Conduction mechanisms and temperature dependence of the electroluminescence in ZnO varistors

The electroluminescence of ZnO varistors have been measured at various current levels and temperatures between 20K and 300K. Their dependence on photon energy, temperature, voltage and current indicates that, initially, the holes are generated by impact ionisation in the reverse biased grain. Moreover, the electroluminescence has been always found to start from a barrier height value lower than the gap energy. This result is attributed to the presence of deep levels in the depletion region near the grain boundaries.