The barrier height formation in ZnO varistors

Abstract

The electrical characteristics of ZnO ceramic varistors are interpreted as being due to the existence of potential barriers at the grain boundaries. The two main models describing the origin of these barriers, surface states at the interface, and surface oxidation, are discussed in this work, where the microstructure and the electrical properties at various temperatures (up to 160 °C) of a standard varistor composition, are used in an attempt to separate both contributions. The temperature dependence of both the distribution of surface states and the Fermi level, when extrapolated to high temperatures, indicates that the surface states control mainly the shape of the I–V curves, and that the surface oxidation contribution to the barrier, although as high as 0.25 eV, is not sufficient to account for the total energy of the barrier. Values for the density of carriers calculated using the dependence of the Fermi level on temperature, were in good agreement with reported values determined by other techniques.