The University of British Columbia

An attempt is made to reconcile the various approaches that have recently been used to estimate the maximum frequency of oscillation fmax in high-performance AlGaAs/GaAs HBTs. fmax 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 fmax 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 fmax, involving f T and effective base resistance and collector capacitance, is remarkably accurate for frequencies below those at which transit-time effects become important