Reconciliation of methods for estimating f/sub max/ for microwave heterojunction transistors

An attempt is made to reconcile the various approaches that have recently been used to estimate the maximum frequency of oscillation f/sub max/ in high-performance AlGaAs/GaAs HBTs. f/sub max/ is computed numerically from the full expression for Mason's invariant gain using y-parameters derived from the different approaches, i.e., the hybrid- pi equivalent circuit, the T-equivalent circuit, and the drift-diffusion equations. It is shown that the results for f/sub max/ are essentially the same, irrespective of the source of the y-parameters, provided that the phase delays due to transit of carriers across the base and the collector-base depletion region are properly accounted for. It is also shown, for the particular device studied, that the widely used analytical expression for f/sub max/, involving f/sub T/ and effective base resistance and collector capacitance, is remarkably accurate for frequencies below those at which transit-time effects become important.<
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