DEEP-LEVEL TRANSIENT SPECTROSCOPY OF ZINC-OXIDE VARISTORS DOPED WITH ALUMINUM-OXIDE AND OR SILVER-OXIDE

Deep level transient spectroscopy was employed to investigate the effect of two different dopants (aluminum and silver) on the electronic states of ZnO varistors. The base composition was a typical commercial formulation, and specimens were prepared by the mixed oxide route. Two electron traps were detected: trap L1 (E(c) - 0.15 +/- 0.01 eV) and trap L2 (E(c) - 0.25 +/- 0.01 eV). Doping specimens with trivalent Al increased the densities of both traps; such varistors have improved nonlinearity characteristics, but lower stability. In contrast, doping with monovalent Ag decreased the trap densities, reduced the nonlinearity exponents, but increased stability. Specimens containing on optimum combination of Al and Ag doping had a high L1 trap density, a low L2 trap density, and exhibited good nonlinearity and stability characteristics. The L1 trap appears to be associated with extrinsic donor defects, reflecting varistor nonlinearity; L1 trap density increased with nonlinearity exponents. The L2 trap appears to be associated with varistor stability. As the L2 trap density decreased, the sample stability increased; this may reflect the concentration of zinc interstitial defects.