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_t and V_B have been investigated. CANDE, a two-dimensional simulation program, has been used to determine f_T as a function of V_CE and V_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_B than a uniformly doped device for doping levels at which the maximum f_T´s are identical. The range of V_CE over which f_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_CE. The enhancement in f_T and V_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_T on V_CE and VB are consistent with the model presented. Despite processing limitations which currently limit f _T´s to 60% of their theoretical value and V_B´s to 80% of their theoretical value, nonuniformly doped Si PBTs with f_T=22 GHz at V_CE =15 V and f_T=12 GHz at C_CE=26 V have been fabricated