We have evaluated the electrical degradation of the Zincoxide(ZnO) varistors added two kinds of additives by the V-I measurements and the improved ICTS method. Results of the V-I characteristics show that both the characteristics of varistors added Bi2O3 and CoO and those added Bi2O3 and MnO2 were recovered by thermal annealing. This indicates that the distribution of O2-ion changed by applying voltage was returned to the initial condition by thermal annealing. Results of the improved ICTS measurement show that the trap density for varistors added Bi2O3 and CoO decreased after degradation. Trap levels for varistors added Bi2O3 and MnO2 became shallower after degradation. The result of simulation shows that varistors with narrow distribution of trap levels, deep trap levels and the low donor density have excellent nonlinear characteristic.

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