utilizing a 532 nm with laser.
Electrical characterization of the avalanche device was performed using an Agilent B1500A
semiconductor analyzer at room temperature. The photonic characteristics of the FET were measured
under illumination at a wavelength of 520 nm, with the light spot diameter on the FET channel measuring
10 mm. Low-temperature FET mobility measurements were carried out from a AFET with a 285-nm SiO
2
as dielectrics using a low-frequency SR860 lock-in amplifier (Stanford Research Systems) to ensure high
sensitivity and stability. The response time measurement was performed using a voltage-probing setup.
First, the device was mounted on a PCB in series with a load resistor for current-to-voltage (I-V)
conversion. Then, the resulting voltage signal was transmitted to a KEYSIGHT MXR608A oscilloscope
using a low-noise voltage probe for capture and analysis.

E calculation.
b
In this study, the E is obtained by performing a linear fit of multiple V values extracted from the I -V
b b ds ds
curves of devices with varying channel lengths, as shown in Fig. 2c. E is determined as the slope of this
b
fitting curve, such that 𝐸 ≈ 𝑉 /𝐿, based on the assumption of a nearly uniform electric field across the
b 𝑏
channel. This approach is consistent with methods used in other works of 2D-material AFETs.
Accordingly, we also calculated the E for pioneering works that did not provide specific E values,
b b
utilizing this widely accepted equation for benchmarking purposes, as listed in Fig. 2f.

DFT calculations.
DFT simulations were performed using Quantum ATK’s atomic simulation platform. The generalized
gradient approximation (GGA) was employed with the Perdew-Burke-Ernzerhof (PBE) functional to
describe the exchange-correlation interactions. An adequate convergence criterion was set, including a
density mesh cutoff of 105 Hartree, force tolerance of 0.01 eV Å-1, and a self-consistent field convergence
threshold of 1 × 10-6 a.u.. For ReSe , brillouin zone k-point sample of the Monkhorst–Pack grid was 9 ×
2
9 × 9, and Y (0, 0.5, 0), Γ (0, 0, 0), X (0.5, 0, 0), V (0.5, 0.5, 0), U (0.5, 0, 0.5), R (0.5, 0.5, 0.5), Z (0, 0,
0.5), and T (0, 0.5, 0.5), were the high symmetric points here. For MoS , brillouin zone k-point sample of
2
the Monkhorst–Pack grid was 15 × 15 × 15, and Γ (0, 0, 0), M (0, 0.5, 0), K (0.33, 0.33, 0), A (0, 0, 0.5),
L (0, 0.5, 0.5), and H (0.33, 0.33, 0.5), were the high symmetric points. The partial density of states was
extracted to show the electronic behaviour of the orbitals. The m* values are extracted from conduction
band minimum (CBM) along a, b and c axes, which refers to fractional direction (1, 0, 0), (0, 1, 0) and (0,
0, 1), respectively. The CBM is at Z point (0, 0, 0.5) for ReSe while at a point (0.1767, 0.1767, 0) between
2
K and Γ for MoS .
2

TCAD simulation.
The simulation on electric field distribution within ReSe channel was conducted by TCAD silvaco tools.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The author reported a Low breakdown field and high ionization index in ReSe2 avalanche field-effect transistors. The\navalanche breakdown voltage of the device can be as low as around 4 V and the avalanche electric field can be as low as\n2.55 KV/cm.The avalanche detectivity, avalanche photoresponsivity, and avalanche gain can achieve 1.71×104 A/W,\n1.99×1013 Jones, and 173, respectively. Although the innovation of the article needs to be considered, there is no doubt that\nthe performance of the devices is great. In my opinion, this paper can be published once the following concerns can be\nwell addressed.\n1.Please provide a detailed list of the innovative points in your article.\n2.In the fifth page of your article, you said that “The threshold voltage values, extracted from the transfer curves at different\ndrain voltages (Vds) are consistently 0.2 V, indicating an absence of drain-indicated barrier lowering (DIBL) in this ReSe2\nFET.” Could you please to introduce how you determine the turn-on voltage of the device?\n3.A larger effective mass in the c-axis direction means that the migration of charge carriers between different material layers\nis hindered, so charge carriers’ migration within the plane dominates, forming a pathway for charge carrier to migrate in the\nmaterial, which can effectively reduce collision scattering between charge carriers. But a larger effective mass means lower\nmobility, and the effective mass of electrons in the a-axis and b-axis directions (relative to MoS2 material) is actually higher.\nCan we conclude that the excellent performance of the material is due to the difference in effective mass only by analyzing\nthe size of the c-axis electron effective mass?\n4.In avalanche breakdown, when the drain voltage exceeds the breakdown voltage, the current will increase rapidly, often\nexponentially. If you were to plot the current on a logarithmic (log10) scale versus the drain voltage, you would observe a\nsharp, nearly vertical rise in current as the voltage approaches and exceeds the breakdown voltage. This can more\nintuitively illustrate that the current jump observed in the article is an avalanche phenomenon.\n5.The article provides many proofs about the avalanche phenomenon of devices. If you have such conditions, please test\nthe variation of avalanche voltage with temperature. This is a more direct way to prove the avalanche phenomenon.\n6.Could you please to explain how the avalanche voltage is selected and why do you choose the parameter at the\navalanche point to calculate your avalanche gain?\n7.Could you please to introduce the testing method for Figure 3a in the supplementary material and explain why the current\ndecrease sharply at the avalanche point?\n8.Why does the drain voltage required for avalanche significantly increase after replacing the HZO with SiO2 as shown in\nFig 4b in the supplementary material?\n9.Is this phenomenon reliable? Could you please to repeat the measurement of several such devices to determine the\nreliability of this experimental phenomenon？\nLiming Wang", "reviewer_2": "The authors report a room temperature low breakdown voltage (~5V) in ReSe2 device and associate it with the avalanche\nprocess. The intrinsic anisotropic crystal structure is believed to lower the charge carrier scattering and contribute to the\nphenomenon. DFT simulations were performed to extract the electron mass in a, b and c axis. The electron mass of c axis\n(out of plane) is 3.5 times larger than that of a axis (in plane) and is believed as the origin of low charge carrier scattering\nprobability and low breakdown voltage. Finally, the photoresponse of the ReSe2 device was measured. A responsivity of\n1.71×104 A/W and a gain of 173 are reported.\nAfter carefully going through the manuscript, I personally cannot support the publication of it in the highest rated\nmultidisciplinary journal “Nature Communications”. My questions/queries are listed as follows.\n1. There will be more than 4 orders of magnitude increase of current after breakdown. However, the IV curves are all plotted\nin linear coordinate in this work. This is not a rigorous and acceptable way to show the current boost of breakdown. Please\nfind the IV plot in silicon and InGaAs avalanche photodiode (Optical and Quantum Electronics 52 (2020): 1-9; Nature\nCommunications 15, 3639 (2024), graphene/silicon and WSe2 impact ionization field-effect transistor (Nat Commun 15,\n9038 (2024); Nat Commun 13, 6076 (2022))\n2. As is well documented in the textbook, the current break-down could arise from either avalanche or Zener process. There\nis a simple way to distinguish them from each other. Generally, the avalanche process is featured by a positive temperature\ncoefficient of the breakdown voltage (low temperature would freeze the lattice vibration, and thus lead to low frequency of\nelectron-phonon scattering), while the Zener process is featured by a negative coefficient (low temperature would increase\nthe bandgap and thus suppress the band to band tunneling). However, there is no such evidence present in this work.\nBased on the above questions/queries, I cannot support the judgement that the current boast observed in this work is\nbreakdown and associate with the avalanche process.\n3. To calculate the detectivity of the device, the noise is estimated as 2qIds. This is not rigorous and usually overestimates\nthe device performance. The noise should be measured experimentally by the standard measurement method (Nature 542,\n324–327 (2017)).", "reviewer_3": "From this work, the authors present a comprehensive study on ReSe2-based avalanche field-effect transistors, claiming a record-low breakdown field (2.55 kV/cm) and a high ionization index (38.79) among reported 2D-material AFETs. These achievements are attributed to the anisotropic properties of ReSe2 and the incorporation of high-k HZO dielectric. The study also demonstrates the potential of this design through the fabrication of high-performance avalanche phototransistors. The paper is potentially interesting to the community but I believe that the manuscript may contain incomplete supporting evidence for the high performance metrics as they promote in the manuscript. Therefore, considering a high standard set in Nature Communications, I recommend that the manuscript to undergo major revisions after considering the following concerns:\n1. Mechanism for the low breakdown field: while the authors attribute the low breakdown field and high ionization index to anisotropic ReSe2 properties and effective gate modulation, the quantitative relationship between these factors is not clearly established. The manuscript should provide more detailed modeling or theoretical analysis to link the observed device characteristics to the proposed mechanisms, particularly concerning the reduction in carrier scattering. I would like to suggest the following aspects to be included in their work:\na. Experimental evidence for reduction in carrier scattering at different regimes of gate voltages: mobility, carrier lifetime, etc.\nb. Detailed modeling or theoretical analysis to link the observed carrier multiplication to the proposed mechanisms, particularly quantitatively linking the experimental the reduction in carrier scattering to the theoretical work.\nc. Detailed description of device operation:\ni. What is the carrier accumulation layer thickness? ii. For the thickness of the channel that they use, at what thickness away from the dielectric are the main transport channels?\nd. Disregarding artifact: I am concerned that the authors are neglecting the depletion regions that can form from employing high-k dielectric for the operation of the device (see ACS Nano 2017, 11, 12, 12583–12590). Rather than avalanche multiplication, I wonder that the authors may have formed lateral p-n junctions that could show up as similar device characteristics as the author’s measure.\n2. Mechanism for the fast response time: the authors claim impressive rise/fall times of their photocurrent characteristics.\na. Supporting evidence and mechanism for the fast photo response: Since the device is likely to accumulate minority carriers at the interface, the authors need to carefully characteristics and discuss why the fast photoresponse is expected in their devices.