Effect of Active Layer Scaling on the Performance of Ge 1– x Sn x Phototransistors

The impact of scaling of the base layer on the noise performance, spectral responsivity,and frequency response of Ge/Ge1-xSnx/Ge p-n-p heterojunction phototransistor (HPT) is presented and evaluated by simulation. The proposed structure consists of Ge1-xSnxalloy 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 (fmax) 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 fT> 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.