Effect of Active Layer Scaling on the Performance of Ge1–xSnx Phototransistors

The impact of scaling of the base layer on the noise performance, spectral responsivity,and frequency response of Ge/Ge
_1-x
Sn
_x
/Ge p-n-p heterojunction phototransistor (HPT) is presented and evaluated by simulation. The proposed structure consists of Ge
_1-x
Sn
_x
alloy in the base layer, allowing extension of the photodetection to the mid-infrared (MIR) region, enabling photodetection over a wide range. This paper also includes the effect of Sn concentration and base bias voltage on the frequency performance of the HPT. In addition, various noise components and signal-to-noise ratio (SNR) are estimated as a function of base layer thickness and base resistance. The simulated results show that cutoff frequency (fT) and maximum frequency (f
_max
) are not only strongly dependent on the base layer thickness but also on the Sn composition in the base layer and applied base-bias voltage. The results show that the proposed HPT provides higher f
_T
> 45 GHz and SNR of >70 dB for the base layer thickness of 50 nm (with Sn = 9%) can be achieved. Therefore, the GeSn p-n-p HPT is promising for future fiber-optic telecommunication and MIR applications.