A new ac MODFET model including distributed effects is presented. We have solved the wave equation of the MODFET, which automatically accounts for the propagation delay, capacitances, and charging resistances. Using a frequency power series proposed by Ziel, we derive an approximate analytic expression for the four intrinsic Y parameters. We verify that the truncated frequency power expansion used is accurate for frequencies up to twice the unilateral power-gain cutoff-frequency. The Y parameters derived hold from the threshold region to the edge of saturation. The theory was used to reproduce the dc characteristics of a 1-μm GaAlAs/GaAs MODFET together with its microwave characteristics measured from 2 to 18.4 GHz. Device parameters so obtained by fitting the I-V characteristics were used to calculate the MODFET Y parameters. They yield a closer fit to the data than the Y parameters obtained from conventional equivalent-circuit models. A reasonable prediction of the scattering parameters measured from 2 to 18.4 GHz is achieved with those device parameters. Further improvements can be obtained by optimizing the parasitics.