Profile design considerations for minimizing base transit time in SiGe HBTs for all levels of injection before onset of Kirk effect

An iteration scheme to calculate the base transit time (/spl tau//sub b/) for a given collector current density is developed in order to determine the optimal doping profile and Ge profile in the neutral base for minimizing the /spl tau//sub b/ of SiGe HBTs under all levels of injection before the onset of the Kirk effect. We adopt a consistent set of SiGe transport parameters, tuned to measurement data, and include important effects such as the electric-field dependency of the diffusion coefficient and plasma-induced bandgap narrowing in our study. The scheme has been verified with simulation results reported in the literature. Our study shows that under both low and high injection, for a given Ge dose, intrinsic base resistance, and base concentration near the emitter, a retrograde doping profile with a trapezoidal Ge profile gives the minimum /spl tau//sub b/.