Preparation and characteristics of ZnO varistors

The nonohmic properties of ZnO varistors having additives of Bi, Sb, and several other three-valence metal oxides were investigated. It is determined that the very high nonlinearity of the electrical properties of these varistors is related to the additive content, sintering temperature, and ambient temperature. Results of scanning electron microscopy indicate that the characteristics of ZnO varistors can be attributed to the electrical barriers in the vicinity of the grain boundary within the ZnO grains, which are mostly found in direct contact with each other. The barrier height at zero bias was found to be 1.57 V and the donor concentration of ZnO grains was determined to equal 3 x 10 to the 18th/cu cm. The V-I characteristics in the prebreakdown region at very low voltage are shown to be temperature sensitive, which is consistent with thermionic emission. The breakdown field in the depletion layer is found to be above 10 to the 6th V/cm. In addition, it is determined that tunneling through the barrier dominates in the breakdown regions and can adequately account for values of alpha in excess of 60.