Hole-spreading effect on upward current gain in n-p-n transistors

The upward current gain and stored charge for future small-sized n-p-n transistors have been studied. Included is the effect of hole spreading from a narrow base region to a wide emitter region, whether buried layer or substrate. When base dimensions are comparable with or shorter than the hole diffusion length in the emitter, the hole current is concentrated near the fringe of the base so that the base current tends to be proportional to the base length instead of the base area. The same tendency appears regarding the stored charge in the emitter. Hence, the scaling down of lateral dimensions gives rise to a decline in the upward current gain and cutoff frequency. The calculations introduced here are in quite fair agreement with the observed base currents in sidewall base contact structure (SICOS) n-p-n transistors, where dimensions for the emitter-base junction are almost the same as for the collector-base junction.