Modélisation des mécanismes de conduction non linéaires dans les varistances ZnO

Abstract

A calculation method has been developed for the current-voltage characteristic of zinc-oxide varistors. This method calls for the standard models of double Shottky barrier and thermionic emission in the low current range where the non-linearity coefficient α is below 30. The novel part of the model lies in the treatment of the intermediate current range which corresponds to the highest values of α. An impact ionization mechanism generated by high-energy electrons is put forward to explain the appearance of a hole current directed to the interface between two zinc-oxide grains. The ionization probability is calculated by a Monte-Carlo method and compared with analytic relations which form both a lower bound and an upper bound for the result. Hole recombination is taken into account using a number of assumptions on the energy distribution and the capture cross-section of the localized states at the interface. The positive charge resulting from hole trapping at the interface brings about a sudden lowering of the potential barriers between zinc-oxide grains. The model provides nonlinearity coefficients higher than 60, in agreement with the experiments Calcul des caracteristiques I-V en utilisant le modele de double barriere de Schottky et d'emission thermoionique, dans la zone des faibles courants ou le coefficient de non-linearite α est inferieur a 30 et un mecanisme d'ionisation d'impact initie par des electrons d'energie elevee pour expliquer l'apparition d'un courant de trous dirige vers l'interface entre deux grains de ZnO, pour la zone des courants intermediaires