Gain in electrophotography—I: Phototransistor configuration

The potential of the phototransistor or "hook" structure for realizing gain in the electrophotographic process is considered. By considering the phototransistor to be an emission-limited device, it can be shown that gain in excess of unity can only be produced in the external circuit. The charge necessary to produce the image is accumulated on an external capacitor whose size limits the gain to the ratio between the external and device capacitances. The predictions of this analysis are supported by experiments using a commercial phototransistor in simple circuits that simulate electrophotographic plates.