Conduction mechanisms in ZnO varistors

Abstract
The thermionic emission enhanced by barrier lowering and the Zener tunnelling in ZnO varistors are discussed. Capacitance versus voltage, reverse current versus temperature, and 1/f noise measurements have been performed on commercial varistors. A model is proposed from which follows a diffusion voltage VD which is consistent both with the 1/C2 vs V plot and with the thermal activation energy obtained from the reverse current versus 1/T. The empirical law β=d log V/dlog I= constant, is explained by Zener breakdown. The 1/f current noise is proportional to the square of the current and independent of the conduction mechanism across reversely biased barriers.

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