AC Properties Of ZnO Varistors

ZnO varistors with highly nonlinear U-I characteristic, excellent surge withstandingcapability are widely used to protect electrical circuits in the fields of electrical andelectronics. In this paper, the AC properties of ZnO varistors was investigated and theresults are presented, which include the leakage current and AC U-I characteristic curve.The samples’ impedance spectroscopy in a wide temperature range are measured andanalyzed. Under a small AC signal, electrons will be emitted to the conduction band andrecaptured by the donor. This process is a Debye-type relaxation. Tow loss peaks withactivation energy0.33eV and0.26eV were found, which are consistent with the electroniclevels of the ionized oxygen vacancy and sub-stable interstitial Zn in the depletion layer,respectively.We study the AC response of the ZnO varistors in the low electrical field region andhigh nonlinear region.In the low electrical field region, the leakage current signal is similar to the sine waveunder the sinusoidal voltage. The phase difference between the current signal and thevoltage signal is less than2. The simple RC equivalent circuit can not reflect ACcharacteristic of the ZnO varistors correctly. The AC equivalent circuit of the ZnO varistorsis made of parallel equivalent conductance, equivalent capacitance and nonlinear resistor,all in parallel connection. The equivalent conductance and equivalent capacitance is derivedfrom the donor trap relaxation response. The equivalent conductance increases with theincrease of frequency, while the equivalent capacitance decreases. The equivalentcapacitance has an less influence on voltage amplitude, but the equivalent conductanceincreases with the increase of voltage amplitude.In the high nonlinear region, AC U-I characteristic curve presents cross hysteresisphenomenon, showing not only capacitive characteristic, but inductive characteristic. Theinductive characteristic is not derived from the lead inductance, because it is more obviouswith the decrease of frequency. The inductive characteristic showed in AC U-I characteristiccurve is derived from the impact ionization of the the deep donor.