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 (τ_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 τ_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 τ_b