Optical characteristics of InGaP/GaAs HPTs

We examined the optical characteristics of InGaP/GaAs heterojunction phototransistor (HPT) directly compared with AlGaAs/GaAs HPT for the first time. Because of its inherent good electrical properties, the InGaP/GaAs HPT produced a high optical gain of about 61 at V/sub C/=3 V, I/sub B/=2 μA, for an input optical power of 1.23 μW. This is 2.5 times as high as that of the AlGaAs/GaAs HPT. In the transient response, the InGaP/GaAs HPT was a little inferior to the AlGaAs/GaAs HPT. This is due to the longer time delay caused by the photo-generated hole accumulation at the interface of heterojunction. The extended response time can be overcome by using a small load resistance in conjunction with the advantage of the superior optical gain.