ASYMMETRIC PHOTOCURRENT CHARACTERISTICS IN GAAS ALGAAS PHOTOTRANSISTORS WITH VERY SHORT CARRIER-DIFFUSION LENGTHS/

Asymmetric optical gain was observed in a structure that was unusual f or transistors. Symmetric p(-)/n(+)/p(-) layers on p(+)- substrate wer e formed using selective overcompensation of n(-)-GaAs and n(-)-AlGaAs by diffusion of deep accepters of iron. Both GaAs homojunction and Al GaAs/GaAs double-heterojunction (DH) devices demonstrated asymmetric p hotocurrent with bias polarity; the current is larger and constant whe n the hole injection is along the carrier profile in the base built up by photoabsorption, while for the reverse injection, the current is s maller and increases with bias voltage. A gain of 10 was observed for the larger photocurrents of homojunction devices. Since the hole diffu sion length is greatly reduced due to recombination centers of iron, a nd the absorption length is comparable to the base width of 0.8 mu m, the photocarrier profiles built up in the base region are believed to be responsible for the asymmetric gain. Possible rapid response is als o indicated.