Effects of the field-dependence of the carrier mobility on the base transit time of SiGe HBTs having non-uniformly doped base

This work develops an analytical model of the base transit time for the SiGe-heterojunction bipolar transistors (HBT) having non-uniformly and heavily doped base considering the field-dependence of the carrier mobility under moderate-level injection. The proposed model considers all the non-ideal effects found in the literature owing to the non-uniformity and the heavy levels of the base doping. The evolving mathematical intractability has been resolved by applying an exponential approximation technique and the concept of the perturbation theory. Simulation results of the proposed model for the exponentially doped base with three types of Ge-dosing profile (box, trapezoidal and triangular) show that the effects of the field-dependent carrier mobility on the base transit time is significant.