A number of configurations can be used to form p-n junctions in semiconductors, by applying an electrical potential difference to them under conditions where dopants, native or foreign, show ionic conductivity, In all these cases the electrical potential difference guides the dopants to create compositional inhomogeneities. This can lead to actual type conversion and thus to p-n junction formation, Junction formation can be localized on a scale that is small compared to the sample size, by working under conditions far from equilibrium and by limiting thermal effects.