Effect of Defects on the Performance of Si-Based GeSn/Ge Mid-Infrared Phototransistors

In this paper, we discuss the impact of interfaces and defects on the frequency performance of Si-based GeSn heterojunction phototransistors (HPTs) at room temperature. Furthermore, the effect of base-emitter (B-E) voltage on the spectral responsivity (SR) and detectivity (D*) has also been studied. The presented device consists of Ge1-xSnxalloy in the active layer to enhance the photodetection range. However, increasing Sn content in Ge1-xSnxcauses a lattice mismatch between Ge1-xSnxand Ge virtual substrate (VS), which may increase defect density at the heterointerfaces. As a result, the diffusion length (Lp) and minority carrier lifetime (τp) may decrease, thereby, degrading the performance of the device. Three major noise components such as shot noise current components at the base-emitter (B-E) and base-collector (B-C) junctions and Johnson noise have been considered for the first time to calculate the detectivity and noise-equivalent-power (NEP) of the presented device. The calculated results show that the decrease in diffusion length does not cause a significant impact on the SR, D* and internal quantum efficiency (IQE), however, it significantly degraded the cut-off frequency (fT), maximum frequency (fmax), and sensitivity (S) of the GeSn HPT at 300 K.