To obtain an insight of the factors limiting the current gain, minority-carrier lifetime (τn), base transport factor (αT), and emitter injection efficiency (γ) have been studied in npn AlGaAs/GaAs heterojunction bipolar transistors using a base width modulation technique. It is found that in addition to γ, αT is also a function of current injection level. At low injection αT increases 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 γ, surface recombination of the minority carriers in the base of a mesa device is also important as it greatly reduces αT and hence the current gain.