Velocity‐field curves forn‐type silicon on sapphire have been measured at fields up to 60 kV/cm using pulsedI‐Vtechniques and specially designed test structures. Successive etching was used to determine the dependence of drift velocity on distance from the silicon‐sapphire interface. Within experimental error, the drift velocity of electrons is found to saturate at the same value as in bulk silicon, although the field required to achieve saturation is greater than in bulk Si and increases with decreasing Ohmic mobility near the interface. Sample heating can have a large effect on the experiment. The results, which are important for the design of high‐speed SOSFET’s are explained using simple hot‐electron physics.