\nb. Gate-voltage and incident power dependence of photo response times over a wide range: the authors should provide power-dependence of the photo response times (not just pW scales) to elaborate on the photophysical processes at different incident powers and gate voltages by discussing the relevant timescales for carrier injection, transport and trapping/detrapping"}, "author_response": "Reviewer #1:\nThe author reported a low breakdown field and high ionization index in ReSe\n2\navalanche field-effect transistors. The avalanche breakdown voltage of the device can\nbe as low as around 4 V and the avalanche electric field can be as low as 2.55 KV/cm.\nThe avalanche detectivity, avalanche photoresponsivity, and avalanche gain can achieve\n1.71×104 A/W, 1.99×1013 Jones, and 173, respectively. Although the innovation of the\narticle needs to be considered, there is no doubt that the performance of the devices is\ngreat. In my opinion, this paper can be published once the following concerns can be\nwell addressed.\nQ1 Please provide a detailed list of the innovative points in your article.\nAuthors’ response:\nThanks for your feedback. As suggested, we have now clearly outlined the key\ninnovations in both abstract (Page 1, Paragraph 1, Line 5-10) and introduction sections\nto address your comment.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Introduction\nPage: Page 3, Paragraph 3, Line 12-17.\nQ2 In the fifth page of your article, you said that “The threshold voltage values,\nextracted from the transfer curves at different drain voltages (V ) are consistently 0.2\nds\nV, indicating an absence of drain-indicated barrier lowering (DIBL) in this ReSe FET.”\nAuthors’ response:\nThe threshold voltage (V ) of the device was extracted using the constant current\nth\nmethod, with the turn-on voltage (0.2 V) defined as the point where the drain-source\ncurrent (I ) reaches 1 nA. In response to your feedback, we have included a detailed\nds\ndescription of this V extraction method in the revised Supplementary information to\nth\nensure clarity for readers.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary information\nPage: Page 4, Paragraph 1, Line 2-4.\nQ3 A larger effective mass in the c-axis direction means that the migration of charge\ncarriers between different material layers is hindered, so charge carriers’ migration\nwithin the plane dominates, forming a pathway for charge carrier to migrate in the\nmaterial, which can effectively reduce collision scattering between charge carriers. But\na larger effective mass means lower mobility, and the effective mass of electrons in the\na-axis and b-axis directions (relative to MoS material) is actually higher. Can we\n2\nconclude that the excellent performance of the material is due to the difference in\neffective mass only by analyzing the size of the c-axis electron effective mass?\nAuthors’ response:\nYes, you are correct that the anisotropic property of the material arises from effective\nmass difference along a-, b- and c-axis. This reduces collision scattering and contributes\nto the outstanding performance of this AFET. In addition to the different effective mass,\nthe effective gate control enabled by the high-k HZO-based dielectrics further enhanced\nthe device’s avalanche characteristics, as demonstrated in Fig. 2b and 2e. We have\nexplicitly emphasized these two critical factors-anisotropic material properties and gate\nmodulation in the abstract of the revised manuscript to clarify and alignment your\ninquiry.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Abstract\nPage: Page 2, Paragraph 1, Line 6-7.\nQ4 In avalanche breakdown, when the drain voltage exceeds the breakdown voltage,\nthe current will increase rapidly, often exponentially. If you were to plot the current on\na logarithmic (log10) scale versus the drain voltage, you would observe a sharp, nearly\nvertical rise in current as the voltage approaches and exceeds the breakdown voltage.\nThis can more intuitively illustrate that the current jump observed in the article is an\navalanche phenomenon.\nAuthors’ response:\nThanks for your feedback. In response to your suggestion, we have included a\nlogarithmic-scale plot of I versus V in Fig. 2a of the revised manuscript to explicitly\nds ds\ndemonstrate the abrupt current jump resulting from the avalanche breakdown effect.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 5, Paragraph 2, Line 12-13; Page 6, Fig. 2a.\nQ5 The article provides many proofs about the avalanche phenomenon of devices.\nIf you have such conditions, please test the variation of avalanche voltage with\ntemperature. This is a more direct way to prove the avalanche phenomenon.\nAuthors’ response:\nThanks for your suggestion. We have added temperature-dependent avalanche\ncharacteristics and extracted temperature coefficient of breakdown voltage in the\nrevised Supplementary information to further validate the avalanche effect intrinsic to\nthe AFET design in this work.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 5, Paragraph 1, Line 18.\nSection: Supplementary information\nPage: Page 5, Paragraph 1, Line 19-26; Page 6,\nSupplementary Fig. 3d and e.\nQ6 Could you please to explain how the avalanche voltage is selected and why do\nyou choose the parameter at the avalanche point to calculate your avalanche gain?\nAuthors’ response:\nThe avalanche voltage, defined as the threshold voltage at which the I exhibits a\nds\nnonlinear surge due to avalanche breakdown, is a widely adopted metric in analogous\nstudies [R1–R4]. To answer your question, we have incorporated this extraction method\nin both the revised manuscript and Supplementary information.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 5, Paragraph 1, Line 25; Page 6, Paragraph 1,\nLine 1.\nSection: Supplementary information\nPage: Page 7, Section 4, Supplementary Fig. 4a-g\nQ7 Could you please to introduce the testing method for Figure 3a in the\nsupplementary material and explain why the current decrease sharply at the avalanche\npoint?\nAuthor’s response:\nThe breakdown curve in Supplementary Fig. 3a was generated by applying a voltage\nsweep (0 to 6 V) to the drain electrode. The drain current initially increases rapidly to\na peak, attributed to carrier multiplication under high electric fields. Beyond this peak,\nthe abrupt current surge triggers thermal breakdown within the material, causing\nirreversible physical damage to the device. Consequently, the current exhibits a sharp\ndecline post-peak, as depicted in Supplementary Fig. 3a. In response to your question,\nwe have expanded the discussion of this mechanism including carrier multiplication,\nthermal breakdown, and device degradation in the revised Supplementary information\nto ensure methodological clarity.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary information\nPage: Page 5, Line 2-5\nQ8 Why does the drain voltage required for avalanche significantly increase after\nreplacing the HZO with SiO as shown in Fig 4b in the supplementary material?\nThe AFET employing the high-κ HZO-based dielectric exhibits enhanced gate control\nefficacy compared to its SiO₂ counterpart, owing to the high dielectric constant of HZO.\nThis improved gate modulation and suppressed carrier scattering, as elaborated in Fig.\n3c of the manuscript. The resultant reduction in scattering-induced energy loss enables\na lower breakdown voltage (V ) for triggering avalanche breakdown in the HZO-\nb\nintegrated device, underscoring its performance advantage. In response to your question,\nwe added the reason in the revised manuscript.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 6, Paragraph 1, Line 6-8.\nQ9 Is this phenomenon reliable? Could you please to repeat the measurement of\nseveral such devices to determine the reliability of this experimental phenomenon?\nAuthors’ response:\nThank you for your feedback. In response to your suggestion, we have incorporated\nstatistical data from 59 devices, detailing their avalanche characteristics, into the\nrevised Supplementary information to ensure statistical rigor.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 6, Paragraph 1, Line 10-11.\nSection: Supplementary information\nPage: Page 9, Supplementary Fig.6a.\nReviewer #2:\nThe authors report a room temperature low breakdown voltage (~5 V) in ReSe device\n2\nand associate it with the avalanche process. The intrinsic anisotropic crystal structure\nis believed to lower the charge carrier scattering and contribute to the phenomenon.\nDFT simulations were performed to extract the electron mass in a, b and c axis. The\nelectron mass of c axis (out of plane) is 3.5 times larger than that of an axis (in plane)\nand is believed as the origin of low charge carrier scattering probability and low\nbreakdown voltage. Finally, the photo response of the ReSe device was measured. A\n2\nresponsivity of 1.71×104 A/W and a gain of 173 are reported.\nAfter carefully going through the manuscript, I personally cannot support the\npublication of it in the highest rated multidisciplinary journal “Nature\nCommunications”. My questions/queries are listed as follows.\nQ1 There will be more than 4 orders of magnitude increase of current after\nbreakdown. However, the IV curves are all plotted in linear coordinate in this work.\nThis is not a rigorous and acceptable way to show the current boost of breakdown.\nPlease find the IV plot in silicon and InGaAs avalanche photodiode (Optical and\nQuantum Electronics 52 (2020): 1-9; Nature Communications 15, 3639 (2024),\ngraphene/silicon and WSe2 impact ionization field-effect transistor (Nat Commun 15,\n9038 (2024); Nat Commun 13, 6076 (2022))\nAuthors’ response:\nThank you for your feedback and the recommended references. As suggested, we have\nincluded logarithmic-scale I -V curves in the revised manuscript, demonstrating a\nds ds\ncurrent surge spanning over four orders of magnitude to highlight the pronounced I\nds\nenhancement due to the avalanche breakdown effect.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 5, Paragraph 2, Line 12-13; Page 6, Fig. 2a.\nQ2 As is well documented in the textbook, the current break-down could arise from\neither avalanche or Zener process. There is a simple way to distinguish them from each\nother. Generally, the avalanche process is featured by a positive temperature coefficient\nof the breakdown voltage (low temperature would freeze the lattice vibration, and thus\nlead to low frequency of electron-phonon scattering), while the Zener process is\nfeatured by a negative coefficient (low temperature would increase the bandgap and\nthus suppress the band to band tunneling). However, there is no such evidence present\nin this work. Based on the above questions/queries, I cannot support the judgement that\nthe current boast observed in this work is breakdown and associate with the avalanche\nprocess.\nAuthors’ response:\nThanks for your valuable feedback. In response to your question, we have measured\nand plotted the temperature-dependent breakdown characteristics in the revised\nSupplementary information. These curves exhibit a positive temperature coefficient, a\nhallmark of avalanche-driven breakdown mechanisms in semiconductors. This\nconfirms that the observed breakdown behavior originates from the avalanche process.