Numerical and Experimental Study on the Device Geometry Dependence of Performance of Heterjunction Phototransistors

Heterojunction phototransistors (HPTs) with scaling emitters have a higher optical gain compared to HPTs with normal emitters. However, to quantitatively describe the relationship between the emitter-absorber area ratio (A_\rm e/A_\rm a) and the performance of HPTs, and to find the optimum value of A_\rm e/A_\rm a for the geometric structure design, we develop an analytical model for the optical gain of HPTs. Moreover, five devices with different A_\rm e/A_\rm a are fabricated to verify the numerical analysis result. As is expected, the measurement result is in good agreement with the analysis model, both of them confirmed that devices with a smaller A_\rm e/A_\rm a exhibit higher optical gain. The device with area ratio of 0.0625 has the highest optical gain, which is two orders of magnitude larger than that of the device with area ratio of 1 at 3 V. However, the dark current of the device with the area ratio of 0.0625 is forty times higher than that of the device with the area ratio of 1. By calculating the signal-to-noise ratios (SNRs) of the devices, the optimal value of A_\rm e/A_\rm a can be obtained to be 0.16. The device with the area ratio of 0.16 has the maximum SNR. This result can be used for future design principles for high performance HPTs.