InGaAs/InP heterojunction bipolar transistors with a carbon-doped base are shown to have lower gain that those with a Be-doped base. In order to keep advantage of the low diffusivity of the carbon dopant, alternative carbon-doped materials have been tested as the base material. A quaternary InGaAsP alloy with a lower band gap than InGaAs to reduce Auger recombination proved to be unsatisfactory, because of low doping (<2 x 10(19) cm(-3)) and dramatically lower electron lifetime than in InGaAs. The compositionally graded base proved to be an attractive solution, allowing significantly improved gain in HBTs, and leading to some advantageous side effects, such as higher doping level, constant gain over large current range, and higher frequencies.