A theoretical treatment of a Schottky barrier dynamic response is developed on the basis of a general model of a semiconductor with thickness comparable in length to the space charge region width. It is shown that, when the space charge region approaches the metal/semiconductor interface, the electric field at this interface, induced by the charge accumulated on the metal, becomes significant with respect to the electric field induced by the charge accumulated on the semiconductor. Under this condition, the total capacitance of the Schottky barrier becomes independent of the polarization potential and tends to the value ε/L, like in a pure dielectric insulator. The term thin film is intended to be with respect to the screening length, which is a function of the volumetric charge density. In amorphous materials the transition potential at which the semiconducting to insulating behaviour is observed is dependent on the frequency. An approximated analytical solution for the capacitance of the junction is calculated. The model for finite thickness semiconductors is successfully applied to the study of anodic Nb2O5, formed in phosphate buffer 0.5 M aqueous solution up to different formation potentials (namely 5, 10 and 20 V vs. Ag/AgCl). The finite thickness semiconductor model permits extrapolation to a general behaviour of the oxide in a wide range of frequencies, potentials and thicknesses, and identification of the electron transfer between adsorbed surface species and the conduction band of Nb2O5 at potentials near to the flat band value.