Barrier voltage and its effect on stability of ZnO varistor

Abstract

The major voltage drop in ZnO varistors occurs across the grain boundaries, which behave generally as Schottky barriers supporting a barrier voltage Vg. This voltage is proportional to the device voltage. Experimental evidence shows that there is a time-dependent reduction in the barrier voltage combined with an increase in resistive current iR when the varistor is subjected to a continuous ac voltage stress. The phenomena is reversed when the applied voltage is removed, showing nearly complete recovery. Instabilities of the resistive current and of the barrier voltage are shown to be manifestations of the same phenomenon and are attributed to a metastable component in the Schottky barriers. It is proposed that this metastable component is due to interstitial zinc ions that are capable of migration under thermal and electrical driving forces. When these ions are removed or stabilized by a suitable heat treatment, the instability of the device is reduced. This paper presents experimental data and analysis to support this hypothesis.