ABSTRACTThe paper resumes essential grain boundary phenomena which were found experimentally by microcontact measurements on adjacent grains. The results suggest a ZnO homojunction model. Calculations of defect equilibria in the Schottky defect model show that homojunctions can be formed during the cooling period under the influence of an extrinsic donor species by a shift of the bulk defect equilibrium and intersecting diffusion profiles of oxygen- and zinc vacancies near the grain boundary.Recent experiments indicate that heterogeneous Bi2O3-segregation layers with a dissolved zinc-antimony-oxide compound are transformed to ZnO epitaxial layers plus a fine grained pyrochlore phase of zinc-bismuth-antimony oxide during the cooling period. Thus epitaxial ZnO layers eliminate thin Bi2O3-films and form homogeneous connections of adjacent grains.