$Co_2O_3$ additive effects on the electrical properties in ZnO varistor are investigated. As the Co cations being substituted and ionized normally to a divalent state for Zn in the lattice site of ZnO increase, the leakage current in the prebreakdown region decrease while so-called "upturn" moves into the lower current region.
These phenomena may be explained by both the increase of the activation energy and the decrease of the donor density in ZnO grains, which are experimentally confirmed from the temperature dependency of V-I relationships and C-V measurements.
It is likely that the number of interstitial Zn cation (or oxygen vacancy) should be reduced by the incorporation of $Co_2O_3$ additive, according to the reaction of zinc interstitial(or oxygen vacancy) with residual excess oxygen formed by the substitution effect of Co cation for Zn cation.
