%0 Journal Article %J IEEE Trans. Electron Devices (USA) %D 1999 %T Extrapolated fmax of heterojunction bipolar transistors %A Vaidyanathan, M %A Pulfrey, D L %K extrapolation;heterojunction bipolar transistors;parameter estimation;semiconductor device models; %P 301 - 9 %U http://dx.doi.org/10.1109/16.740894 %V 46 %X It is shown that the extrapolated fmax of heterojunction bipolar transistors (HBT's) can be written in the form fmax=√fT/8π(RC)eff, where fT 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/gm, where gm 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 fmax %Z extrapolated fmax;heterojunction bipolar transistors;common-emitter unity-current-gain frequency;general time constant;base resistance;collector-base junction capacitance;parasitic emitter resistance;parasitic collector resistance;dynamic resistance;effective base-resistance-collector-capacitance product; %9 article