The investigations on hydrogen in semiconductors have shown that this element could be inadvertently introduced in many materials and could modify their electrical properties by forming bonds with host atoms as well as with impurities and dopants. The presence of hydrogen is due to the fact that many precursors or forming gases used in epitaxial growth do contain hydrogen or that water vapour is present in some steps of the bulk crystal growth. The activity and introduction is due to the fact that isolated hydrogen has a donor level in the forbidden gap of many semiconductors and that it can

diffuse easily as a proton till trapped by an atom or a defect. Hydrogen can also be introduced deliberately in semiconductors by implantation, plasma exposure or by annealing in a hydrogen atmosphere, so that many quantitative investigations of its properties in semiconductors have been performed.