Recently, microwave performance has been reported for PNP InAlAshGaAs HBTs [l]. Although some simulations have been performed for the optimization of GaAs-based PNP HaTs [2], little has been reported on the optimization of PNP HBTs in the InP material system.In ithis work, various layer structures for InAlAshGaAs PNP HBTs were simulated using a 2dimensional drift-diffusion simulator in order to determine the effect of the emitter-base junction design, the base thickness, the base doping, and the collector thickness on both DC and microwave performance. The results show that the most significant performance improvement can1 be obtained through a thin base (-300 A) with low base doping and a built-in drift electric field to accelerate the holes toward the collector. Two wafers of InAlAshGaAs PNP HBTs were fabricated from MBE-grown epilayers, one with a 500-A base doped uniformly at 5~1 0 '~ ~m -~, and the other with a 500-A base with linearly graded doping. 5x10 pm2 HBTs from the uniform-base wafer had a large-signal current gain of 12, an fT of 11 GHz, and an f , , , of 3 1 GHz. Similar HBTs from the graded-base wafer had a large-signal current gain of 4.2, an f~ of 13 GHz, and anf,,, of 26 GHz. The graded doping in the base decreased zb by 25%; however, the resulting 85% increase in RB caused a reduction off,,,.These results demonstrate the highest publishedf,, for InP-based PNP HBTs.