Two-dimensional effects in the bipolar polysilicon self-aligned transistor

There has been a great deal of interest recently in the performance enhancements afforded by self-aligned silicon transistors. Concomitantly, it has been observed that the current gain and collector drive capability of these devices sometimes exhibit a strong dependence an the geometrical size and shape of the emitter, In this study, it has been shown that this two-dimensional effect is explicable in terms of a parasitic p-n diode in parallel with the emitter-base junction. An accurate model has been developed to predict the geometry-dependent current gain under low to moderate bias. It is further shown how the model can be extended to the high-bias regime. Lastly, it is shown, by means of an emitter-base debiasing model, that the geometry of the emitter contact has a more significant effect on the emitter-base debiasing than does the intrinsic base sheet resistance itself.