For the first time, the effect of base thickness and parasitic collector resistance on InP-based PNP HBTs was determined experimentally. HBTs with 350-angstroms and 900-angstroms base layers and self-aligned collector contacts demonstrated DC gain of 21.3 and 5.9, fT of 18.6 and 8.5 GHz, and fmax of 27.3 and 20.8 GHz, respectively. This is the highest fT reported for any InP-based PNP HBT. Analysis of these HBTs demonstrated that recombination of holes in the neutral base limited the DC gain, and hole transit across the base was the most significant component of τec. In addition, comparison of HBTs with and without self-aligned collectors demonstrated that collector series resistance had a minor but noticeable impact on fT as well as the gain and power-added efficiency at 8 GHz.