Performance optimization of PNP InAlAs/InGaAs HBTs

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 (~300 Å) 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-Å base doped uniformly at 5×10¹⁸ cm^-3, and the other with a 500-Å base with linearly graded doping. 5×10 μm² HBTs from the uniform-base wafer had a large-signal current gain of 12, an f_T of 11 GHz, and an f_max of 31 GHz. Similar HBTs from the graded-base wafer had a large-signal current gain of 4.2, an f_T of 13 GHz, and an f_max of 26 GHz. The graded doping in the base decreased τ_b by 25%; however, the resulting 85% increase in R_B caused a reduction of f_max. These results demonstrate the highest published f _max for InP-based PNP HBTs