GaAs homojunction phototransistor with minority carrier transport assisted by photo-generated carrier profile in the base

Optical gain was observed in a structure that was unusual for transistors. Symmetric /spl pi/n/sup +/x layers on p/sup +/-substrate were formed using selective overcompensation by iron-diffusion. Devices of 100 /spl mu/m diameter showed asymmetric photocurrent with bias polarity: the gain of 10 is nearly constant when the hole injection is along the carrier profile built up by photoabsorption, while for the reverse injection a smaller gain decreases with incident optical power. A double heterojunction device with /spl pi/-AlGaAs layers also showed similar asymmetric photocurrent. Since the hole diffusion length is less than the base width, the photocarriers generated in the base region is believed to be responsible for the asymmetric gain.