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 Ge
_1-x
Sn
_x
alloy in the active layer to enhance the photodetection range. However, increasing Sn content in Ge
_1-x
Sn
_x
causes a lattice mismatch between Ge
_1-x
Sn
_x
and Ge virtual substrate (VS), which may increase defect density at the heterointerfaces. As a result, the diffusion length (L
_p
) and minority carrier lifetime (τ
_p
) 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 (f
_T
), maximum frequency (f
_max
), and sensitivity (S) of the GeSn HPT at 300 K.