Fabrication and characteristics of Al/sub x/Ga/sub 1-x/As heterojunction phototransistors with wide-gap window

The fabrication process and device characteristics of Al/sub 0.7/Ga/sub 0.3/As/Al/sub 0.2/Ga/sub 0.8/As heterojunction phototransistors are described. The interface between the emitter and base layers becomes rough with the increase in the Al content of the emitter layer, and it causes a decrease in the optical conversion gain of the heterojunction phototransistors. A spacer layer with the same Al content as that in the emitter layer is inserted between the emitter and base layers. Both the interface morphology and the optical conversion gain are improved. Enhanced by the avalanche multiplication in the base-collector junction, an optical conversion gain of 1500 is achieved in spite of the shorter diffusion length in the Al/sub 0.2/Ga/sub 0.8/As base layer. The device exhibits a photoresponse in the range of 520-780 nm, i.e., from green to red light.<
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