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 5, Paragraph 1, Line 18.\nSection: Supplementary information\nPage: Page 5, Paragraph 1, Line 19-26; Page 6,\nSupplementary Fig. 3d and e.\nQ3 To calculate the detectivity of the device, the noise is estimated as 2qI . This is\nds\nnot rigorous and usually overestimates the device performance. The noise should be\nmeasured experimentally by the standard measurement method (Nature 542, 324–327\n(2017)).\nAuthors’ response:\nYes, you are correct. The noise current in this work was estimated using the expression\nI = √2qI ∆f, where f denotes the bandwidth. This formulation only considers the\nnoise ds ∆\nwhite noise characteristic dominated at high frequency, which inherently overestimates\nthe detectivity. Regrettably, we could not obtain the accurate noise due to measurement\nequipment limitation. Furthermore, the detectivity is not the parameter that we mainly\nclaim in this work, and we did not outline it in abstract and conclusion parts in the\nmanuscript initially. To avoid confusing, we described this overestimation and deleted\nthe detectivity column in the revised manuscript and Supplementary information.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: APT characteristics\nPage: Page 11, Paragraph 1, Line 4-5, Line 11-12.\nSection: Reference\nPage: Page 15, Reference 38.\nSection: Supplementary information\nPage: Page 17, Supplementary Table 1.\nReviewer #3:\nFrom this work, the authors present a comprehensive study on ReSe -based avalanche\n2\nfield-effect transistors, claiming a record-low breakdown field (2.55 kV/cm) and a high\nionization index (38.79) among reported 2D-material AFETs. These achievements are\nattributed to the anisotropic properties of ReSe and the incorporation of high-k HZO\n2\ndielectric. The study also demonstrates the potential of this design through the\nfabrication of high-performance avalanche phototransistors. The paper is potentially\ninteresting to the community but I believe that the manuscript may contain incomplete\nsupporting evidence for the high performance metrics as they promote in the manuscript.\nTherefore, considering a high standard set in Nature Communications, I recommend\nthat the manuscript to undergo major revisions after considering the following concerns:\nQ1 Mechanism for the low breakdown field: while the authors attribute the low\nbreakdown field and high ionization index to anisotropic ReSe properties and effective\n2\ngate modulation, the quantitative relationship between these factors is not clearly\nestablished. The manuscript should provide more detailed modeling or theoretical\nanalysis to link the observed device characteristics to the proposed mechanisms,\nparticularly concerning the reduction in carrier scattering. I would like to suggest the\nfollowing aspects to be included in their work:\na. Experimental evidence for reduction in carrier scattering at different regimes of gate\nvoltages: mobility, carrier lifetime, etc.\nAuthors’ response:\nThanks for your question. Initially, we extracted the scattering probability β under\ndifferent gate voltages based on experimental data to quantitively analyze the gate\nmodulated scattering, as shown in Fig. 3c. Based on your question, we extracted the\nmobility from the measured I-V curves under different gate voltages to evidence the\nreduced carrier scattering as gate voltage increases negatively and added this curve in\nthe revised Supplementary information.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Mechanism analysis\nPage: Page 9, Paragraph 1, Line 6-12.\nSection: Supplementary information\nPage: Page 14, Paragraph 3, Line 10-13, Supplementary Fig.\n12c\nb. Detailed modeling or theoretical analysis to link the observed carrier multiplication\nto the proposed mechanisms, particularly quantitatively linking the experimental the\nreduction in carrier scattering to the theoretical work.\nAuthors’ response:\nThanks for your feedback. We added theoretical analysis on scattering rate by a model\nconsidering ionization impurity scattering and photon scattering in detail. Moreover,\nthe extracted scattering probability β from experimental data is linked with the\ncalculated one from the proposed model in the revised manuscript and Supplementary\ninformation.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Mechanism analyses\nPage: Page 9, Paragraph 1, Line 12-16. Page 10, Fig. 3c.\nSection: Abstract\nPage: Page 2, Paragraph 1, Line 2-3,\nSection: Introduction\nPage: Page 3, Paragraph 3, Line 19-20\nSection: Supplementary information\nPage: Page 15-16, Section 11\nc. Detailed description of device operation:\ni. What is the carrier accumulation layer thickness? ii. For the thickness of the channel that they use, at what thickness away from the dielectric are the main transport channels?\nAuthors’ response:\nThanks for your advice. The ReSe FETs work in depletion mode, in which I increases\n2 off\nand V decreases as the ReSe layer number increases [R5, R6], as illustrated in\nth 2\nSupplementary Fig. 2c in the revised Supplementary information. The layer number of\nReSe in the AFET is 5, as descripted in Page 4, Paragraph 1, Line 1. Therefore, all the\n2\n5 layers of ReSe in this work are accumulation layer and main transport channels. We\n2\nhave clarified the device operation mode in the revised manuscript in response to your\nquestion.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 5, Paragraph 2, Line 6-7.\nSection: Supplementary information\nPage: Page 4, Supplementary Fig 2c.\niii. In this case, is the trapping at the interface expected to be significant?\nAuthors’ response:\nThanks for your question. The trapping at the interface is not significant, which was\nproved by the hysteresis characteristics, as illustrated in Supplementary Fig. 11 in the\nSupplementary information.\nCorresponding change in manuscript: No\niv. What do the authors expect as the thickness dependence of the channel, ReSe ?\nThe transfer characteristics of the ReSe FET exhibit dependence on the ReSe\n2 2\nthickness, as plotted in Supplementary Fig. 2c. While the avalanche characteristics of\nthe ReSe₂ FET show an independence on the ReSe₂ thickness, as demonstrated in\nSupplementary Fig. 9. This phenomenon arises from weak interlayer interactions that\nconstrain carrier transport along the out-of-plane direction[R6], a mechanism\ncorroborated by the cross-sectional carrier dynamics analysis in Fig. 3a of the\nmanuscript. We added the thickness-independent avalanche characteristics in the\nrevised manuscript based on your advice.\nCorresponding change in manuscript: Yes\nSection: Mechanism analyses\nPage: Page 8, Paragraph 1, Line 8-9.\nSection: Supplementary information\nPage: Page 11, Supplementary Fig. 9.\nd. Disregarding artifact: I am concerned that the authors are neglecting the depletion\nregions that can form from employing high-k dielectric for the operation of the device\n(see ACS Nano 2017, 11, 12, 12583–12590). Rather than avalanche multiplication, I\nwonder that the authors may have formed lateral p-n junctions that could show up as\nsimilar device characteristics as the author’s measure.\nAuthors’ response:\nThank you for your insightful observation. The device operates in the depletion regime\nunder negative gate bias (V < 0), which suppresses excessive electron density and\ngs\nminimizes scattering during the avalanche multiplication process. In response to your\nquestion, we have expanded the discussion of this mechanism in the revised manuscript\nto ensure clarity for readers.\nThe observed avalanche multiplication in this design, as opposed to p-n junction-\nmediated behavior, is supported by two key points:\n1. Material and device properties: The ReSe₂ channel exhibits n-type doping\n(see Supplementary Fig. 2a), with unipolar n-type transport behavior distinct\nfrom the ambipolar characteristics reported in ACS Nano 2017, 11, 12, 12583–\n12590. This precludes p-n junction formation.\n2. Breakdown characteristics: Logarithmic-scale I -V curves reveal an abrupt\nds ds\ncurrent surge exceeding four orders of magnitude, a hallmark of avalanche\nbreakdown rather than gradual junction-based conduction.\nIn summary, the sharp nonlinear current escalation, coupled with the absence of\nambipolarity, confirms that the observed behavior is attributable to avalanche\nbreakdown mechanisms.\nCorresponding change in manuscript: Yes\nSection: AFET characteristics\nPage: Page 6, Paragraph 1, Line 2-3;\nQ2 Mechanism for the fast response time: the authors claim impressive rise/fall\ntimes of their photocurrent characteristics.\na. Supporting evidence and mechanism for the fast photo response: Since the device is\nlikely to accumulate minority carriers at the interface, the authors need to carefully\ncharacteristics and discuss why the fast photoresponse is expected in their devices.\nAuthors’ response:\nAt negative gate voltages (V <0), minority carriers (electrons) act as the primary\ngs\ncontributors accelerated under high electric fields, initiating impact ionization to\ngenerate secondary electron-hole pairs. The observed fast photoresponse stems from\nthree synergistic mechanisms:\n1. High ionization coefficient (n): The elevated n value in this design enhances\nthe carrier multiplication rate, directly accelerating the photogenerated carrier\ndynamics [R7].\n2. Defect suppression: Effective gate modulation minimizes defect-mediated hole\ntrapping, thereby improving photo-response efficiency [R8].\n3. Optimized contact interfaces: As evidenced in Supplementary Fig. 2d, the\nlow-resistance ohmic contacts between the source/drain electrodes and the\nchannel facilitate rapid carrier extraction, further boosting photo-response speed\n[R9].\nIn response to your feedback, we have expanded the discussion of these mechanisms in\nthe revised manuscript to ensure clarity and rigor.\nCorresponding change in manuscript: Yes\nSection: APT characteristics\nPage: Page 11, Paragraph 1, Line 21-22.\nb. Gate-voltage and incident power dependence of photo response times over a wide\nrange: the authors should provide power-dependence of the photo response times (not\njust pW scales) to elaborate on the photophysical processes at different incident powers\nand gate voltages by discussing the relevant timescales for carrier injection, transport\nand trapping/detrapping\nAuthors’ response:\nThanks for your advice. We provided the power-dependence of photo-response time\nwith a brief discussion in the revised manuscript and Supporting information. Decay\ntime shows a decreasing trend with increasing irradiation power. This behavior is\nattributed to trap-state saturation caused by the higher density of photogenerated\ncarriers under strong optical excitation, which enhances the photo-response dynamics.\nCorresponding change in manuscript: Yes\nSection: APT characteristics\nPage: Page 12, Paragraph 1, Line 4-7.\nSection: Supplementary information\nPage: Page 18, Supplementary Fig. 13."