Opto-Electric Effects in Ge-GaAs p-n Heterojunctions

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Abstract
In an abrupt Ge‐GaAs p‐n heterojunction, a plot of conduction band edge vs position shows a discontinuity at the material interface. As a result, a ``notch'' occurs in the band‐edge profile which strongly affects the opto‐electric characteristics of such a junction. The conversion efficiency is much less than that of a homojunction of similar doping levels. At room temperature the photocurrent is limited by the rate at which electrons can be thermally excited from the notch into the GaAs. At 78°K this contribution to current is negligible. For reverse bias, the photocurrent results from electron tunneling from the notch into the GaAs. In forward bias, the space charge resulting from the presence of the optically excited electrons in the notch changes the potential profile in such a way so as to reduce the forward current, and to cut it off entirely for sufficiently high light intensity. Because of the low conversion efficiency, it is concluded that such a heterojunction collector in an optical transistor is not practical.