Zinc oxide, a wide-band-gap semiconductor with many technological applications, typically exhibits n-type conductivity. The cause of this conductivity has been widely debated. A first-principles investigation, based on density functional theory, produces strong evidence that hydrogen acts as a source of conductivity: it can incorporate in high concentrations and behaves as a shallow donor. This behavior is unexpected and very different from hydrogen's role in other semiconductors, in which it acts only as a compensating center and always counteracts the prevailing conductivity. These insights have important consequences for control and utilization of hydrogen in oxides in general.

We discuss commonalities and differences for point-defect formation in various semiconductors. Point defects do not act as sources of conductivity, and self-compensation is not necessarily more severe in wide-band-gap semiconductors than it is in Si or GaAs. Deviations from stoichiometry are discussed. Effects of lattice constant and size mismatch tend to be more pronounced than trends with ionicity or band gap.

We review a theoretical approach for studying defects and impurities in wide-bandgap semiconductors, focusing on mechanisms that limit doping. Among native defects, vacancies play a prominent role. They cause compensation, and also give rise to luminescence bands. The behavior of gallium and nitrogen vacancies in GaN is compared with cation and anion vacancies in other wide-band-gap semiconductors, including ZnO.