}, {"round": 2, "reviewer_comments": {"reviewer_2": "In the revised manuscript, the authors have incorporated several important additions, including an analysis of the temperature coefficient, logarithmic-scale I -V\nds ds\ncharacteristics, and corrections to the previously inaccurate detectivity calculations. These improvements have undoubtedly enhanced the overall quality of the work. However, I remain concerned about certain aspects of the newly presented data, particularly regarding the temperature coefficient and avalanche mechanism. If not addressed meticulously, these issues could lead to significant misinterpretations and critical errors in the interpretation of the results. Given these unresolved concerns, I am unable to recommend acceptance of this manuscript in its current form for publication in Nature Communications. Below, I outline specific suggestions for further clarification and refinement:\n1. Temperature coefficient. The authors have supplemented their manuscript with experimental data characterizing the breakdown voltage across a temperature range of 300 K to 160 K, as presented in Supplementary Figures 3d and 3e. While this additional data provides valuable insights into the device’s thermal behavior, two critical questions emerge:\n1) With the decrease in temperature, both scattering and thermal excitation effects in semiconductor (including ReSe2 FET in this work) should be suppressed, leading to an expected reduction in dark current. The current-voltage characteristics presented here should follow the conventional behavior observed in avalanche photodiodes, as extensively documented in prior literature (Nature Photonics 2020, 14, 559-563). However, the authors observed a completely opposite experimental phenomenon. In the provided figures (Supplementary Fig. 3d), the dark current in the linear region actually increased with rising temperature, growing from approximately 3*1e-10 A at 300 K to about 9*1e-10 A at 160 K. This finding is difficult to reconcile with the current theoretical framework. The authors are requested to provide a rigorous explanation for this observation.\n2) Why does the breakdown voltage drop sharply from approximately 2 V to 1.4 V when the temperature changes from 300 K to 280 K (only a 20 K variation), while further decreases in temperature result in much less pronounced changes in breakdown voltage, even showing a trend toward saturation? Considering these issues, I believe the authors should include more temperature-point data, particularly in the range from 300 K to 280 K, as well as extend testing to lower temperatures (e.g., 77 K), in order to provide a more detailed explanation and determine the minimum breakdown voltage.\n2. Working mechanism. I find the working mechanism proposed in this paper quite perplexing. Conventional avalanche devices rely on “junctions” to establish strong electric fields, thereby accelerating carriers and enabling the avalanche multiplication process through impact ionization. Recently, some studies (e.g. Nat. Nanotechnol. 2019, 14, 217-222; Nature Communications 2024, 15, 3639) have achieved localized electric fields and reduced scattering by constructing novel structures, such as vertical junctions or van der Waals stepwise homojunctions, leading to lower avalanche threshold voltages. Given these considerations, I have two questions:\n1) The proposed structure in this work lacks junction-induced localized electric fields (as the authors stated in Supplementary Figure 2d), which are typically crucial for avalanche multiplication, yet it achieves a relatively low avalanche threshold voltage compared to conventional designs. Could the authors provide further explanations in light of recent literature (e.g. Nat. Nanotechnol. 2019, 14, 217-222; Nature Communications 2024, 15, 3639)? Additionally, what is the electric field distribution within the proposed device structure? Could the authors support their findings using TCAD simulations—a widely accepted theoretical approach in previous studies—to validate the proposed avalanche multiplication mechanism?\n2) According to the avalanche mechanism proposed by the authors, since both terminals form ohmic contacts with the material, the two electrodes should be electrically equivalent. Therefore, shouldn't the device exhibit symmetric I-V characteristics and avalanche multiplication processes under bidirectional bias? Specifically, when the left terminal is grounded, avalanche multiplication should theoretically occur under both positive and negative bias applied to the right terminal. The authors should provide complete I-V characteristics spanning from negative to positive bias, such as the -5V to +5V range in Figure 2a, to demonstrate this symmetry.\n3. Gate Voltage Modulation Characteristics. The authors present the breakdown voltage versus gate voltage curve in Figure 2. Here, I have two questions:\n1) Negative Gate Bias Dependence: As the gate voltage becomes more negative, does the breakdown voltage continue to decrease? What is the minimum achievable breakdown voltage (i.e., the performance limit of the breakdown voltage)?\n2) Positive Gate Bias Dependence: How does the breakdown voltage behave when the gate voltage is increased in the positive direction? Does it exhibit saturation, increase, or an unexpected trend?\nTo strengthen the credibility of the findings, the authors should provide extended gate voltage sweep data, covering both deeper negative and positive bias regimes. This would help clarify the full gate-dependent breakdown behavior and validate the proposed mechanism.\n4. Rate of increase in the current. The authors have not fully addressed my previous question (Q1 from the first round of review). They should provide a comprehensive comparison of the I-V characteristics between ReSe and commercial devices (such as InGaAs, Si, etc. Hamamatsu G8931 series. https://www.hamamatsu.com.cn/cn/zh-cn/product/optical-sensors/apd/ingaas-apd/G8931-04.html), with particular focus on extracting the avalanche current rise rate (dVds/dlog(Ids)). In conventional devices like InGaAs APDs, the avalanche current rise rate typically exceeds 400 mV/dec. This quantitative comparison represents a critical benchmark for assessing avalanche performance, as originally highlighted in Reviewer #1’s comments (Q4). The authors must rigorously address this fundamental issue to validate their claims regarding avalanche current.", "reviewer_3": "Throughout the revision, the authors have attempted to address my previous comments for the manuscript. However, there are still major remaining concerns for the manuscript for which I find concerning to be published in Nature Communications:\n1. Relating the mobility values to ionization coefficient in only specific voltage regimes: In SI Section 10, the authors present a qualitative correlation between mobility at different Vg and ionization rate. Although the authors show the results between Vg=0V to -2.5V for the ionization rate data, they somehow omit the Vg < -2.5 V regime for which the mobility drops (see Fig. S12c). The full range of the data should be shown to demonstrate the correlation between the impact ionization rate and the carrier mobility.\n2. Justificaiton by an established mode: the authors claim a large ionization index of around 39. Instead of qualitative correlation, the authors need to justify better how they can achieve a large ionization coefficient, ionization index and carrier multiplication values. They can refer to RPL model to justify quantiatively whether their mobility of the order of only 1(0.2~5.5 cm^2/Vs) is suffcient to cause such significant impact ionization rate to initiate carrier multiplication, along with the justificaiton of the parameters used for the model.\n3. Scattering model in SI section 11 need better justification: The authors quote scattering rates estimated from parameters predicted from DFT calculations. I am concerned that these are based on weak assumptions and there are no discussion based on whether these obtained values are physically reasonable. For example, I do not believe that 5.18 x 10^17 cm^-3 of the trap density nor the ~10^12 cm^-2 of the ionized impurity density will be in agreement with the claim of the authors that the carrier multiplication rate can be high due to the low degre of trapping and good transport properties (and that there is no hysterisis). The inconsistency in the authors’ claims in different aspects of the device characteristics support that the physical models used are not in support of the experimental observations.\n4. Experimental supporting data for scaterring model in SI Section 11 required: I strongly sugest that the authors determine by measuring the temperature dependence of mobility from FET devices to directly estimate the mobility and scattering rates. They can distinguish the ionized impurity and phonon contribution from the temperature dependence. The agreement with the transport parameters extracted from the experimental device characteristics should be made."}, "author_response": "Reviewer #2:\nIn the revised manuscript, the authors have incorporated several important additions,\nincluding an analysis of the temperature coefficient, logarithmic-scale I -V\nds ds\ncharacteristics, and corrections to the previously inaccurate detectivity calculations.\nThese improvements have undoubtedly enhanced the overall quality of the work.\nHowever, I remain concerned about certain aspects of the newly presented data,\nparticularly regarding the temperature coefficient and avalanche mechanism. If not\naddressed meticulously, these issues could lead to significant misinterpretations and\ncritical errors in the interpretation of the results. Given these unresolved concerns, I am\nunable to recommend acceptance of this manuscript in its current form for publication\nin Nature Communications. Below, I outline specific suggestions for further\nclarification and refinement:\nQ1 Temperature coefficient. The authors have supplemented their manuscript with\nexperimental data characterizing the breakdown voltage across a temperature range of\ndata provides valuable insights into the device’s thermal behavior, two critical questions\nemerge:\n1) With the decrease in temperature, both scattering and thermal excitation effects in\nsemiconductor (including ReSe FET in this work) should be suppressed, leading to an\n2\nexpected reduction in dark current. The current-voltage characteristics presented here\nshould follow the conventional behavior observed in avalanche photodiodes, as\nextensively documented in prior literature (Nature Photonics 2020, 14, 559-563).\nHowever, the authors observed a completely opposite experimental phenomenon. In the\nprovided figures (Supplementary Fig. 3d), the dark current in the linear region actually\nincreased with rising temperature, growing from approximately 3*1e-10 A at 300 K to\nabout 9*1e-10 A at 160 K. This finding is difficult to reconcile with the current\ntheoretical framework. The authors are requested to provide a rigorous explanation for\nthis observation.\nAuthors’ response:\nWe thank the reviewer for the thoughtful comment and for pointing us to the avalanche-\nphotodiode (APD) literature (Nat. Photonics 2020, 14, 559–563). The APD expectation\n(dark current decreases with decreasing temperature due to suppressed thermal\ngeneration) applies to reverse-biased p–n junctions operated in the avalanche regime,\nwhere the dominant mechanism is thermally assisted carrier generation in the depletion\nregion. By contrast, the specific dataset highlighted by the reviewer corresponds to our\nFET operated in the linear region at low drain bias and in the off state (V < 0)—i.e., a\ngs\nregime where avalanche physics is not operative and the current is not set by depletion-\nregion generation. In this linear-regime FET operation, the dark current can be written,\nto first order, as\nI ∝ μ(T) n (T) V (R1)\ndark eff ds\nwhere μ(T) is the carrier mobility and n (T) is the small residual carrier density set by\neff\ngate-induced depletion.