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When tin oxide is doped with either Sb2O3 and CoO or Ta2O5 and CoO, it shows highly nonlinear current (I)–voltage (V) characteristics. Addition of CoO leads to creation of oxygen vacancies and helps in sintering of SnO2. Both tantalum and antimony act as donors and increases the conductivity. The change in nonlinear coefficient (α) and the breakdown voltage (EB) are studied as function of the quantity of the donor introduced. The magnitudes of grain size and grain boundary barrier height (ΦB) are calculated as a function of duration of sintering. The observed nonlinear coefficient (α) is in the range of 3–34 and the breakdown voltage varies from 110 V/cm to 4400 V/cm. Incorporation of 5 mole% titanium leads to increase in both α and EB values. All the samples show single phase as found by X-ray diffraction. These samples exhibit low breakdown voltages as compared to commercial ZnO varistors.