An improved approach of electrical characterization of varistors

Varistors are one of the oldest technologically important electronic devices. These systems are generally produced by sintering ZnO powder together with small amounts of other oxide additives whose main component is bismuth oxide Bi₂O₃ and mostly used as protection against transients both in power transmission and in electronics. The DC current-voltage (I-V) characteristics of varistors are highly nonlinear and have three main distinguishable regions: linear (pre-breakdown) with very high resistance of the order of giga-ohm, highly nonlinear (breakdown) and again linear with resistance not exceeding tens of ohms. In the breakdown region, I-V characteristic is described by a traditional empirical power-law expression of the form: I = A V^β, where A is a constant whose value depends on the type of varistor and β is a nonlinearity exponent which measures the strength of nonlinearity of the I-V characteristic curve. β may have values ranging from 2 to few hundreds. In the literature, from the electrical point of view, varistors are frequently characterized by two main empiric parameters: the nonlinearity exponent β and an electric field EV determined at a constant current density j=1mAcm^-2. However, such description suffers from several drawbacks which would be discussed in details. We propose a new scaling formalism to analyze the highly nonlinear I-V data of various types of varistors. This method is based on the existence of a scaling function g(y), an onset field E₀ and the nonlinearity exponent x