Current crowding effects in GaAs/AlGaAs heterojunction phototransistors

The heterojunction phototransistor (HPT) has applications as an optical detector/amplifier in receivers for fibre optic communications systems. It is well known that the provision of an external bias, either optically or electrically, via a third (base) terminal, can enhance the performance of the HPT in terms of gain, speed of response and signal to noise ratio. This is due principally to an increase in emitter efficiency and a reduction in the RC time constant of the emitter base junction. In the paper we show that crowding of the bias current and the signal current can affect very significantly the high frequency performance of phototransistors and will limit the advantage given by the external bias in three-terminal devices but improve the performance of two-terminal devices. The implications for the design of high speed phototransistors are discussed.