The understanding of the electronic process taking place in the energy conversion devices based on nanodimensioned electrolyte-metal-oxide-semiconductor contacts requires a familiarization with basic aspects of the thermodynamics and electronic of the formed contact. The ¢rst is necessary to determine the energetic frame established upon electronic equilibrium among the different phases constituting the contact. The conversion process involves essentially the trapping of minority charge carriers generated upon illumination of the semiconductor. These reach charge carriers by diffusion, the limits of the electric ¢eld created at the semiconductor-oxidemetal contact and are further transported to the metal-electrolyte interface through the stabilizing ¢lm (oxide). Maximal conversion ef¢ciencies require the selection of materials and processes that achieve an optimal semiconductor band bending (electric ¢eld) and an ef¢cient electron transfer at the different interfaces and through the oxide. The ¢nal step, the electron transfer at the metal-electrolyte interface, depends on the electrocatalytic properties of the metal islands and will be treated in Chapter 4.