The base transit time of Si/SiGe heterojunction bipolar transistor is analyzed including the effects of minority carrier recombination lifetime and velocity saturation. The reduction of recombination lifetime in the neutral base region increases the base transit time as compared to the infinite recombination lifetime, and the finite saturation velocity also degrades the base transit time, as compared to the infinite saturation velocity. This analytical analysis can obtain the optimum design of Ge profiles in the base to minimize the base transit time. The extremely heavy doping of the base can degrade the base transit time significantly if the base width is larger than the diffusion length. (C) 2003 Elsevier Ltd. All rights reserved.