The common emitter current gain of InP/Ga0.47In0.53As heterojunction bipolar transitors having a base doping level of 7 X 10(19) cm-3 is found to increase monotonically with decreasing base thickness in the range of 200-1000 angstrom. The variation of the gain with base thickness W(B) is proportional to 1/W(B)2, as expected for diffusive base transport, and a high injection efficiency.