New analytic determination of f/sub T/, f/sub MAX/ and the frequency dependence of current gain and power gain in HBTs

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 f/sub T/, with an experimental example presented here with error of 50%. The frequency dependence of G/sub MAX/ shows it to be unsuitable for extrapolation to find f/sub MAX/, while extrapolation of Unilateral Gain (U) will generally in the absence of resonances yield accurate values for f/sub MAX/. 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 f/sub T/ and f/sub MAX/ intercept frequencies of HBTs.<>