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=√f_T/8π(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