MODELING THE INFLUENCE OF HIGH CURRENTS ON THE CUTOFF FREQUENCY IN SISIGE/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 t he base-collector region of Si/SiGe/Si and SiC/Si heterojunction bipol ar transistors (HBTs) with wide collectors, to study the variation of the cutoff frequency f(T) with collector current density J(C). Our res ults show that while the presence of strain enhances the electron mobi lity, the scattering from alloy disorder and from ionized impurities r educes the electron mobility so much that it is less than that of Si a t the same doping level, leading to larger base transit times tau(B) a nd hence poorer f(T) performance for large Je for an Si/SiGe/Si HBT th an for an SiC/Si HBT. At high values of J(C), we demonstrate the forma tion of a parasitic electron barrier at the base-collector interface w hich 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 mechan ism for base pushout after barrier formation that differs somewhat fro m that given for the analytical model.