A simple iterative procedure for approximation of the germanium composition profile minimizing base transit time in an SiGe alloy base HBT is developed. Two variants of the procedure are presented: one applicable when the germanium concentration is specified at the edges of the base, and the other when the total germanium content in the base is given. In the latter case the optimum Ge profile has a smaller slope near the emitter and a larger slope near the collector than does a linear ramp. In the former case the optimum profile approximates a linear ramp over the central portion of the base, but becomes retrograde near the collector.

Solid State Electronics
Department of Electronics

Winterton, S.S., Peters, C.J., & Tarr, N.G. (1993). Composition grading for base transit time minimization in SiGe-base heterojunction bipolar transistors. Solid State Electronics, 36(8), 1161–1164. doi:10.1016/0038-1101(93)90197-X