Unified minority-carrier transport equation for polysilicon or heteromaterial emitter contact bipolar transistors

An analytical minority-carrier transport equation is derived that unifies the various models for polysilicon-emitter contact bipolar transistors. The theory covers drift and diffusion in the arbitrary doped crystal-silicon-emitter region, recombination and tunneling at the polysilicon-silicon interface and the potential barrier associated with heteromaterial, and diffusion in the polysilicon emitter region. The ratio of recombination current at the interface and diffusion current in the polysilicon to the total hole current is formulated using this theory. The influence of the two-dimensional structure of the oxide at the polysilicon-silicon interface on current gain is also analyzed. The theory is applied to the heterojunction bipolar transistor (HBT), and it is found that the recombination velocity at the heteromaterial-silicon interface should be suppressed to less than 10⁴ cm/s