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 _T and the breakdown voltage V_B for a variety of doping profiles. These range from a uniform profile (4×10¹⁶ cm^-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_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_B than uniform doping profiles for a given f_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-Ω-cm) epitaxial layer. Although present processing technology limits the f_T and V _B to 60% and 80% of the simulated values, respectively, an f_T>20 GHz at a base-to-collector bias of 16 V and an f_T of 12 GHz at a base-to-collector bias of 26 V have been obtained