Bulk electron traps in zinc oxide varistors

Three types of commercial zinc oxide varistor materials were examined using admittance spectroscopy between 30 and 350 K. Maxima in the ac conductance at frequencies from 1 to 100 kHz are observed. The maxima are interpreted as arising from electron traps located within the depletion regions of double Schottky barriers at ZnO‐ZnO grain boundaries. Two traps are observed in each material and are likely to be from common origin. The trap energies are found to be 0.17 and 0.33 eV below the conduction‐band edge. The traps are likely to be associated with native defects in ZnO and may influence device characteristics such as voltage overshoot.

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