@article { 4016626, title = {Reconciliation of methods for estimating fmax for microwave heterojunction transistors}, journal = {IEEE Trans. Electron Devices (USA)}, volume = {38}, number = {8}, year = {1991}, note = {semiconductors;maximum frequency of oscillation estimation;microwave heterojunction transistors;HBTs;Mason's invariant gain;y-parameters;hybrid-π equivalent circuit;T-equivalent circuit;drift-diffusion equations;analytical expression;fT;effective base resistance;collector capacitance;transit-time effects;AlGaAs-GaAs;}, pages = {1685 - 92}, type = {article}, abstract = {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-π 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 fT and effective base resistance and collector capacitance, is remarkably accurate for frequencies below those at which transit-time effects become important}, keywords = {aluminium compounds;equivalent circuits;gallium arsenide;heterojunction bipolar transistors;III-V semiconductors;semiconductor device models;solid-state microwave devices;}, URL = {http://dx.doi.org/10.1109/16.119002}, author = { Laser, A.P. and Pulfrey, D.L.} }