A connection has been established between device processing and field-effect mobility in 4H-SiC MOSFETs. Processing conditions were investigated that reproducibly and predictably yielded mobilities above 40 cm(2)/Vs or below 10 cm(2)/Vs. A relationship between experimental field-effect mobility and average drift mobility in SIC inversion layers is presented. It is shown that high held-effect mobility is obtained when the interface state density is decreased to a value below the inversion-layer capacitance at the same Fermi energy.