A mechanism of degradation in leakage currents through ZnO varistors

Abstract

Thermally stimulated currents through ZnO varistors subjected to the voltage stresses, dc, and square-wave voltages are analyzed on the base of carrier trappings in the depletion layers of ZnO grains. The analysis indicates that the field ionization of trapped carriers in a reverse-biased depletion layer plays an important role in the degradation phenomena, thereby suggesting that the current increase with time under the dc and ac stresses is due to the same origin. An Arrhenius relation to predict the life of varistors and a dependence of leakage current Jc on time t, ln Jc ∝t1/2, are derived from a carrier trapping model.