Zinc oxide based varistors: A possible mechanism

We propose a theoretical explanation of the highly nonlinear current-voltage (I–V) characteristics of ZnO based varistor ceramics, with maximum values of $\text{α =}\text{dlogI}\text{dlogV}$ of over 70 at T = 300 K. Starting with symmetrical Schottky barriers at the junctions between ZnO grains, we show that the presence of deep interface states with a very slow recombination can lead to a control mechanism for the V-dependence of the barrier height that is essential for the occurence of very high α values. The model accounts for the observed voltage and temperature dependence of α and I, and for the capacitance of ZnO varistors.