Extrapolated f/sub max/ of heterojunction bipolar transistors

It is shown that the extrapolated f/sub max/ of heterojunction bipolar transistors (HBT's) can be written in the form f/sub max/=/spl radic/f/sub T//8/spl pi/(RC)/sub eff/, where f/sub T/ is the common-emitter, unity-current-gain frequency, and where (RC)/sub eff/ is a general time constant that includes not only the effects of the base resistance and collector-base junction capacitance, but also the effects of the parasitic emitter and collector resistances, and the dynamic resistance 1/g/sub m/, where g/sub m/ is the transconductance. Simple expressions are derived for (RC)/sub eff/, and these are applied to two state-of-the-art devices recently reported in the literature. It is demonstrated that, in modern HBT's, (RC)/sub eff/ can differ significantly from the effective base-resistance-collector-capacitance product conventionally assumed to determine f/sub max/.