Design and Analysis of Ge/Ge1-xSnx/Ge Heterojunction Phototransistor for MIR Wavelength Biological Applications

This work presents the influence of lateral scaling such as emitter layer thickness and base layer thickness on the responsivity spectra and frequency response of Ge/Ge
_1-x
Sn
_x
/Ge p-n-p heterojunction phototransistor (HPT), a generalized detector for biological sensing and defense applications. The proposed structure contains Ge
_1-x
Sn
_x
alloy in the base layer, allowing extension of the photodetection to the mid-infrared (MIR) applications. The study takes account of the effect of base-emitter voltage and Sn concentration on the frequency performance of the HPT. In addition, we have also estimated gain as a function of lateral scaling of the HPT. The calculated results show that cutoff frequency (f
_T
)and maximum frequency (f
_max
) are not only strongly dependent on the lateral scaling of HPT but also on the Sn concentration and applied base-emitter voltage. Furthermore, gain is less dependent on the emitter layer thickness, however, it is strongly dependent on the base layer thickness. With the GeSn base (with Sn = 9%), fT > 65 GHz for the optimized emitter and base layer thickness of 100 nm and 50 nm, respectively, can be achieved compared with that of InGaAs based HPT. The proposed GeSn HPT exhibits the high spectral responsivity >100 A/W at 1550 nm and >32 A/W at 2100 nm and response time ~2.41 ps. Therefore, the proposed HPT may open up a potential candidate for mid-infrared photodetection in bio-sensing applications.