Base capacitor coupled photosensor with emitter tunnel oxide for very wide dynamic range in a contactless imaging array

摘要

A technique for decreasing the effective gain of a bipolar phototransistor at high light levels makes the image usable over a greatly extended range of illumination conditions. The effective current gain at high light levels is reduced by fabricating a "non-ideal" emitter, such as by inserting a thin 20 521 tunnel oxide between the emitter and base junction. The tunnel oxide between the emitter and base serves as a variable resistor as well as a good junction for carrier injection from the emitter. The total base voltage is the sum of the oxide voltage and the intrinsic base voltage. At high image intensity, the bipolar phototransistor will gradually enter into the saturation mode, i.e., the base to collector junction is forward biased. The beta is thus reduced. The bias of the collector should be about 0.3-0.8 V higher than the emitter at the 20 ANGSTROM tunnel oxide thickness for optimum operation.