Numerical and Experimental Study on the Device Geometry Dependence of Performance of Heterjunction Phototransistors**Supported by the National Natural Science Foundation of China under Grant Nos 11574362, 61210014, 11374340 and 11474205, the Innovative Clean-Energy Research and Application Program of Beijing Municipal Science and Technology Commission under Grant No Z151100003515001, and the National Key Technology R&D Program of China under Grant No

Abstract

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 (Ae/Aa) and the performance of HPTs, and to find the optimum value of Ae/Aa for the geometric structure design, we develop an analytical model for the optical gain of HPTs. Moreover, five devices with different Ae/Aa 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 Ae/Aa 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 Ae/Aa 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.