At low temperatures the thermal energy kBT is insufficient for thermionic emission to occur and the electron transport through the oxide takes place by quantum mechanical tunnel effect. Considering the zero temperature case, if cL > c0, tunneling of electrons from the filled part of the metal conduction band which lies above the O- level takes place (Fig. 1.3). If the metal Fermi level EF and the O- level of adsorbed oxygen are equalized, the current drops to zero and the system is in equilibrium with the potential VM existing across the film. The tunnel electron current during the establishment of this potential