Firstly, the frequence dependency of the impedance of ZnO varistor on the applied bias voltage has been investigated in the frequency range from 1Hz to 1GHz and in the voltage range from 0 to 35volts.
From these results, we can find that the most suitable barrier model is the double Schottky barrier model with sandwiched intergranular barrier, although many barrier models have been proposed previously, such as intergranular barrier model, double Schottky barrier model, and double Schottky barrier model with sandwiched intergranular barrier. We can also find that the most of the applied bias voltage appears on the reverse biased Schottky barrier in the prebreakdown region. Thus we believe that the holes can be generated in the depletion region of the reverse biased Schottky barrier.
Secondly, it is found experimentally that the nonlinear coefficient of the current-voltage characteristics of ZnO varistor increases with CoO concentration. This result is interpreted in terms of the enhanced electron-hole generation rate due to the Co states in the band gap of ZnO in a new model proposed in this paper.
In the new model which enables us to explain the highly nonohmic behavior, C-V characteristics, temperature dependence of the breakdown voltage, the transient conduction phenomena, and the role of CoO additive, the highly nonohmic varistor conduction process is associated with the hole diffusion current which flows due to the gradient of the hole concentration which is caused by the holes generated in the depletion region of the reverse biased Schottky barrier and tunnel through the interface layer to the depletion region of the forward biased Schottky barrier.
As the hole diffusion current is strongly limited by the electron-hole generation rate, it is necessary to increase the electron-hole generation rate for the enhancement of the nonlinearity of ZnO varistors. Co ions do this role, i.e., the electron-hole generation rate is increased with CoO concentration in ZnO varistors.
