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An improved large-signal modeling approach of GaN on Si devices for RF high-power applications is presented. This approach accounts for the parasitic buffer loading effect under microwave RF operation in addition to the self-heating and trap ping effects associated with high-power operation conditions. A hybrid optimization (genetic and Simplex) technique based procedure is used to determine the optimal values of the model extrinsic elements. These elements are de-embedded from multibias S-parameter measurements to find the intrinsic part of the device, which is then used to construct a nonlinear current-charge (represented by nonlinear charge and current sources) based model for the gate current of the device. Pulsed and static dc IV characteristics are used for modeling of the drain current. The validity of the developed modeling approach is verified by comparing simulated large-signal single- and two-tone simulation with measured data of a 2-mm (10 × 200 μm) GaN HEMT on Si substrate. The model has been employed for designing a class-AB power amplifier. Very good agreement between the amplifier simulation and measurement shows the validity of the model.