Recent semiconductor devices require both introduction of many materials and aggressive scaling. As a result, present semiconductor devices contain various nano-scale interfaces, and a lot of unusual interface phenomena that cannot be explained by ordinary concepts of interfaces have been observed. In this paper, we have pointed out that ordinary concepts of interfaces cannot be applied directly to these interfaces, and propose two new concepts of “Generalize charge neutrality level (GCNL)” and “Fixing of Fermi level based on the generation of oxygen vacancies by interfacial reactions”.