The heterojunction phototransistor(HPT)is a new type of optoelectronic device which has got rapidly development recent years.It can be used as a good detector in a longer wavelength range competing with Ge and Ⅲ-Ⅴ compound APD.Furthermore

there are several special functions

such as optical bistability

switching and amplification

by means of combination or integration of HPT and other semiconductor devices.For instance

DH laser etc.The operating mechanism and some properties of HPT are introduced in this paper.It is shown that HPT's can provide a much larger optical gain without the high bias voltage necessary as APD and without excess nois due to avalanching effects.This results from its heterostructure emitter and."window" effect.The interesting characteristics and some recent progress in GaAlAs/GaAs and InGaAsP/InP HPT are reviewed.It is believed that the characteristics of HPT could be improved by improving design and fabrication technique of devices.