Analytical base transit time of integrated bipolar transistors in quasi-saturation and hard saturation

Based on the assumption of negligible recombination within the thin epitaxial collector layer of an integrated bipolar transistor switch in quasi-saturation, solutions to the collector minority carrier profile and transit time in the induced base are derived. In contrast to Dai and Yuan´s analysis (1997), the present analysis takes both the drift and diffusion currents into account and is valid for all levels of injection. Dependence of transit time on characteristics of the epitaxial-substrate interface and recombination at the interface is studied for the transistor driven into hard saturation. At high effective surface recombination velocity, recombination at the interface cannot be neglected. The study shows that transit time increases more rapidly with collector current when the transistor operates in hard saturation and the interface is highly reflecting