N2O processes at temperatures over 1200degreesC are shown to substantially reduce 4H-SiC MOS interface states, and quadruple the effective surface channel mobility of inversion-mode 4H-SiC MOSFETs. Accumulation-mode 4H-SiC MOSFETs exhibit mobilities as high as 99 cm(2)/V-s for normally-off devices and 240 cm(2)/V-s for normally-on devices.