\nUpon lowering T, phonon scattering is suppressed, so μ(T) typically increases in 2D\nTMD channels (often μ ∝ T−α with α ∼ 0.6-1). Meanwhile, because the device is already\nstrongly depleted (V < 0), the residual n (T) is only weakly thermally activated. Over\ngs eff\n300 → 160 K, the mobility enhancement outweighs the modest reduction in n , so the\neff\nproduct μ(T) n (T) increases and the measured dark current in the linear region rises as\neff\ntemperature decreases, exactly as observed in Supplementary Fig. 3d.\nTo avoid confusion, we have revised the Supplementary Information. We hope this\nclarification resolves the apparent discrepancy and makes the temperature dependence\nof the linear-region dark current transparent to readers.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary information\nPage: Page 5, Paragraph 2, Line 1-6. Page 6, Paragraph 1\nLine 1-6.\n2) Why does the breakdown voltage drop sharply from approximately 2 V to 1.4 V\nwhen the temperature changes from 300 K to 280 K (only a 20 K variation), while\nfurther decreases in temperature result in much less pronounced changes in breakdown\nvoltage, even showing a trend toward saturation? Considering these issues, I believe\nthe authors should include more temperature-point data, particularly in the range from\nprovide a more detailed explanation and determine the minimum breakdown voltage.\nAuthors’ response:\nWe thank the reviewer for this helpful suggestion. In the initial submission, the 300 K\ndata and the low-temperature data were taken months apart, which can introduce\ndevice-to-device or environment-induced drift and obscure fine temperature trends. To\neliminate this source of variability, we re-measured the same device on the same day\nacross a continuous temperature sweep from 300 K down to 77 K, using a fixed\nbreakdown criterion and identical bias protocol. We also densified the sampling\nbetween 300–280 K (two additional points) and extended the sweep to 77 K as\nrequested. The revised dataset and protocol are now included in the Supplementary\nInformation (revised Supplementary Fig. 3d).\nWhat the new data show:\n(i) Between 300 K and ~280 K, the breakdown voltage V decreases steeply and\nb\nmonotonically when sampled with finer temperature steps.\n(ii) Below ~160 K, further cooling produces a much smaller change in V ,\nb\napproaching a saturation/plateau; down to 77 K the minimum breakdown\nvoltage remains within the measurement scatter of this plateau.\nThe observed two-regime behavior is consistent with the temperature dependence of\nimpact-ionization physics in a field-effect device:\n(i) The onset of avalanche is governed by the ionization coefficients α (E,T),\nn,p\nwhich are highly sensitive to carrier energy gain between scattering events.\nWhen the device is cooled modestly from 300 K to ~280 K, phonon scattering\ndrops sharply, increasing the mean free path and enabling carriers to acquire\nthe ionization threshold energy at a lower electric field.\n(ii) At deeper cryogenic temperatures, two effects counteract the continued\nmobility gain: (a) the bandgap widens with decreasing T, raising the ionization\nthreshold energy; and (b) carrier freeze-out / reduced free-carrier density and\ncontact transmission changes alter the field profile and multiplication\ndynamics. The net result is a progressive flattening of V (T) and the emergence\nb\nof a saturation regime below ~160 K. The new 77 K data confirm that V does\nb\nnot continue to drop indefinitely.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary Information\nPage: Page 6, Supplementary Fig. 3d. Page 6, Paragraph 2,\nLine 1-7.\nQ2. Working mechanism. I find the working mechanism proposed in this paper quite\nperplexing. Conventional avalanche devices rely on “junctions” to establish strong\nelectric fields, thereby accelerating carriers and enabling the avalanche multiplication\nprocess through impact ionization. Recently, some studies (e.g. Nat. Nanotechnol. 2019,\n14, 217-222; Nature Communications 2024, 15, 3639) have achieved localized electric\nfields and reduced scattering by constructing novel structures, such as vertical junctions\nor van der Waals stepwise homojunctions, leading to lower avalanche threshold\nvoltages. Given these considerations, I have two questions:\n1) The proposed structure in this work lacks junction-induced localized electric fields\n(as the authors stated in Supplementary Figure 2d), which are typically crucial for\navalanche multiplication, yet it achieves a relatively low avalanche threshold voltage\ncompared to conventional designs. Could the authors provide further explanations in\nlight of recent literature (e.g. Nat. Nanotechnol. 2019, 14, 217-222; Nature\nCommunications 2024, 15, 3639)? Additionally, what is the electric field distribution\nwithin the proposed device structure? Could the authors support their findings using\nTCAD simulations—a widely accepted theoretical approach in previous studies—to\nvalidate the proposed avalanche multiplication mechanism?\nAuthors’ response:\nWe thank the reviewer for the thoughtful comments and the helpful pointers to recent\nliteratures (Nat. Nanotechnol. 2019, 14, 217–222; Nat. Commun. 2024, 15, 3639). Our\ndevice does not employ a doped junction; however, two effects create a junction-like,\nnanoscale high-field region that enables avalanche at comparatively low applied\nvoltages:\n1. Localized field at the contact edge (finite Schottky barrier).\nAs clarified in the revised Supplementary Information, the output curves in\nSupplementary Fig. 2d are linear but not perfectly ohmic. Consistent with 2D\nAFETs1, there is a small Schottky barrier at the metal/ReSe₂ interface (extracted\nbarrier ∼120 meV from TCAD simulation). This finite barrier leads to field\ncrowding at the metal edge on the drain side under the avalanche-bias polarity,\nproviding a compact region where carriers experience a large lateral field and\ncan gain the ionization energy over sub-mean-free-path distances.\n2. Gate-induced pinch-off (“virtual junction”) under off-state bias.\nWith V < 0V, the channel is strongly depleted, and a pinch-off region forms\ngs\nnear the drain at the onset of current saturation. The combination of a thin high-\nk gate dielectric (strong electrostatics and fringing fields) and the drain bias\nproduces a steep potential drop over a short length scale-functionally analogous\nto the depletion region of a junction-thereby concentrating the electric field\nwhere impact ionization initiates.\nThese two effects are fully compatible with the strategies highlighted by the reviewer:\nprior works reduce V either by structurally focusing the field (e.g., vertical or stepwise\nb\nhomojunctions) or by minimizing energy loss over the acceleration length (e.g., sub-\nmean-free-path geometries). In our case, contact-edge field enhancement + gate-\ndefined pinch-off supplies the former, while the anisotropic ReSe₂ channel (transport\nalong the low-scattering axis) and nanoscale acceleration region mitigate energy loss,\njointly lowering the avalanche threshold without a chemically formed junction.\nBesides, the TCAD simulation has been added in the revised Supplementary\ninformation to show the electric field distribution in ReSe AFET.\nLocation of Change:\nSection: Introduction\nPage: Page 3. Paragraph 1, Line 3-6\nSection: AFET characteristics\nPage: Page 5, Paragraph 1, Line 8-9; Page 6, Paragraph 1,\nLine 15-19\nSection: Methods\nPage: Page 18, Paragraph 3, Line 1-2.\nSection: Supplementary Information\nPage: Page 4, Supplementary Fig. 2e and f; Page 8,\nSupplementary Section 4.\n2) According to the avalanche mechanism proposed by the authors, since both terminals\nform ohmic contacts with the material, the two electrodes should be electrically\nequivalent. Therefore, shouldn’t the device exhibit symmetric I-V characteristics and\navalanche multiplication processes under bidirectional bias? Specifically, when the left\nterminal is grounded, avalanche multiplication should theoretically occur under both\npositive and negative bias applied to the right terminal. The authors should provide\ncomplete I-V characteristics spanning from negative to positive bias, such as the -5V to\n+5V range in Figure 2a, to demonstrate this symmetry.\nAuthors’ response:\nThank you for the feedback. The revised manuscript now includes the full I–V sweep\nfrom −5 to +5 V. As noted previously, the source/drain contacts to the channel are low-\nresistance but not perfectly ohmic; a finite Schottky barrier at the metal–semiconductor\ninterface accounts for the slight asymmetry observed in the I–V curves.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 6, Paragraph 1, Line 15-19. Page 7, Figure 2a.\nQ3. Gate Voltage Modulation Characteristics. The authors present the breakdown\nvoltage versus gate voltage curve in Figure 2. Here, I have two questions:\n1) Negative Gate Bias Dependence: As the gate voltage becomes more negative, does\nthe breakdown voltage continue to decrease? What is the minimum achievable\nbreakdown voltage (i.e., the performance limit of the breakdown voltage)?\nAuthors’ response:\nThanks for your question. As analyzed in the manuscript (Figure 3d), the breakdown\nvoltage initially decreases when the gate voltage is swept from 0 to -2.5V, reaching a\nminimum value of 2.05 V at V = -2.5 V. While further decreasing the gate voltage to\ngs\n-3.5 V leads to an increase in breakdown voltage.\nCorresponding change in manuscript: No\n2) Positive Gate Bias Dependence: How does the breakdown voltage behave when the\ngate voltage is increased in the positive direction? Does it exhibit saturation, increase,\nor an unexpected trend? To strengthen the credibility of the findings, the authors should\nprovide extended gate voltage sweep data, covering both deeper negative and positive\nbias regimes. This would help clarify the full gate-dependent breakdown behavior and\nvalidate the proposed mechanism.\nAuthors’ response:\nThanks for your advice. Under the positive gate voltage, the AFET sequentially\noperates in the linear, saturation, and finally avalanche breakdown regimes. Therefore,\nthe avalanche breakdown voltage at positive V is significantly higher than that\ngs\nobserved under negative V , as shown in the Supplementary Fig. 7 in the revised\ngs\nSupplementary information.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 7, Paragraph 1, Line 4-7\nSection: Supplementary Information\nPage: Page 11, Supplementary Fig.7\nQ4. Rate of increase in the current. The authors have not fully addressed my previous\nquestion (Q1 from the first round of review). They should provide a comprehensive\ncomparison of the I-V characteristics between ReSe and commercial devices (such as\ncn/product/optical-sensors/apd/ingaas-apd/G8931-04.html), with particular focus on\nextracting the avalanche current rise rate (dVds/dlog(Ids)). In conventional devices like\nInGaAs APDs, the avalanche current rise rate typically exceeds 400 mV/dec. This\nquantitative comparison represents a critical benchmark for assessing avalanche\nperformance, as originally highlighted in Reviewer #1’s comments (Q4). The authors\nmust rigorously address this fundamental issue to validate their claims regarding\navalanche current.