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 (/spl sim/300 /spl Aring/) 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-/spl Aring/ base doped uniformly at 5/spl times/10/sup 18/ cm/sup -3/, and the other with a 500-/spl Aring/ base with linearly graded doping. 5/spl times/10 /spl mu/m/sup 2/ HBTs from the uniform-base wafer had a large-signal current gain of 12, an f/sub T/ of 11 GHz, and an f/sub max/ of 31 GHz. Similar HBTs from the graded-base wafer had a large-signal current gain of 4.2, an f/sub T/ of 13 GHz, and an f/sub max/ of 26 GHz. The graded doping in the base decreased /spl tau//sub b/ by 25%; however, the resulting 85% increase in R/sub B/ caused a reduction of f/sub max/. These results demonstrate the highest published f/sub max/ for InP-based PNP HBTs.