Gain and frequency response of a graded-base heterojunction bipolar phototransistor

A study has been made of the carrier transport and optical gain of a heterojunction bipolar phototransistor in the presence of a drift field in the base region. The computational method employed is based on transfer matrices for minority-carrier density in the base region in the presence of ionizing radiation. Scattering matrices for the electron and hole currents are used for computing the avalanche multiplication in the collector junction. The effect of varying the ratio of absorption in the base region to that in the collector has been examined. The expressions derived have been used to estimate the frequency response of the bipolar heterojunction phototransistor.