To obtain an insight of the factors limiting the current gain, minority‐carrier lifetime (&tgr;n), base transport factor (&agr;T), and emitter injection efficiency (&ggr;) have been studied innpnAlGaAs/GaAs heterojunction bipolar transistors using a base width modulation technique. It is found that in addition to &ggr;, &agr;Tis also a function of current injection level. At low injection &agr;Tincreases at a fast rate with increasing injection. It is felt that, in addition to the surface recombination current at the periphery of the emitter‐base space‐charge region which affects &ggr;, surface recombination of the minority carriers in the base of a mesa device is also important as it greatly reduces &agr;Tand hence the current gain.