The process-related surface state effect is investigated the fabrication of SiC devices, and a nonlinear model for 4H-SiC power metal-semiconductor field effect transistor (MESFET) is proposed, which takes into account the surfacerelated parameters. The frequency- and temperature- dependent transconductance dispersion is readily demonstrated in terms of the improved model. Simulation results show that larger dispersion and higher transition frequency occur in 4H-SiC MESFET than in GaAs MESFET. The advantage of this analytical model over the two-dimensional numerical simulation is the simplicity of calculations, therefore it is suitable for the processing improvement of SiC devices.