\nAuthors’ response:\nWe appreciate the request for a quantitative comparison. Following the reviewer’s\nmetric, we extracted the avalanche current rise rate.\ndV /dlog (I ) (R2)\nds 10 ds\nby linearly fitting V vs. log I over the two decades of current immediately preceding\nds 10 ds\nbreakdown, at fixed V and temperature. For our ReSe AFET, the representative value\nGS 2\nis 250 mV/dec at V = −2.5 V. This is sharper (lower mV/dec) than the ~400 mV/dec\ngs\ncommonly reported for conventional InGaAs APDs. We include a brief benchmark\nbelow for comparison.\nTable R1. Benchmarking between the ReSe and commercial devices\nChannel material Gain Ref\n(V) rise rate (mV/dec) (A/W)\nReSe2 2.05 250 1.71×104 173 This work\nSi 500 - ~20 50~100 a\nInGaAs 55~65 400 10 ~30 a\na: the data of commercial Si and InGaAs devices are from: https://www.hamamatsu.com.cn/cn/zh-\ncn/product/optical-sensors/apd.html.\nOur device’s rise rate (250 mV/dec) indicates a more abrupt avalanche onset than the\n~400 mV/dec benchmark for conventional InGaAs APDs, while operating at much\nlower drain bias due to the gate-defined high-field region.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 6, Paragraph 1, Line 1-4.\nSection: Conclusion\nPage: Page 14, Paragraph 1, Line 6-7.\nSection: Supplementary Information\nPage: Page 23, Supplementary Table 3.\nReviewer #3:\nThroughout the revision, the authors have attempted to address my previous comments\nfor the manuscript. However, there are still major remaining concerns for the\nmanuscript for which I find concerning to be published in Nature Communications:\nQ1. Relating the mobility values to ionization coefficient in only specific voltage\nregimes: In SI Section 10, the authors present a qualitative correlation between mobility\nat different Vg and ionization rate. Although the authors show the results between\nVg=0V to -2.5V for the ionization rate data, they somehow omit the Vg < -2.5 V regime\nfor which the mobility drops (see Fig. S12c). The full range of the data should be shown\nto demonstrate the correlation between the impact ionization rate and the carrier\nmobility.\nAuthors’ response:\nWe appreciate the suggestion. In the revision, we now include the previously omitted\nV < −2.5 V regime. Specifically, we extended the ionization-rate extraction to V =\ngs gs\n−3.0 V and −3.5 V using the same Shockley-type fits as in the main set (updated\nSupplementary Fig. 14a and b), and we plot the mobility across the full gate range\nalongside (updated Supplementary Fig. 14c). For clarity, the mobility panel the\nreviewer referred to in the earlier version (their “Fig. S12c”) now appears with the\nionization analysis so the correlation can be read at a glance.\nWhat the added data show:\n1. The qualitative correlation holds over the full range: as |V | becomes more\nGS\nnegative beyond -2.5 V, the mobility decreases, and the extracted ionization rate\nα (from the linear fits of V vs. logI immediately prior to breakdown)\nds ds\ncorrespondingly rolls off/saturates relative to its peak near -2.5 V.\n2. The fit quality remains high (coefficients of determination R2>0.9), consistent\nwith the Shockley form used throughout (see captions in Supplementary Fig.\n14a,b).\nWe believe these additions address the request to “show the full range of the data” and\ndemonstrate the mobility–ionization correlation across both the original (0 to -2.5 V)\nand extended (-2.5 to -3.5 V) gate regimes.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary Information\nPage: Page 18, Supplementary Fig. 14a and b.\nQ2. Justification by an established mode: the authors claim a large ionization index of\naround 39. Instead of qualitative correlation, the authors need to justify better how they\ncan achieve a large ionization coefficient, ionization index and carrier multiplication\nvalues. They can refer to RPL model to justify quantitively whether their mobility of\nthe order of only 1(0.2~5.5 cm^2/Vs) is sufficient to cause such significant impact\nionization rate to initiate carrier multiplication, along with the justification of the\nparameters used for the model.\nAuthors’ response:\nThank you for your valuable feedback and recommendation. The mobility value shown\nin Supplementary Fig. 14c was extracted from transfer characteristics and used to\nanalyze the scattering behavior under different gate voltages. However, this value is\nunderestimated due to the inclusion of contact resistance. To establish a more\nquantitative link to the impact ionization rate, we also performed four-terminal\nmeasurements on ReSe FET, which eliminate contact resistance. Using this corrected\n2\nmobility in the RPL model, the calculated ionization rate closely matches the\nexperimental results, thereby validating the ionization index reported in this work. The\ncorresponding data and analysis have been incorporated into the revised manuscript and\nSupplementary Information.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary Information\nPage: Page 13-14, Section 9.\nSection: AFET characteristics\nPage: Page 8, Paragraph 3, Line 8-10.\nSection: Conclusion\nPage: Page 14, Paragraph 1, Line 5-6.\nQ3. Scattering model in SI section 11 need better justification: The authors quote\nscattering rates estimated from parameters predicted from DFT calculations. I am\nconcerned that these are based on weak assumptions and there is no discussion based\non whether these obtained values are physically reasonable. For example, I do not\nbelieve that 5.18 x 10^17 cm^-3 of the trap density nor the ~10^12 cm^-2 of the ionized\nimpurity density will be in agreement with the claim of the authors that the carrier\nmultiplication rate can be high due to the low degree of trapping and good transport\nproperties (and that there is no hysteresis). The inconsistency in the authors’ claims in\ndifferent aspects of the device characteristics support that the physical models used are\nnot in support of the experimental observations.\nAuthors’ response:\nThanks for your valuable feedback. The trap density of 5.18 1017 cm-3 was obtained\n×\nby the following equation:\n𝑁 =\n𝐶𝑜𝑥 |∆𝑉\n𝑡ℎ\n|\n………………………………….(R3)\ntp\net\nwhere oxide capacitance C , ReSe thickness t, and hysteresis V were all extracted\nox 2 ∆ th\nfrom experimental data. Both calculated interface ionized impurity density of 1.76\n1011 cm-2 (based on experimental data) and the value of 9.23 1011 cm-2 used in the\n× ×\nfitting model are physically reasonable for 2D material devices, consistent with state-\nof-the-art reports2,3.\nYes, you are correct that the N alone could not determine the fitting scattering rate or\ni\nsolely account for the high multiplication rate. Form our model, in addition to N, the\ni\ncarrier concentration (n) and Debye screening length (λ ), which are V dependent also\ns gs\ncontribute to the high multiplication rate. We have incorporated these explanations and\nprovided a detailed table of all parameters used in the fitting model in the revised\nSupplementary Information for clarity.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary Information\nPage: Page 19-21, Section 13.\nQ4. Experimental supporting data for scattering model in SI Section 11 required: I\nstrongly suggest that the authors determine by measuring the temperature dependence\nof mobility from FET devices to directly estimate the mobility and scattering rates.\nThey can distinguish the ionized impurity and phonon contribution from the\ntemperature dependence. The agreement with the transport parameters extracted from\nthe experimental device characteristics should be made.\nAuthor’s response:\nThanks for your suggestion. We have measured the mobility under different\ntemperatures and plotted this curve in the revised Supplementary information. From\nthat figure, the mobility increases with reduced temperature due to suppressed phonon\nscattering at low temperature. Additionally, the mobility initially increases with more\nnegative V and then reduces when the V further reduces. This non-monotonic\ngs gs\nbehaviour, which aligns with the trend of transport parameters, is consistent with the\nscattering mechanisms discussed in scattering model.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary Information\nPage: Page 21, Paragraph 3, Line 1-8, Supplementary Fig. 15b\nSection: Mechanism analyses\nPage: Page 10, Paragraph 1, Line 16-17.\nSection: Conclusion\nPage: Page 14, Paragraph 1, Line 12.\nSection: Methods\nPage: Page 17, Paragraph 3, Line 7-8.\nReferences\n1. Seo, J. et al. Ultrasensitive photodetection in MoS avalanche phototransistors. Adv.\n2. Joe, A. et al. Transport Study of Charge-Carrier Scattering in Monolayer WSe .\n3. Zeng, S., Liu, C. & Zhou, P. Transistor engineering based on 2D materials in the\npost-silicon era. Nat. Rev. Electr. Eng. 1, 335–348 (2024)."}, {"round": 3, "reviewer_comments": {"reviewer_2": "In the revised manuscript, while the authors have addressed some of the previous points, major concerns persist regarding several critical issues, notably the identification of the avalanche mechanism and the origin of the dark current. I have serious reservations on these points, which I believe constitute fundamental flaws that could easily lead to significant misinterpretation of the results. My specific comments are as follows:\n(1) In connection with the dark current issue (Q1) I raised in the previous review round, I still have concerns. While I appreciate your effort to clarify the temperature-dependent behavior of the dark current in the linear region of your ReSe₂ FET, I find the explanation provided still unsatisfactory and inconsistent with your own experimental data.\nYou argue that the increase in dark current with decreasing temperature is due to a mobility increase that outweighs the change in carrier density. However, your extracted mobility values from Supplementary Fig. 15a show only a minor change (<10%) in the temperature range above 200 K. In contrast, the dark current in Supplementary Fig. 3d increases by several times over the same temperature range. This significant discrepancy clearly contradicts your proposed mechanism and suggests that the mobility change alone cannot account for the pronounced increase in dark current.\nMoreover, I remain deeply concerned about the presentation of your results. As a study on an avalanche photodiode (APD), it is essential to include temperature-dependent characterizations-especially those related to dark current-in the main text, rather than relegating them to the Supplementary Materials. The temperature dependence of key parameters is central to the understanding of avalanche mechanisms and device performance. Its omission from the main text severely limits the readers’ ability to evaluate the validity and novelty of your claims.