A new analytic formalism describing the frequency dependence of current gain and power gain of HBTs based on the T-Model equivalent circuit is presented. It is found that while extrapolation serves as a conservative benchmark for HBT high-speed device comparison, it often significantly underestimates the actual intercept frequency for fT, with an experimental example presented here with error of 50%. The frequency dependence of GMAX shows it to be unsuitable for extrapolation to find fMAX, while extrapolation of Unilateral Gain (U) will generally in the absence of resonances yield accurate values for fMAX. The conditions for resonance and resonance frequencies for U are analytically investigated here, and new analytic expressions are developed and experimentally demonstrated for highly accurate determination of the fT and fMAX intercept frequencies of HBTs.
