Switching properties of polysilicon emitter transistor operating in saturation

Saturation behaviour of a polysilicon emitter transistor is studied using the analysis of the most fundamental parameter, the saturation time constant, and correlated with the physical structure. The analysis takes into account the effect of the polysilicon/monosilicon interface in the emitter and n/n ⁺ type transition interface due to be buried layer diffusion in the collector. The effect of the polysilicon/monosilicon interface becomes crucial to the transistor behaviour at smaller emitter junction depths. The interface contributes to a significant amount of charge storage in the emitter region and is independent of charge storage in the collector region bounded by the n/n⁺ interface recombination velocity. The increase in saturation time constant because of charge storage in the emitter and collector regions should be taken into consideration in the design of BiCMOS VLSI circuits