The authors report the simultaneous achievement of record current gain cutoff frequency f/sub /spl tau// and high power gain cutoff frequency f/sub max/ in a transferred-substrate HBT. The device parasitics are examined, and inferences made about the relative value of various approaches toward further improvements in f/sub /spl tau// and f/sub max/. Transferred-substrate HBTs with record f/sub /spl tau// of 254 GHz are reported.
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Mensa et al. (1999) studied this question.