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 l~ ~Hz are obs~rved. ~he maxima are interpreted as arising from electron traps located wlthm 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 e V 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.4186

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Admittance spectroscopy was used to characterize the bulk electron traps in polycrystalline ZnO-Bi203 and ZnO-BaO systems. Temperatures from 30 to 350 K and a frequency range from 1.0 to 33.3 kHz were used. Admiittance spectra for the ZnO-Bi,03 system showed large variatiions with sintering atmosphere and heat treatment. A trap is olbserved at 0.33 eV below the conduction band edge in both systems. This trap is likely to be associated with an ionized oxygen vacancy. A possible explanation of primitive varistor characteristics in terms of the roles of the electronic defects is presented.