The origin of the frequency-dependent Mott–Schottky behavior observed in a wide range of ZnO-Bi2O3 varistor systems has been investigated. Lumped parameter/complex plane analysis of two-probe ac electrical data indicates that several trapping relaxations contribute to the measured MOV grain-boundary admittance in the frequency range, 10−2 Hz≤f≤107 Hz. Furthermore, this approach allows the development of an equivalent circuit representation which incorporates these trapping phenomena in a systematic manner.

Electrical transport in zinc oxide varistors is simulated using two-dimensional Voronoi networks. The networks are assumed to contain randomly distributed grain boundaries of three electrical types: (1) high nonlinearity (i.e., ‘‘good’’) junctions; (2) poor nonlinearity (i.e., ‘‘bad’’) junctions; and (3) linear with low-resistivity (i.e., ohmic) junctions. These type classifications are those found in experimental measurements. By varying the type concentrations, the simulated current density versus electric field (J–E) characteristics can be made to conform to the different experimentally observed characteristics of ZnO varistors. These characteristics include the sharpness of switching at the transition between ohmic and nonlinear J–E response (i.e., knee region), as well as the degree of nonlinearity. It is shown that the reduction of the nonlinearity coefficient of bulk varistors, relative to that of isolated grain boundaries, can be explained only by the presence of ‘‘bad’’ varistor junctions.

In this paper a detailed nature of the electrical potential barriers in ZnO based varistors are presented. The excellent nonlinear current–voltage (I–V) characteristic of these varistors is attributed to the potential barriers formed between the successive ZnO grains during the processing cycle. These grain boundaries in the microstructure results in a few device‐related parameters such as built‐in‐potential, barrier height, barrier width, grain carrier density, Fermi level position in the grains, etc. Thermal activation energy is determined from the status of the leakage current of this device. Substantial studies conducted to comprehend possible conduction processes in the grain boundary regions under applied bias (electric field). It is understood that electron transport dominate the grain boundary regions based on electrical potential barrier consideration across the grain boundaries. The free electrons overcome potential barrier at elevated temperatures as well as under bias. These carriers fill trap sites (charged defect states) both in the bulk ZnO and in the grain surfaces (surface states). The formation of space charge region within the successive ZnO grains across the grain boundaries is a result of the equilibrium state of the device. Based on the findings an expression of the Schottky potential barrier width is proposed satisfying relevant expression of the current caused by the application of the electric field. It is observed that the potential barrier width is sharply reduced in the temperature range 320 K ≤ T ≤ 350 K. This reduction in the potential barrier width is likely to be associated with the relaxation process of the trap filled (trapped) carriers, usually monitored by the dielectric response (in the form of differential capacitance) of the device. Thus, the results provide an improved understanding of the nonlinear conduction behavior in ZnO varistors. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A realistic model of transport properties of zinc oxide varistors is constructed from two-dimensional Voronoi networks and studied via computer simulations. In agreement with experimental microcontact measurements made on individual junctions, the networks are assumed to contain randomly distributed microjunctions of two types: (1) electrically active with highly nonlinear current-voltage (I-V) characteristics and (2) ohmic, i.e., with linear I-V characteristics. Effects of the ohmic grain boundaries in the network are simulated for various concentrations and resistivities. Shapes of the simulated I-V characteristics and current dependence of the coefficient of nonlinearity of the network are in good agreement with those experimentally observed for thin varistor samples and in the measurements employing various surface electrode patterns. It is found that the breakdown voltage of the networks increases with the number of the ohmic grain boundaries, except when their resistivity is so low that it becomes comparable with that of the ZnO grains. The maximal value of the coefficient of nonlinearity of the network is shown to be insensitive to the presence of the ohmic grain boundaries, regardless of their resistivity and concentration.