\n(2) Regarding the relationship between avalanche threshold voltage and temperature (Q2) that I raised in the previous review round, I still have concerns. In the last review, I pointed out that the breakdown voltage drops sharply from approximately 2 V to 1.4 V when the temperature decreases from 300 K to 280 K (a mere 20 K variation). The authors attributed this discrepancy to the fact that \"the 300 K data and the low-temperature data were taken months apart.\" I find this explanation highly unsatisfactory, as all data should ideally be collected in a single, continuous measurement session to ensure consistency. Presenting data acquired under such inconsistent conditions may easily mislead readers.\nI appreciate that the authors have addressed this issue by re-measuring the same device in one day using a continuous temperature sweep from 300 K down to 77 K (as shown in Supplementary Fig. 3d). However, I regret to say that the new data still appear insufficient to support definitive conclusions. First, although the figure caption mentions I-V curves at nine temperature points from 300 K to 77 K, only seven curves are actually displayed-data for 160 K and 100 K are missing. Second, I have reservations about how the threshold voltage was determined. While the authors describe the extraction method in Supplementary Section “5. Avalanche breakdown voltage (Vb) extraction,” the procedure applied in Fig. 3b does not seem fully consistent with this description. For instance, the data at 220 K and 120 K appear nearly identical, yet the extracted threshold voltages (Supplementary Fig. 3e) differ by approximately 0.3 V. These issues raise concerns about the validity of the data.\n(3) In Figure 4, the authors present the optoelectronic performance characterization. I find this part of the analysis lacking in professionalism.\ni) Firstly, calculating D* solely based on dark current (Idark) may introduce significant errors. It is important to note that the dark current (Ids) is primarily influenced by shot noise. The photogain (avalanche gain) should be considered when calculating the detectivity (D*), as the noise current will also be amplified by the gain, which is approximately 173 in this work. Additionally, thermal noise and generation-recombination noise-two significant sources of noise-appear to have been overlooked. I recommend testing the noise current using standard methods (e.g., Nature, 2017, 542, 324) before recalculating the detectivity;\nii) Please use the international units \"cm Hz^1/2 W^-1\" for detectivity;\niii) The decay time should be presented as a data point plot. Furthermore, the testing method and definition of response time should be clearly specified in the article.", "reviewer_3": "Throughout the revision, the authors have made significant efforts to address many of my concerns. Although I believe that there are some remaining issues with a clear phenomenological interpretation of the avalanche breakdown effect in ReSe2 (as the other reviewer also pointed out) and transport analysis (e.g. credibility in the T-dependence of mobility data, shown in Fig. S15b, that show sudden sharp transitions), I think that the work presents interesting results and supporting evidence that should be open to be discussed with the entire community. Therefore, I would like to recommend publication of this article."}, "author_response": "REVIEWER COMMENTS\nReviewer #2 (Remarks to the Author):\nIn the revised manuscript, while the authors have addressed some of the previous points,\nmajor concerns persist regarding several critical issues, notably the identification of the\navalanche mechanism and the origin of the dark current. I have serious reservations on\nthese points, which I believe constitute fundamental flaws that could easily lead to\nsignificant misinterpretation of the results. My specific comments are as follows:\nQ1 In connection with the dark current issue (Q1) I raised in the previous review round,\nI still have concerns. While I appreciate your effort to clarify the temperature-dependent\nbehavior of the dark current in the linear region of your ReSe₂ FET, I find the\nexplanation provided still unsatisfactory and inconsistent with your own experimental\ndata.\n1) You argue that the increase in dark current with decreasing temperature is due to a\nmobility increase that outweighs the change in carrier density. However, your extracted\nmobility values from Supplementary Fig. 15b show only a minor change (<10%) in the\ntemperature range above 200 K. In contrast, the dark current in Supplementary Fig. 3d\nincreases by several times over the same temperature range. This significant\ndiscrepancy clearly contradicts your proposed mechanism and suggests that the\nmobility change alone cannot account for the pronounced increase in dark current.\nAuthors’ response:\nWe thank the reviewer for the careful and valid observation. We agree that, as originally\nwritten, the explanation could be perceived as inconsistent without clearer clarification\nof the experimental context.\nFirst, we clarify that the mobility data and the dark-current data were extracted from\ndifferent device configurations, optimized for different measurement purposes.\nSpecifically, the temperature-dependent mobility in Supplementary Fig. 15b was\nextracted at 𝑉 = 0.1 V from a four-terminal ReSe FET fabricated on 285-nm SiO ,\n𝐷𝑆 2 2\nwhich is required for reliable wire-bonding and contact de-embedding in low-bias\ntransport measurements. In contrast, the dark current in Supplementary Fig. 3d was\nmeasured from a device employing an 8-nm HZO dielectric, which exhibits\nsubstantially stronger electrostatic control and a different interfacial environment.\nAs a result, the absolute magnitude and temperature sensitivity of the dark current\ncannot be quantitatively compared directly with the mobility extracted from the SiO -\n2\nbased device. The purpose of the low-temperature mobility analysis was not to establish\na one-to-one quantitative correlation with the dark current magnitude, but rather to\nidentify the dominant scattering trend, for which the temperature dependence (rather\nthan the absolute mobility value) is the key indicator.\nQuantitatively, for the SiO -based device at 𝑉 = 0.1 V, the drain current and\n2 𝐷𝑆\nextracted mobility decrease by ~46% and ~28.7%, respectively, when the temperature\nincreases from 220 K to 300 K, indicating a mobility-limited transport regime. In the\nto a more pronounced temperature response of the dark current, consistent with the\ncombined effects of mobility enhancement and dielectric-interface-mediated transport\nrather than mobility change alone.\nTo avoid any potential misinterpretation, we have explicitly stated the difference in\ndevice structures, dielectrics, and measurement conditions in the revised manuscript\nand Supplementary Information, and we have clarified that the mobility analysis is\nintended to elucidate the transport trend rather than to serve as a direct quantitative\nexplanation for the magnitude of the dark current.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Methods\nPage: Page 17, Paragraph 2, Line 7-8.\nSection: Supplementary Information\nPage: Page 5, Paragraph 4, Line 2-5; Page 20, Paragraph 2,\nLine 2-3.\n2) Moreover, I remain deeply concerned about the presentation of your results. As a\nstudy on an avalanche photodiode (APD), it is essential to include temperature-\ndependent characterizations-especially those related to dark current-in the main text,\nrather than relegating them to the Supplementary Materials. The temperature\ndependence of key parameters is central to the understanding of avalanche mechanisms\nand device performance. Its omission from the main text severely limits the readers’\nability to evaluate the validity and novelty of your claims.\nAuthors’ response:\nWe thank the reviewer for this important comment. We fully agree that, for a study\ncentered on avalanche photodiodes, temperature-dependent characteristics—\nparticularly dark current and breakdown behavior—are essential for evaluating the\navalanche mechanism and overall device performance, and should therefore be\npresented in the main text.\nIn response, we have moved the temperature-dependent current–voltage characteristics\nand breakdown voltage analysis from the Supplementary Information into the main\nmanuscript. These data are now explicitly discussed in the Results section, where their\nrelevance to the avalanche process, dark current evolution, and device stability can be\ndirectly assessed by the reader. Correspondingly, we have expanded the accompanying\ndiscussion to clarify how the observed temperature trends support the proposed\navalanche mechanism and reinforce the validity of our interpretation.\nWe believe this revision significantly improves the clarity and completeness of the\nmanuscript, and enables readers to more readily evaluate both the physical origin of the\n2\navalanche behavior and the novelty of the reported device performance.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 6, Paragraph 1, Line 6-14; Page 7, Figure 2b-d\nSection: Supplementary Information\nPage: Page 6, Supplementary Fig.3d\nQ2 Regarding the relationship between avalanche threshold voltage and temperature\n(Q2) that I raised in the previous review round, I still have concerns. In the last review,\nI pointed out that the breakdown voltage drops sharply from approximately 2 V to 1.4\nV when the temperature decreases from 300 K to 280 K (a mere 20 K variation). The\nauthors attributed this discrepancy to the fact that \"the 300 K data and the low-\ntemperature data were taken months apart.\" I find this explanation highly unsatisfactory,\nas all data should ideally be collected in a single, continuous measurement session to\nensure consistency. Presenting data acquired under such inconsistent conditions may\neasily mislead readers. I appreciate that the authors have addressed this issue by re-\nmeasuring the same device in one day using a continuous temperature sweep from 300\nK down to 77 K (as shown in Supplementary Fig. 3d). However, I regret to say that the\nnew data still appear insufficient to support definitive conclusions. First, although the\nfigure caption mentions I-V curves at nine temperature points from 300 K to 77 K,\nonly seven curves are actually displayed, data for 160 K and 100 K are missing.\nAuthors’ response:\nWe thank the reviewer for carefully examining the revised temperature-dependent\nmeasurements and for pointing out this issue. We confirm that all nine I–V curves (300,\n260, 220, 180, 160, 140, 120, 100, and 77 K) were indeed measured during a single\ncontinuous temperature sweep on the same device within one day, as stated in the\ncaption.\nThe apparent absence of the 160 K and 100 K curves in the original version of\nSupplementary Fig. 3d was due to the fact that their electrical characteristics are nearly\nidentical to those at adjacent temperatures, resulting in curve overlap when plotted on\nthe same scale. This overlap made the individual traces visually indistinguishable,\nalthough they were present in the dataset.\nTo eliminate any ambiguity and to avoid misleading the reader, we have revised the\nfigure by adding a zoomed-in view of the relevant voltage range, which clearly resolves\nand displays all nine temperature-dependent I–V curves. The figure caption and\ncorresponding text have been updated accordingly to explicitly clarify this point.