Base transit time of shallow-base bipolar transistors considering velocity saturation at base-collector junction

The authors studied the influence of the velocity saturation in the base-collector depletion layer and compared the injected electron concentration profile, collector current density, and base transit time with velocity saturation to those without. The collector current with velocity saturation is only a little smaller than the current without saturation, but the injection electron concentration profile changes substantially when comparing currents with and without velocity saturation. The base transit time is increased by velocity saturation, and the ratio of base transit time with velocity saturation to that without velocity saturation increases as the base width decreases. Thus, velocity saturation must be considered in order to evaluate the base transit time of shallow-base bipolar transistors. The dependence of the base transit time on the doping profile was also analyzed, revealing that the base transit time of a box doping profile is increased more than that of a Gaussian doping profile