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A new model for selectively doped heterostructure two-dimensional electron gas (2DEG) FET's has been proposed. In order to take into account the strong field dependence of the 2DEG mobility, a parabolic approximation is employed for a velocity-field curve below a velocity saturation field. The nonlinear field dependence of parasitic resistances has also been considered, which is of great importance for a more accurate description of actual FET characteristics. The proposed FET model is very useful for a digital IC design, since it has fewer fitting parameters and gives a smooth fit to measured data. Good agreement between the calculated drain current-voltage characteristics and the experimental characteristics, both for short-gate FET's and for long-gate FET's, demonstrates the validity of the present model. In addition, it has been recently found from the analysis that a transconductance compression is possible caused by a current limitation, due to hot electrons in the source-to-gate region, even though the n-(AlGa)As layer is totally depleted.