Evolution of Physical Models for ZnO-Varistors

Abstract

The highly nonlinear conduction mechanism in ZnO based ceramics has attracted growing interest during the past decade. With increasing experimental experience model approaches have drifted from mechanisms related to massive non-zinc oxide intergranular layers like space charge limited current or Nordheim-Fowler tunneling to grain boundary effects where ZnO itself plays the dominant role. Firstly several attempts were made with surface state density distributions of foreign atoms. To allow for the measured high nonlinearity coefficient α in the range of α = 70 or even more, additional effects have been taken into account arising from the generation of minority carriers — voltage dependent shortening of the spatial distance of energy states which are involved in quantum mechanical tunneling, or the barrier height control by capture of holes at surface states. In fact recent experimental work gives strong evidence for the presence of minority carriers above threshold voltage.