Investigation of the effect of different dopants on the trap states of ZnO-based and SnO₂-based varistors

The electrical properties of ZnO varistors and novel SnO2 varistors have been attributed to their grain boundary barriers. In this report, the techniques of capacitance-voltage (C-V) measurement and deep level transient spectroscopy (DLTS) were employed to investigate the effect of different dopants on the electronic states of ZnO-based and SnO2-based varistors. Two trap levels were determined in typical ZnO varistors: trap L1 at Ec-0.15±0.01eV and trap L2 at Ec-0.25±0.01eV. The different dopants affected the parameters of those traps significantly. Two deeper trap levels, trap S1 at Ec-0.30±0.03eV and trap S2 at Ec-0.69±0.03eV, were identified in both SnO2-CoO-Nb2O5 and SnO2-CoO-Nb2O5-Cr2O3 varistors. The variations in the donor density and trap density appear to be associated with the addition of trivalent Cr3+. The features of these trap levels and the defect theory related to the SnO2 varistors need to be further studied.