The saturation behavior of the polysilicon emitter transistor has been studied through the analysis of a single most fundamental parameter, the saturation time constant, and correlated with the physical structure. It is shown that the current transport in the polysilicon emitter transistor is significantly affected by the polysilicon-monosilicon interface of the emitter and n-n/sup +/ interface of the collector region in comparison to the conventional transistor structure. The effect of the polysilicon-monosilicon interface becomes crucial for the transistor behavior at shallower emitter junction depths. The interface leads to a significant amount of charge storage in the emitter region and is apart from charge storage in the collector region bounded by n-n/sup +/ interface recombination velocity. It is noted that the increase in saturation time constant due to charge storage in the emitter and collector region should be taken into consideration in the design of BiCMOS VLSI circuits.
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