The capacitance of ultrathin-oxide (3.420.9 nm) large-area square metal-oxide-semiconductor field-effect-transistor devices has been measured at temperatures ranging from 4.2 to 300 K and for a wide range of gate voltages and substrate voltages. The contribution of the inversion layer to the capacitance is as large as 10%. The spatial extent of the lowest-lying inversion-layer subband wave function from the interface, the average inversion-layer thickness, can be determined from these measurements. The results are in semiquantitative agreement with calculations. The carriers are pushed toward the interface with an increase in the carrier density and/or the application of a negative substrate voltage. At elevated temperatures excitation of electrons to higher-lying subbands increases the average inversion-layer thickness and results in a decrease in the measured capacitance. © 1988 The American Physical Society.