Investigation of GaAs MESFET Small Signal Equivalent Circuits for use in a Cell Library

More than 20 different linear equivalent circuits can be found in the literature for modeling GaAs MESFETs. Five models with 7 to 13 elements were selected and investigated for use in a cell library. S-parameters were measured on wafer (Cascade) up to 20 GHz for 1 and 0.5 ¿m GaAs MESFETs. The element values for each of the five equivalent circuits were optimised by a least-squares fitting of the S-parameters calculated from the equivalent circuit model to those measured. The influence of the number of elements and of the topology on the accuracy of the models was determined. The requirement to give a fit within the estimated measurement errors was fulfilled by the models with 13 elements. One of these was selected for use in the cell library. Large uncertainties (e. g. 100 % for Rg, Ri, Rs, Rd, Ls, Ld) were found in the element values. Thereby the relatively small errors associated with S-parameter measurements on wafer were considered for the first time. Measures to reduce these uncertainties in parameter extraction are discussed.