The multiple-layer epitaxial wafer is widely adopted to promote the performance of 4H-SiC MESFET in microwave applications, in which the p-type buffer layer underlying the n-type active layer affects both the DC and the RF output characteristics significantly. An equivalent parameter is proposed in this paper to describe the effect of doping and thickness of the p-buffer layer on the pinch off voltage and the drain conductance of 4H-SiC MESFET. Also the trapping effects at the channel/buffer interface and in the buffer layer are discussed qualitatively. The simulations of the improved I-V model are compared with the measurements from the 4H-SiC MESFETs on semi-insulating and conductive substrates and good agreements are obtained. (C) 2004 Elsevier Ltd. All rights reserved.