The gate capacitance of a nearly pinched‐off semiconducting channel located between gate and substrate depletion layers is derived for the high‐frequency regime in which the substrate charge does not change with ac channel charge. The distance corresponding to the inverse gate capacitance exceeds the gate depletion layer width, and theR‐Ctime constant per unit channel length approaches the inverse of the product of channel carrier mobility and voltage equivalent of temperature. OrdinaryC‐Vanalysis at high frequency provides an artifactitious impurity profile which is fairly independent of the doping concentration and of the substrate and which varies in proportion to absolute temperature. Reconstruction of the true impurity profile from this artifactitious profile is discussed.