The paper provides upper and lower bounds on the base transit time of a bipolar transistor which hold regardless of the shape of the base doping profile. These bounds apply, for the case of conventional bipolar transistors, when both the diffusion coefficient of the mobile carriers and the effective bandgap narrowing depend on doping density. They would also apply to composition grading in heterojunction bipolar transistors.

Additional Metadata
Journal International Journal of Electronics
Winterton, S.S., & Tarr, N.G. (1996). Bounds on bipolar transistor base transit time. International Journal of Electronics, 81(6), 643–646.