Device physics analysis of parasitic conduction band barrier formation in SiGe HBTs

This paper presents a physics-based model describing the current-induced formation of a parasitic barrier in the conduction band at the base-collector heterojunction in n-p-n SiGe heterojunction bipolar transistors (HBTs). Due to the valence band discontinuity /spl Delta/E/sub v/, hole injection into the collector at the onset of base pushout is impeded, which gives rise to the dynamic formation of a barrier to electron transport which degrades the device's performance at high frequencies. In this paper, we present results from an analytical model for the height of the barrier calculated from the device's structure as a function of the collector junction bias and collector current density. The effects of barrier formation on electron buildup at the collector end of the quasi-neutral base are also described.