In this work we present a comparison between the field-effect (FE) and conductivity (inv) mobilities calculated from ID-VG measurements on a 4H-SiC MOSFET. A compact device model is used to determine inv. The conductivity mobility is found to be larger than FE near room temperature, but less than FE at 500K. These results are due to a reduction in charge trapping at higher temperatures. In strong inversion, inv decreases markedly with increasing temperature. Modeling indicates that surface phonon scattering dominates in this regime.