A generalized expression of current gain for nonuniformly doped bipolar transistor considering the effects of energy‐gap narrowing, doping‐dependent diffusivity, contact‐surface and bulk recombinations, is derived by using generalized current flow equations and continuity equations. Based on the derived expression, the generalized expressions of current gain and transit times across the neutral base and emitter regions in the case of uniformly doped transistor structure are calculated in detail. Numerical results have shown that the recombination in heavily doped emitter may not be neglected and the conventional charge control expression of current gain does give erroneous predictions. Moreover, the current gain and transit time for a transparent emitter bipolar transistor have been studied, which are shown to be quite sensitive to the contact‐surface recombination velocity, and are found to be useful in the study of microwave bipolar transistor.