Physical properties of the electrical barriers in varistors

The nonlinear relationship between current and voltage in the zinc oxide varistor arises from electrical barriers at the grain boundaries in the polycrystalline ceramic. At very low fields, thermionic emission accounts for the majority of the current, with Schottky emission becoming more important as the field increases and assists in lowering the barrier. Tunneling through the barrier dominates in the presence of high fields, giving the highly nonlinear behavior. The bismuth (oxide) which segregates at the grain boundries occurs in two forms: an adsorbed layer present on the interfaces between grains, and a discrete phase which is continuous along grain edges. The latter is in the form of triangular prisms which exhibit a dihedral angle of about 60° as they approach a ZnO–ZnO interface, and thus this phase does not completely wet the ZnO grains. The adsorbed layer can account for the electrical barrier. Its chemical thickness is less than 50 Å as determined by Auger electron analysis. A depletion layer is also present in a manner analogous to a Schottky barrier, giving an electrical thickness of 200 to 400 Å. The capacitance, and its dependence on voltage, is in agreement with this model.