\nLocation of Change:\nSection: AFET characteristics\nPage: Page 7 Figure 2b.\n2) Second, I have reservations about how the threshold voltage was determined. While\nthe authors describe the extraction method in Supplementary Section “5. Avalanche\nbreakdown voltage (V ) extraction,” the procedure applied in Fig. 3b does not seem\nb\nfully consistent with this description. For instance, the data at 220 K and 120 K appear\nnearly identical, yet the extracted threshold voltages (Supplementary Fig. 3e) differ by\napproximately 0.3 V. These issues raise concerns about the validity of the data.\nAuthors’ response:\nWe thank the reviewer for this careful observation. We agree that the apparent\ninconsistency between the overlapping I–V characteristics and the extracted threshold\nvoltages require clarification.\nFirst, the I–V curves in Supplementary Fig. 3d of the previous Supplementary\nInformation appear nearly coincident because they were plotted on a logarithmic\ncurrent scale over a broad 𝑽 range, which compresses small but systematic\n𝑫𝑺\ndifferences near the avalanche onset region. In particular, the curves at 220 K and 120\nK, while visually similar on the logarithmic plot, exhibit subtle but reproducible\ndeviations in slope and curvature close to the breakdown regime that are not readily\ndistinguishable at that scale.\nSecond, as correctly noted by the reviewer, the avalanche threshold voltage (𝑽 )\n𝒃\nextraction procedure described in Supplementary Section 5 is performed on linearly\nplotted I–V characteristics, where the onset of the sharp current increase can be resolved\nwith higher sensitivity. To ensure full consistency between the plotted data and the\nextraction method, we have re-plotted all temperature-dependent I–V curves on a linear\nscale and re-extracted 𝑉 for all temperatures using the same criterion as described in\n𝑏\nSupplementary Section 5.\nThese revised linear-scale plots and the corresponding 𝑉 extraction results are now\n𝑏\nprovided in Figure R1(a–k). The re-analysis confirms that the observed ~0.3 V\ndifference in 𝑉 between 220 K and 120 K arises from genuine, temperature-\n𝑏\ndependent shifts in the avalanche onset rather than from plotting artifacts or inconsistent\nextraction procedures.\nTo further improve clarity and avoid potential confusion, we have:\n1. Added a zoomed-in linear-scale view of the avalanche onset region in the\nrevised manuscript, and\n2. Explicitly stated the scale used for 𝑽 extraction and its distinction from the\n𝒃\nlogarithmic plots used for wide-range visualization.\n77K. b-j V extraction from avalanche I-V curves on a linear scale and k summarization\nb\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: AFET characteristics\nPage: Page 7 Figure 2b. Page 7, Paragraph 1 Line 1-2.\nQ3 In Figure 4, the authors present the optoelectronic performance characterization. I\nfind this part of the analysis lacking in professionalism.\n1) Firstly, calculating D* solely based on dark current (I ) may introduce significant\ndark\nerrors. It is important to note that the dark current (I ) is primarily influenced by shot\nds\nnoise. The photo gain (avalanche gain) should be considered when calculating the\ndetectivity (D*), as the noise current will also be amplified by the gain, which is\napproximately 173 in this work. Additionally, thermal noise and generation-\nrecombination noise-two significant sources of noise-appear to have been overlooked.\nI recommend testing the noise current using standard methods (e.g., Nature, 2017, 542,\n324) before recalculating the detectivity;\nAuthors’ response:\nWe thank the reviewer for this insightful and technically important comment. We agree\nthat a rigorous evaluation of detectivity (D*) in avalanche photodetectors should, in\nprinciple, account for gain-dependent noise amplification as well as multiple noise\n5\na\ne\ni\nb\nf\nj\nc\ng\nk\nd\nh\nsources, including shot noise, thermal noise, and generation–recombination (G–R)\nnoise.\nIn the original manuscript, the noise current was estimated using\n𝐼 = √2𝑞𝐼 Δ𝑓,\nnoise ds\nwhich assumes shot-noise-dominated white noise and does not explicitly incorporate\nthermal or G–R noise contributions. We acknowledge that this simplified approach may\noverestimate the detectivity, particularly under avalanche operation where both the\nphotocurrent and noise current are amplified by the avalanche gain (≈173 in this work),\nas correctly pointed out by the reviewer.\nWe would like to clarify that the detectivity calculation was included only as a\nsecondary, indicative metric, and was not intended to be a central claim of the study.\nAccordingly, D* was not highlighted in the Abstract, Conclusions, or benchmark\ncomparison table (Supplementary Table 2). This limitation was already briefly noted in\nthe original manuscript (Page 12, Paragraph 1, Lines 10–11).\nWe fully agree that direct noise spectral density measurements, as performed using\nspecialized low-noise setups (e.g., Nature 542, 324 (2017)), would provide a more\ncomplete and quantitative evaluation. However, due to instrumentation bandwidth\nlimitations and the current device geometry, such measurements are not feasible at\nthis stage.\nTo avoid any potential misunderstanding or overinterpretation by readers, we have\ntaken the following corrective actions:\n1. Removed the detectivity (D*) discussion from the main text, and\n2. Relocated the D* estimation and its associated assumptions to the revised\nSupplementary Information, where the limitations of the simplified noise model\nare explicitly stated.\nWe believe this revision improves the scientific rigor and presentation quality of the\nmanuscript, and ensures that the reported conclusions remain fully supported by\nexperimentally validated quantities.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: APT characteristics\nPage: Page 12, Paragraph 1, Line1, 9-11.\nSection: Supplementary Information\nPage: Page 24, Supplementary Section 16\n6\n2) Please use the international units \"cm Hz^1/2 W^-1\" for detectivity;\nAuthors’ response:\nThanks for your advice. We updated the detectivity unit to be cm Hz1/2W-1 in the revised\nversion.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Supplementary Information\nPage: Page 24, Paragraph 1, Line 6.\n3) The decay time should be presented as a data point plot. Furthermore, the testing\nmethod and definition of response time should be clearly specified in the article.\nAuthors’ response:\nThank you for your valuable suggestion. Following your advice, we have updated\nFigure 4e and Supplementary Fig. 16a by replacing the line-connected plots with\ndiscrete data-point representations in the revised manuscript and Supplementary\nInformation. In addition, we have expanded the description of the measurement\nmethodology and explicitly clarified the definition of the response time in the revised\nmanuscript to improve clarity and reproducibility.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: APT characteristics\nPage: Page 12, Paragraph 1, Line 18-20; Page 13, Figure 4e\nSection: Method\nPage: Page 18, Paragraph 1, Line 1-4.\nSection: Supplementary Information\nPage: Page 25, Supplementary Fig. 17a\nThroughout the revision, the authors have made significant efforts to address many of\nmy concerns. Although I believe that there are some remaining issues with a clear\nphenomenological interpretation of the avalanche breakdown effect in ReSe (as the\n2\nother reviewer also pointed out) and transport analysis (e.g. credibility in the T-\ndependence of mobility data, shown in Fig. S15b, that show sudden sharp transitions),\nI think that the work presents interesting results and supporting evidence that should be\nopen to be discussed with the entire community. Therefore, I would like to recommend\npublication of this article.\nAuthors’ response:\nWe sincerely thank the reviewer for the careful evaluation of the revised manuscript\nand for the positive recommendation for publication. We appreciate the recognition that,\ndespite some remaining open questions, the work presents results that are of interest to\nthe broader community and worthy of discussion.\nRegarding the phenomenological interpretation of the avalanche breakdown in ReSe₂,\nwe have further clarified the underlying temperature-dependent behavior by moving\nthe temperature-dependent I–V characteristics from the Supplementary Information\ninto the main text, where they are now discussed explicitly in the context of avalanche\nonset and breakdown evolution. This revision improves transparency and allows\nreaders to more directly assess the physical trends relevant to avalanche behavior.\nConcerning the apparent discrepancy between the temperature dependence of mobility\nand current above 200 K, we clarify that these data were obtained from different device\nconfigurations optimized for different measurements. Specifically, the low-temperature\nmobility was extracted from a four-terminal device using 285-nm SiO₂ as the dielectric,\nwhich is required for reliable wire-bonding and contact de-embedding. In contrast, the\ndark current and avalanche measurements were performed on the proposed device\nemploying an 8-nm HZO dielectric, which provides much stronger electrostatic control\nand a different interfacial environment. This dielectric difference accounts for the\ndiffering temperature sensitivities and has now been explicitly stated in the revised\nmanuscript and Supplementary Information.\nWith respect to the temperature-dependent mobility behavior and the sharp transition\nobserved in Fig. S15b, we have expanded the discussion to clarify the underlying\ntransport physics. At low temperatures (T ≲ 100 K), ionized impurity scattering\ndominates and weakens with decreasing temperature, whereas at higher temperatures\n(T ≳ 100 K), phonon scattering becomes dominant and strengthens with increasing\ntemperature. During the transition between these two regimes, the contributions from\nthe two scattering mechanisms become comparable, leading to an inevitable crossover\npoint (around ~100 K) where a pronounced change in the mobility trend is observed.\nSuch behavior has been widely reported in 2D material systems (Refs. R1–R3), and the\n8\ncorresponding explanation has now been added to the revised Supplementary\nInformation to improve clarity and credibility.\nWe thank the reviewer for the constructive assessment and thoughtful feedback, which\nhave helped us strengthen the presentation and interpretation of our results while clearly\narticulating the scope and limitations of the current study.\nCorresponding change in manuscript: Yes\nLocation of Change:\nSection: Methods\nPage: Page 17, Paragraph 2, Line 7-8.\nSection: AFET characteristics\nPage: Page 6, Paragraph 1, Line 6-14; Page 7, Figure 2b-d\nSection: Supplementary Information\nPage: Page 5, Paragraph 4, Line 2-5;\nPage 20, Paragraph 2, Line 2-11.\nReferences\nR1 Pradhan, N. R. et al. Phase modulators based on high mobility ambipolar ReSe\n2\nfield-effect transistors. Sci. Rep. 8, 12745 (2018).\nR2 Pack, J. et al. Charge-transfer contacts for the measurement of correlated states\nin high-mobility WSe . Nat. Nanotechnol. 14, 948-954 (2024).\nWaals heterostructure device platform. Nat. Nanotechnol. 10, 534-540 (2015).\n9"}]

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