NUS Engineers ReSe2 Avalanche Photodetector with Record Performance
In a groundbreaking achievement for Singapore's higher education landscape, researchers at the National University of Singapore (NUS) have developed an innovative avalanche field-effect transistor (AFET) using rhenium diselenide (ReSe₂), a two-dimensional (2D) material. Published in Nature Communications on February 26, 2026, this work demonstrates a photodetector with an exceptionally low breakdown electric field of 2.55 kV/cm and a high ionization index of 38.79, setting new benchmarks for ultrasensitive light detection.
The device, known as a ReSe₂ avalanche phototransistor, achieves a responsivity of 1.71 × 10⁴ A/W and an avalanche gain of 173 under 658 nm illumination. This positions NUS at the forefront of 2D materials research, aligning with Singapore's push for advanced photonics and semiconductor technologies through initiatives like the Research, Innovation and Enterprise 2025 plan.
Understanding Avalanche Field-Effect Transistors and ReSe₂
An avalanche field-effect transistor (AFET) is a specialized semiconductor device that amplifies photocurrent through impact ionization, where high-energy carriers generate additional electron-hole pairs under a strong electric field. Traditional avalanche photodiodes (APDs) suffer from high breakdown fields (often >10 kV/cm) and low ionization ratios (μ ≈ 1-10), leading to excess noise and high power consumption.
ReSe₂, a transition metal dichalcogenide (TMDC) with a distorted 1T' phase, offers anisotropy—different properties along in-plane (a/b axes) and out-of-plane (c-axis) directions. Density functional theory (DFT) calculations reveal electron effective masses of 0.63 mₑ along a/b axes and 2.43 mₑ along c-axis, minimizing interlayer scattering.
- Anisotropy advantage: Reduces phonon scattering, enabling lower breakdown fields.
- High-k dielectric: HfZrO₂ (HZO) gate insulator enhances electrostatic control, filling trap states and modulating carrier density.
This combination allows operation at room temperature with minimal noise, ideal for integrated photonics.
The NUS Research Team Behind the Innovation
Led by Associate Professor Kah-Wee Ang from NUS's Department of Electrical and Computer Engineering, the team includes PhD candidates Jiaona Zhang (first author), Jinyong Wang, Maksim Andreev, Samarth Jain, and Haofei Zheng. Collaborators from NUS Materials Science and Engineering (Ahmet Enes Bozcali, Ahmet Avsar) contributed mobility analysis, while international partners from CUHK Shenzhen and HKUST provided simulations.
Ang's group specializes in 2D semiconductors for optoelectronics, building on NUS's Centre for Advanced 2D Materials (CA2DM). This publication underscores NUS's role in Singapore's ecosystem, fostering talent for faculty positions in emerging tech.
Key Technical Breakthroughs: Low Breakdown and High Ionization
The ReSe₂ AFET's breakdown voltage decreases with negative gate bias (V_GS), reaching a minimum at -2.5 V due to optimal carrier modulation. The breakdown field E_B = 2.55 kV/cm is over 4x lower than typical 2D APDs (e.g., WSe₂ ~10 kV/cm).
Ionization index μ, defined as the ratio of electron-to-hole impact ionization rates, peaks at 38.79—enabling electron-initiated, low-noise multiplication. Confirmed by temperature-dependent measurements (positive coefficient 1.80 × 10⁻³ °C⁻¹) and McIntyre's model fits.
| Metric | ReSe₂ AFET (NUS) | Typical 2D APDs |
|---|---|---|
| Breakdown Field (kV/cm) | 2.55 | >10 |
| Ionization Index μ | 38.79 | 1-10 |
| Responsivity (A/W) | 1.71e4 | 10²-10³ |
| Gain | 173 | 10-100 |
TCAD simulations validate reduced scattering probability.
Photo by GEE MENG WAH on Unsplash
Fabrication Process: Step-by-Step Innovation
- Substrate prep: Heavily p-doped Si with 30 nm HfZrO₂ via atomic layer deposition (ALD).
- ReSe₂ transfer: CVD-grown flakes mechanically exfoliated and aligned.
- Contacts: Au/Ti source/drain, gate on top.
- Encapsulation: hBN for protection.
- Testing: Four-terminal for accurate mobility, photocurrent under low power (57-285 pW).
Decay time: 40 μs, improving with gate tuning. Scalable for integration.
Performance Metrics and Comparisons
Under avalanche bias (>V_B), photocurrent amplifies via unipolar electron multiplication. Compared to MoS₂/WSe₂ APDs, ReSe₂ offers 10x higher gain-bandwidth potential due to low noise (high μ).
- 3 dB bandwidth potential: >10 GHz (estimated from low capacitance).
- Detectivity: High due to low dark current.
Outperforms bulk Si/GaAs APDs in noise figure, suitable for telecom (1.55 μm potential via bandgap tuning).Read the full paper.
Applications in Photonics and Beyond
This innovation enables low-voltage, high-gain photodetectors for:
- Optical communications: Data centers, 6G with >100 GHz bandwidth.
- Imaging/sensing: LIDAR, biomedical, quantum key distribution.
- Nanoelectronics: Integrated circuits with on-chip amplification.
In Singapore, aligns with photonics hub ambitions, complementing NTU/NUS 2D efforts.Explore photonics career advice.
NUS CA2DM overviewSingapore's Leadership in 2D Materials Research
NUS's CA2DM pioneers graphene/TMDCs, with ReSe₂ advancing APDs. Singapore invests S$37B in RIE2030 for quantum/photonics. This bolsters NUS rankings (QS #8 globally), attracting global talent for Singapore university jobs.
Impacts higher ed: Spurs PhD programs, industry ties (e.g., GlobalFoundries).
Photo by Roaming Pictures on Unsplash
Future Outlook and Challenges
Scaling to wafers, mid-IR extension (ReSe₂ bandgap ~1.2-1.5 eV), heterostructures for broadband. Challenges: Uniform CVD growth, noise optimization.
NUS eyes commercialization via startups. Aspiring researchers: Pursue postdoc opportunities in 2D optoelectronics.
Career Opportunities in Singapore Photonics
This breakthrough highlights demand for experts in 2D semiconductors. NUS/NTU offer faculty, research assistant roles. Rate professors like Ang for insights. Explore higher ed jobs, career advice.
- PhD/Postdoc: Quantum photonics.
- Industry: TSMC, Micron Singapore.
