3-D effects in emitter-base junctions in advanced silicon bipolar transistors

The authors report experimental results on double-polysilicon self-aligned n-p-n transistors that clearly indicate that the emitter-base reverse characteristics are inherently limited by three-dimensional effects in the corners of the emitter, and that two-dimensional considerations fail to explain many results for a large variety of experimental conditions. Without any exception it is found that the emitter-base reverse characteristics of narrow emitters (14*0.8 mu m/sup 2/) shown higher reverse leakage currents than large emitters (14*10 mu m/sup 2/), even when compared on an absolute scale. In addition it appears that a large walled emitter (i.e. with two sides overlapping the field oxide) tends to show far better reverse characteristics than a large nonwalled emitter with a common rectangular geometry. The difference in leakage current at a given reverse bias can be as high as two orders of magnitude. The results obtained can be explained qualitatively in terms of the three-dimensional concentration profiles of the boron under the oxide sidewall spacer in the corner of an emitter.