In this work, a short-wave infrared (SWIR) n-MoSe₂/p-GeSn/n-germanium-on-insulator (GOI) heterojunction phototransistor (HPT) with Sn composition-graded GeSn base is proposed for improvement of overall performance at low cost. The Sn composition-graded GeSn base layers are grown using magnetron sputtering epitaxy technique for improvement of crystal quality with a high Sn content of 15.2% in the top layer, rendering the extension of the cutoff wavelength beyond 2400 nm and significant suppression of dark current. The enormous electron/hole injection ratio, resulting from the large bandgap offset between the MoSe₂ emitter and the GeSn base, enables the harvesting of a high photocurrent gain of HPT. By optimizing the device parameters, a considerable responsivity of 23.79 A/W and an excellent specific detectivity of 8.24 × 10¹⁰ Jones at the peak wavelength of 2030 nm were achieved for the HPT with the dark current density of 261 mA/cm² under the emitter-collector bias voltage of 1.0 V at room temperature. The fast response speed is obtained for the HPT in terms of rising/falling times of 2.8 μs/9.3 μs at 1550 nm, surpassing those of most van der Waals (vdW) junction-based devices. Those results demonstrate that GeSn HPTs are suitable for SWIR optoelectronic imaging and microwave photonics applications.

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.