Base-transit-time model considering field dependent mobility for BJTs operating at high-level injection

An analytical model for the base transit time tau b for an exponentially doped base is developed assuming a small change in electron concentration in the base of a bipolar junction transistor at high injection from its low injection value. The model is valid in all levels of injection before the onset of the Kirk effect. In this analysis, bandgap-narrowing effect, high-injection effect, and carrier velocity saturation at the base edge of the base-collector junction, and also doping and field dependence of mobility, are incorporated. The base transit time calculated analytically is compared with simulation and numerical results, and also with experimental data in order to demonstrate the validity of the assumptions made in deriving the expression. The base transit time is found to be different if the field dependent mobility is considered