Volt-ampere characteristic of the double Schottky barrier
The volt-ampere characteristic (I-V characteristic) of the double Schottky barrier located in the contact region of ZnO grains of zinc oxide based varistor ceramics is calculated using the mechanism of the above-barrier electron emission. I-V characteristic is symmetric to the polarity of the voltage U. At U > 2kBT/e (kB is the Boltzmann constant, T is the absolute temperature, e is electron charge) the electric current is saturated. The contact of ZnO grains with a double Schottky barrier behaves like an electrical circuit consisting of two oppositely connected Schottky diodes. A small maximum possible decrease in the height of the double Schottky barrier in an electric field ~ 0.7kBT ≈ 0.018 eV does not allow explaining the high nonlinearity of I – V characteristic of varistor materials by the above-barrier electron emission. The most probable cause of nonlinearity is the tunnel emission of electrons and impact ionization.