The heterostructure phototransistor (HPT) has applications as an optical detector/amplifier in receivers for fibre optic communications systems and analysis has shown that its potential performance may be equal to, or even superior to, that of the PIN/FET combination. 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 principally due to an increase in emitter efficiency and a reduction in the RC time constant of the emitter base junction. In this paper we show that crowding of the bias current and the signal current, due to base resistance, can be a serious problem and will limit the advantage given by the external bias in 3-terminal devices but, on the other hand, will improve the performance of 2-terminal devices. To illustrate this point, the current-crowding has been modelled for 2-terminal and 3-terminal phototransistors with cylindrical geometry (this shape being optimal for low leakage operation). The model is used to relate the gain-bandwidth product of an HPT to the electrical bias current and to the optical signal level.