The mobility of carriers in the channel of silicon carbide is significantly lower than in equivalent silicon devices. This results in a significant increase in on-state resistance in comparison to theoretical predictions and is hindering the uptake of silicon carbide technology in commercial circuits. The density of interface traps at the interface between silicon carbide and the dielectric film is higher and this is often considered to be the primary reason for the low mobility. In this work, we show that the mobility is dominated by the surface roughness of the silicon carbide, especially when the transistor is operating in the strong inversion regime, by careful examination of the characteristics of lateral transistors designed to form complimentary MOS functions.