High gain, broadband p-WSe₂/n-Ge van der Waals heterojunction phototransistor with a Schottky barrier collector

Mixed-dimensional van der Waals (vdW) heterostructures based on two-dimensional transition metal dichalcogenides and three-dimensional semiconductors have led to a new era in next-generation optoelectronics due to the high-quality interfaces and energy band complementation, especially in broadband photodetectors which can be used for all-weather navigation, object identification, etc. However, the reported photodetectors conventionally operated in photodiode mode with low responsivity and a narrow response spectrum. In this study, we report a p-WSe₂/n-Ge vdW heterojunction phototransistor with a Schottky barrier collector on n-Ge for broadband photodetection. Large hole/electron injection ratio from p-WSe₂/n-Ge heterojunction under forward bias due to their large bandgap offset renders the high photocurrent gain, while the Ge Schottky barrier limits the dark current. The responsivities of the phototransistor at 1.0 V emitter-collector bias are 55, 95, and 120 A·W⁻¹ at 405, 1,310, and 1,550 nm, respectively, which is superior to that of the corresponding p-WSe₂/n-Ge photodiodes. The phototransistor shows a high photocurrent gain of 80, a specific detectivity of 10¹¹ Jones, as well as a fast response time of 290 μs at 1,550 nm. The results suggest that the novel phototransistor being implemented with complementary metal-oxide-semiconductor processing is an ideal strategy for high-performance broadband photodetection.