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This investigation offers a technique to predict the ac behavior of mm wavelength GaAs metal semiconductor field effect transistors by using dc characteristics. To predict the intrinsic equivalent circuit parameters of the device from dc data, the measured dc characteristics are first simulated by employing a nonlinear dc model. The effects of biasing on the device ac parameters are evaluated for its low-noise applications. An improvement greater than 10% in predicting the ac response of the device is observed. The concept of depletion layer modification caused by the transverse electric field inside the channel is introduced for accurate Miller's capacitor modeling. It is assumed that with increased device biasing there are more unbalanced positive charges in the gate depletion toward the drain-side of the Schottky barrier. The electric field lines originated by these uncompensated charges induce an opposite charge density in the gate electrode. This modifies the gate biasing and hence the Schottky barrier depletion. As a result, the values of intrinsic ac device parameters change. It is observed that an accurate dc modeling is key to predicting an accurate ac small signal equivalent circuit of a device.