Reconciliation of methods for estimating fmax for microwave heterojunction transistors

TitleReconciliation of methods for estimating fmax for microwave heterojunction transistors
Publication TypeJournal Article
Year of Publication1991
AuthorsLASER, A. P., and D. L. Pulfrey
JournalElectron Devices, IEEE Transactions on
Pagination1685 -1692
Date Publishedaug.
KeywordsAlGaAs-GaAs, aluminium compounds, analytical expression, collector capacitance, drift-diffusion equations, effective base resistance, equivalent circuit, equivalent circuits, fT, gallium arsenide, HBTs, heterojunction bipolar transistors, hybrid- pi, III-V semiconductors, Mason's invariant gain, maximum frequency of oscillation estimation, microwave heterojunction transistors, semiconductor device models, semiconductors, solid-state microwave devices, T-equivalent circuit, transit-time effects, y-parameters

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


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