Doping profile optimization in silicon permeable base transistors for high-frequency, high-voltage operation

The effects of variations in the vertical doping profiles of etched-emitter Si PBTs (permeable base transistors) on f/sub t/ and V/sub B/ have been investigated. CANDE, a two-dimensional simulation program, has been used to determine f/sub T/ as a function of V/sub CE/ and V/sub B/ for a number of profiles. A highly nonuniform doping profile (high-doped emitter, low-doped collector) results in a device with a higher V/sub B/ than a uniformly doped device for doping levels at which the maximum f/sub T/'s are identical. The range of V/sub CE/ over which f/sub T/ remains high is extended for the nonuniformly doped PBT, whereas the uniformly doped device shows a slow degradation from its maximum with increasing V/sub CE/. The enhancement in f/sub T/ and V/sub B/ observed for the nonuniformly doped case should make the device very useful in large-signal operation, particularly in class A. Experimental devices with both nonuniform and uniform doping profiles have been fabricated. The dependence of f/sub T/ on V/sub CE/ and VB are consistent with the model presented. Despite processing limitations which currently limit f/sub T/'s to 60% of their theoretical value and V/sub B/'s to 80% of their theoretical value, nonuniformly doped Si PBTs with f/sub T/=22 GHz at V/sub CE/=15 V and f/sub T/=12 GHz at C/sub CE/=26 V have been fabricated.<
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