Extrapolated fmax of heterojunction bipolar transistors

It is shown that the extrapolated f(max) of heterojunction bipolar transistors (HBT's) can be written in the form f(max) = root(f(T)/8 pi(RC)(eff)) ,where f(T) is the common-emitter, unity-current-gain frequency, and where (RC)(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(m) where g(m) is the transconductance, Simple expressions are derived for (RC)(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)(eff) can differ significantly from the effective base-resistance-collector-capacitance product conventionally assumed to determine f(max).