Electrical Conduction of ZnO Varistors under Continuous DC Stress

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Abstract
The degradation mechanism of I-V curves of ZnO varistors is investigated by means of thermally-stimulated currents and photoconductivity measurements. We postulate that the degradation is due to electrons and holes accumulated across the grain boundary face to face with each other such as polarized charges in a condenser. According to the photoconductivity measurements, the degradation of I-V characteristics is found to be due to the increase in carrier density and field-dependent drift mobility. In addition, the varistor after dc stressing shows n-type conduction. The conduction mechanism for the initial I-V characteristics is also discussed. A model similar to the mobility-gap model is proposed, and it is suggested that the intrinsic conduction should be taken into account for the Schottky-type conduction mechanism.

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