The high frequency performance of type-II InP∕GaAsSb double heterojunction bipolar transistors (DHBTs) with compositionally graded and uniform 25nm base layers and 65nm InP collectors is investigated. The graded base DHBT achieves a peak current gain of 20.5 compared to 14.9 for the uniform base DHBT at current densities near 15mA∕μm2. The peak fT of a graded base device with 0.38×8μm2 emitter dimensions improves from 505GHz at 25°C to 535GHz at −55°C as determined by −20dB∕decade extrapolation. The base grading has reduced the total transit time by 25% compared to the uniform base type-II DHBT.
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Snodgrass et al. (2006) studied this question.
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