Ultrasensitive Photodetection and Metal‐Semiconductor Transition in Bi₂S₃ Phototransistor

Abstract Bismuth sulfide (Bi₂S₃) single crystal shows a great advantage of the direct band gap, high photo‐absorption, and non‐toxic elements. The ideal prerequisites for ultrasensitive photodetector is low noise and high gain, which are however not simultaneously present in existing Bi₂S₃ photodetectors. Herein, a three‐terminal phototransistor based on Bi₂S₃ flakes, achieving both low noise and high photo‐gain for ultrasensitive photodetection capability is developed. By applying negative gate voltage, the Bi₂S₃ channel is depleted, leading to a low noise spectral density of 3.3 × 10⁻¹² A Hz^−1/2. Leveraging the photo‐gating effect, a high photo‐gain of ≈10⁸ is also yielded. These two factors have led to a high responsivity of 3 × 10⁸ A/W and high detectivity up to 1.6 × 10¹⁵ Jones at 635 nm, which have surpassed that in commercial and newly emerging photodetectors. Through the temperature‐dependent electrical transport and photo‐response measurement, the phototransistor demonstrates an interesting metal‐semiconductor transition due to the strong electron‐electron interactions by applying positive gate voltage for high levels of electron doping. A suppressed hysteresis and persistent photoconductivity are observed at low temperatures, suggestive of the existence of trap states. This work develops the Bi₂S₃ photo‐transistor with ultrasensitive photodetection and metal‐semiconductor transition, providing a promising system for strong correlation effects and photodetector applications.