Anomalous current gain degradation in bipolar transistors

The current gain of a bipolar transistor, h/sub FE/, is degraded by the emitter-base reverse-bias stress. The mechanism has been interpreted to be due to an increase in the interface state density, while its dependence on stress time has been found to be proportional to t/sup n/, where n is 0.5 to 1. It was expected that if the stress time was long enough, the increase in interface state density would saturate to a maximum value. However, an anomalous time dependence of base current increase-a slower increase initially followed by a rapid increase in the intermediate stage-was found for bipolar transistors whose junction surface was oxidized in dry ambient conditions. This is explained by the hypothesis that the potential distribution is modulated by the charge in the oxide or at the interface. >