The metal-insulating semiconductor (MIS) Cu/n-GaAs diodes with thin anodic-insulating layer, which is formed by anodic oxidization on the n-GaAs substrate in aqueous 4C(2)H(6)O(2) + 2H(2)O + 0.1H(3)PO(4) electrolyte with pH = 2.02; anodically untreated control Cu/n-GaAs diodes; and anodically treated Cu/nGaAs diodes (several steps of anodization in the same electrolyte followed by a dip in diluted aqueous HCl solution and a subsequent rinse in deionized water) have been prepared. The anodization has increased the barrier heights as well as the ideality factors. We have obtained barrier heights of approximately 0.68 eV, 0.90 eV, and 0.92 eV for the control sample, anodically treated sample, and MIS sample, respectively, adding the contribution caused by image-force effect only. Thus, the barrier height has been increased by at least 140 meV. Furthermore, we have calculated a mean tunneling-barrier height of X = 0.025 eV for the MIS Cu/n-GaAs Schottky barrier diode (SBD).