4H-SiC depletion mode metal oxide semiconductor field effect transistors (MOSFETs) have been fabricated and characterized. The device has a structure similar to that of a metal semiconductor FET (MESFET) where a physical conducting channel already exists. A MOS gate is used to replace the Schottky gate and this allows the device to operate in depletion mode as well as accumulation mode. The oxide layer was formed using thermal oxidation without any nitridation. The average electron mobility of the accumulation layer has been previously deduced to be about 17.5 cm(2)/V s. In this work, we further analyzed the DC current-voltage characteristics of the devices and deduced the electron mobility of the accumulation layer as a function of the gate voltage (V-Gs). A model was developed from which the mobility of the bulk channel layer was estimated as a function of V-Gs. The doping density in the channel layer was also deduced and the result is consistent with that obtained from secondary ions mass spectroscopy. (c) 2007 Published by Elsevier Ltd.