Optimization of the doping profile in Si permeable base transistors for high-frequency, high-frequency, high-voltage operation

The effects of variations in doping profiles along the direction of current flow in Si permeable base transistors have been investigated. Numerical simulations of Poisson's equation and the electron-current-continuity equation have been used to determine the fundamental tradeoff between the unity-current-gain frequency f/sub T/ and the breakdown voltage V/sub B/ for a variety of doping profiles. These range from a uniform profile (4*10/sup 16/ cm/sup -3/) to a highly nonuniform profile in which the doping in the emitter region is greater than 100 times that in the collector region. Although f/sub T/ decreases significantly with increasing collector-to-emitter voltage for the uniformly doped case, it is nearly independent of collector-to-emitter voltage for the nonuniform doping profile. In addition, nonuniform doping profiles produce devices with higher V/sub B/ than uniform doping profiles for a given f/sub T/. A class A power analysis performed using simulated current-voltage characteristics showed that output power, power-added efficiency, and large-signal gain can be increased with devices having nonuniform doping profiles. Experimental devices with nonuniform vertical doping profiles have been fabricated using high-energy (300-400 keV) P implantation into a high-resistivity (4- Omega -cm) epitaxial layer. Although present processing technology limits the f/sub T/ and V/sub B/ to 60% and 80% of the simulated values, respectively, an f/sub T/>20 GHz at a base-to-collector bias of 16 V and an f/sub T/ of 12 GHz at a base-to-collector bias of 26 V have been obtained.<
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