Base transit time of a bipolar transistor considering field dependent mobility

An empirical expression for the base transit time, τb , of a bipolar transistor with exponential base doping profile is derived considering doping and field dependence of mobility. In the analysis, bandgap-narrowing effect, high injection effect and carrier velocity saturation at the base edge of the collector-base junction are also incorporated. The collector current density, Jc , and minority carrier stored charge, Qb , in the base are separately expressed as a function of the injected electron density no in the base in order to find an empirical expression for τb . The modelling of Jc , Qb and τb is essential for the design of a modern bipolar transistor. The base transit time calculated analytically is compared with simulation and numerical results 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.