Prebreakdown conduction in zinc oxide varistors: Thermionic or tunnel currents and one-step or two-step conduction processes

This letter is aimed at understanding the intergranular conduction in zinc oxide varistors. In particular, we have studied current-voltage and Arrhenius plots of the resistive current for the prebreakdown conduction. A current-temperature (I-T) relation has been found to present two breaks at ∼370 and ∼240 K. Although this result can be interpreted as a consequence of distinct regimes, we found that experimental I-T curves can be theoretically reproduced by calculating currents for a double Schottky barrier. Experimental results were fitted with one- and two-step transport process models. It was found that a two-step model better reproduces the electrical properties of these devices.