Modelling the influence of high currents on the cutoff frequency in Si/SiGe/Si heterojunction transistors

A one-dimensional self-consistent bipolar Monte Carlo simulation code has been used to model carrier mobilities in strained doped SiGe and the base-collector region of Si/SiGe/Si and SiC/Si heterojunction bipolar transistors (HBTs) with wide collectors, to study the variation of the cutoff frequency f(T) with collector current density J(C). Our results show that while the presence of strain enhances the electron mobility, the scattering from alloy disorder and from ionized impurities reduces the electron mobility so much that it is less than that of Si at the same doping level, leading to larger base transit times tau(B) and hence poorer f(T) performance for large Je for an Si/SiGe/Si HBT than for an SiC/Si HBT. At high values of J(C), we demonstrate the formation of a parasitic electron barrier at the base-collector interface which causes a sharp increase in tau(B) and hence a dramatic reduction in f(T). Based on a comparison of the height of this parasitic barrier with estimates from an analytical model, we suggest a physical mechanism for base pushout after barrier formation that differs somewhat from that given for the analytical model.