Recently, microwave performance has been reported for PNP InAlAs/InGaAs HBTs. Although some simulations have been performed for the optimization of GaAs-based PNP HBTs, little has been reported on the optimization of PNP HBTs in the InP material system. In this work, various layer structures for InAlAs/InGaAs PNP HBTs were simulated using a 2-dimensional 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 can be obtained through a thin base (approximately 300 angstroms) with low base doping and a built-in drift electric field to accelerate the holes toward the collector. Two wafers of InAlAs/InGaAs PNP HBTs were fabricated from MBE-grown epilayers, one with a 500-angstroms base doped uniformly at 5×1018 cm-3, and the other with a 500-angstroms base with linearly graded doping. 5×10 μm2 HBTs from the uniform-base wafer had a large-signal current gain of 12, an fT of 11 GHz, and an fmax of 31 GHz. Similar HBTs from the graded-base wafer had a large-signal current gain of 4.2, an fT of 13 GHz, and an fmax of 26 GHz. The graded doping in the base decreased τb by 25%; however, the resulting 85% increase in RB caused a reduction of fmax. These results demonstrate the highest published fmax for InP-based PNP HBTs.
