First, the effects of Co oxides on the voltage - current characteristics of ZnO varistor were investigated and, by means of the capacitance-voltage and the high-frequency impedance measurements, their roles on the formation of the grain boundary junction and the conductivity of ZnO grain were analyzed in terms of the surface state density and the donor concentration of ZnO grain.
The electric field - current density measurements showed that additions of Co oxides increase the low-currents nonlinearity, commensurate with the decrease of leakage currents.
When the varistor samples have respectively had the same mole % of Co oxides, $Co_2O_3$-doped sample has showed higher nonlinearity than CoO-doped sample. But as the same atomic % of Co was incoporated, the leakage current was smallest in the $Co_2O_3$-doped sample and largest in the CoO-doped sample.
The onset of vlotage upturn was descended to lower current densities by the addition of $Co_2O_3$. the nonlinear exponent α at high current densities was larger in the CoO-doped sample than in the $Co_2O_3$-doped sample, when the content of Co in the both systems is same.
The capacitance - voltage measurements indicated that the donor density of ZnO grain is decreased with $Co_2O_3$ content.
The high-frequency impedance measurements showed that the addition of $Co_2O_3$ decreases the conductivity of ZnO grain.
In has been also observed that the carrier mobility is increased with the dopant concentration in a content range up to 0.5 mole % of $Co_2O_3$, but decreased beyond 0.5 mole %.
From these experimental results, it was concluded that the Co ions substituted for $Zn^{2+}$ ions in the lattice sites of ZnO can only exist as a divavent state regardless of their valency states before the substitutional reaction. As the energy state of $Co_Zn$ in ZnO grains is located in the forbidden band of ZnO, it can be considered that $Co_Zn$ at the interface region forms a surface state.
During the substitutional reaction of $Co_2O_3$ or $Co_3O_4$ for ZnO, $Zn_i$ and $V_o$ are oxidized so that the donor density is lowered.
It can be also concluded that the lowered carrier mobility in the highly doped ZnO varistor samples containing more than 0.5 mole % $Co_2O_3$ improves the decrease of the grain conductivity.
Second, the effects of heat-treatment on the grain boundary barrier potential and the grain conductivity were observed and interpreted with the change of the surface state density and the donor concentration, respectively due to the phase transformation of $Bi_2O_3$ intergranular layer with a morphological change and the thermal oxidation of the donor defects, $Zn_i^{＼cdot}(Zn_i^{＼cdot＼cdot})$ V_o^{＼cdot}(V_o^{＼cdot＼cdot})$.
The heat-treatment at 750℃ increased the leakage current due to the lowering of the barrier potential. This can be explained by the decrease in the surface state density responsible for the phase transformation of $Bi_2O_3$ from α-to γ-phase.
The treated sample in an atmosphere of ambient air has showed a little higher nonlinearity at the leakage current-ent region than the sample treated in Ar. This can be explained with the decrease in the donor density due to the thermal oxidation of the donor defects.
The heat-treatment in air moved the upturn voltage to lower current. This is also explained by the thermal oxidation, which is confirmed by the C-V measurements.
Finally, we can concluded that the conduction through the thicker $Bi_2O_3$ intergranular phases plays an important role on the V-I characteristics at the prebreakdown region.
