An analytical approach to the modelling of intrinsic base sheet resistance in a SiGe HBT and optimal profile design considerations for its minimization

An analytical model has been developed to calculate the intrinsic base sheet resistance (R_bi), for a given Ge dose and base dopant, with a view to determine the optimal doping profile and Ge profile in the neutral base of a SiGe-base HBT. We have derived an empirical formula for the diffusion constant of holes in a p-type SiGe as a spatial function of mole fraction of Ge as well as base doping concentration. Our model has been verified with experimental and simulated results published in the literature. Our study shows that for a given Ge dose and amount of base dopant, a triangular Ge profile with a retrograde dopant profile yields the lowest R_bi.