Electrical conduction and trapping distributions in ZnO and ZnO-Sn composite varistors

Current density-voltage measurements have been performed on pure ZnO and ZnO containing 2% by weight of Sn. Both types of sample showed ohmic conduction at low fields followed by a power-law dependence with exponent of approximately 3 for the pure samples and approximately 2 for samples containing Sn. The results were interpreted in terms of space-charge-limited conduction controlled by an exponential distribution of traps. The form of the dominant trapping distribution differed between the two types of sample. An analysis of the trapping parameters indicated that the trapping density was significantly higher for the samples containing Sn. Simple extensions to existing theory were sufficient to predict the transition voltage between ohmic and space-charge limited conduction, and to account for the higher values observed for the pure samples than for those containing Sn.