An analysis of the performance of heterojunction phototransistors for fiber optic communications

The theory of operation of the heterojunction phototransistor (HPT) 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, and this dependence results in a power-dependent gain-bandwidth product, fT. Model calculations assuming optimized device structures suitable for multimode and single-mode systems operating in the 1.3-1.55-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 for particular applications if a dc bias (optical or electrical) is used. The noise sources, , and sensitivity of the optimized devices are discussed to clarify system applicability.