Microstructure, electrical properties, and failure prediction in low clamping voltage zinc oxide varistors

Abstract

Preparation and properties are described for ZnO varistors which clamp at sufficiently low voltages (≊10 V) for overvoltage protection of integrated circuits. The desired electrical characteristics have been obtained by microstructure control using alumina as a ZnO grain growth enhancer. Pulse degradation studies on these low voltage varistor compositions suggest that degradation occurs by premature failure of weak depletion layers and excessive current concentration at microstructural flaws. Capacitance–voltage measurements can be employed to detect these artifacts and predict the possibility of varistor failure during high current surges. A nondestructive scheme for varistor evaluation is outlined which utilizes the rate of change of capacitance and dissipation factor with dc bias voltage both above and below the varistor breakdown voltage.