Impact of the emitter and base diffusion capacitances on the a.c. behavior of bipolar transistors

Abstract

The influence of stored minority carrier charge in the emitter and base regions of a bipolar transistor on the small signal a.c. behavior is studied using small signal a.c. numerical simulation. The validity of the quasi-static approach to correctly model the contribution of the emitter diffusion capacitance to the small signal a.c. response, particularly in polysilicon emitter bipolar transistors, is assessed. It is shown that the onset of non-quasi-static behavior in the emitter of polysilicon emitter bipolar transistors can greatly reduce the impact of the emitter diffusion capacitance on the frequency response of the transistor. An analytic expression is presented which can model the impact of non-quasi-static effects in the emitter on the a.c. common emitter current gain, and which involves only quantities that can be obtained from a d.c. analysis.