Establishment of Conduction Mechanism for ZnO Varistors Based on Solid State Chemistry

The tunneling effect model between Schottky-type energy barriers, currently the best-known conduction mechanism for ZnO varistors, is based on the assumption that energy barriers of uniform height exist at the interface inside the ZnO grains. This conduction model does not adequately reflect the compositional heterogeneity and microstructural complexity of actual varistor devices. Therefore, considering newly obtained analytical data on various phenomena, we discuss the “composite barrier model” based on tunneling effects in grain boundary insulating layers with scattered “holes” where ZnO grains come into contact with each other. By calculating each element in the microstructure (thickness of the thin grain boundary layer between ZnO grains, size and occurrence density of “holes” in the grain boundary layer, etc.) and correlating them with electron microscopy observation data and E-J characteristics, the remarkable temperature dependence of E-J characteristics in ZnO varistors can be quantitatively understood. This result cannot be satisfied by conventional models.