Studies on degradation mechanism of ZnO varistor under impulse stress by thermally stimulated current

The degradation mechanism of a ZnO varistor under impulse current stress has been studied. It is found that the impulse degradation, like DC degradation, arises from the variation of double Schottky barriers at the grain boundary which has been investigated by thermally stimulated current (TSC). It is concluded that the differing change of Schottky barriers is due to the high concentration of carriers injected into the grain boundary layer and accumulation of trapping effects in the layer, ultimately resulting in the change of Fermi levels