The theory of operation of the heterojunction phototransistor is reviewed and the limitations on gain and speed-of-response are examined in the context of fiber-optic systems requirements. The response of the base potential is shown to depend on the input optical power yielding a power dependent gain-bandwidth product, fT. Model calculations assuming an optimized device structure with peak response in the 1. μm - 1.55 μm spectral region are used to demonstrate the consequences of this dependence. The results indicate that the HPT can have sufficient gain and speed-of-response if a dc bias current is used. A comparison of the S/N of the HPT and an APD is presented to clarify system applicability.
R. A. Milano,
P. D. Dapkus,
G. E. Stillman,
"Heterojunction Phototransistors For Fiber-Optic Communications", Proc. SPIE 0272, High Speed Photodetectors, (28 July 1981); doi: 10.1117/12.965691; https://doi.org/10.1117/12.965691