A closed-form analysis of f/sub T/ for the bipolar transistor down to liquid nitrogen temperature

A simple closed-form expression for f/sub T/ (cutoff frequency) down to liquid nitrogen temperature has been determined by examining the emitter-collector transit time, t/sub ec/, where f/sub T/=1/(2 pi t/sub ec/). This closed-form expression gives excellent agreement with measured values for polysilicon emitter n-p-n transistors which have been fabricated with Stanford's triple-diffused BiCMOS process. It is shown that the BJT (bipolar junction transistor) f/sub T/ behavior over temperature is primarily determined by only three terms: the base minority carrier storage time, the emitter minority carrier storage time, and the base-collector space-charge-layer storage time. The match of experimental data with the closed-form f/sub T/ expression shows that an increase in the base transit time in compensated BJT base regions at low temperature due to minority carrier trapping does occur and is the main reason for the degradation of f/sub T/ at temperatures lower than 150 K in the devices considered.<
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