A New Technique for Determining Base Transit Time of a Bipolar Junction Transistor

A base transit time model for an npn bipolar junction transistor with exponential-doped base at high level of injection, which is applicable for all levels of injection before the onset of the Kirk effect, is developed. Based on the realistic assumption that a small change in electron concentration in the base at high injection occurs from its modified low injection model incorporating Webster effect, mathematical expressions for minority carrier concentration and current density have been derived. In this work, electric field dependence of mobility in addition to doping dependent mobility, bandgap-narrowing effect, high-injection effect and carrier velocity saturation at the base edge of the base-collector junction are incorporated. The base transit time is found to be different if the field dependent mobility is considered. The analytically calculated base transit time is found to be in good agreement with numerical results available in literature.