Metal‐oxide‐semiconductor field‐effect transistors with very thin (100–400 A˚) gate oxides are fabricated. With improved procedures for extracting the various physical parameters from the capacitance‐voltage curves and carefully controlled experiments, it is confirmed that the fixed oxide charge density increases inversely with the oxide thickness. The surface mobilities at both room temperature and 77 K are also characterized. It is found that the mobility in general decreases as the oxide thickness is reduced. The mobility results are interpreted in terms of the coulomb and surface roughness scattering. A plausible model explaining the correlation of oxide thickness, growth condition, and the above physical parameters is also proposed.