This paper reports on P-n-p HBT’s with an /max of 39 GHz, and an ft of 19 GHz. Power-added efficiency of 31% was obtained in an amplifier at 10 GHz. Design of the high-speed AIGaAs / GaAs P-n-p HBT's takes account of the large degeneracy in the heavily n-type GaAs base. This doping-dependent degeneracy can induce gradients in the valence-band edge to improve the base transit time. High injection efficiency can be maintained in spite of the large degeneracy by increasing the aluminum content of the emitter. HBT’s with emitter aluminum contents of 40% and 75% are described.