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.