Effect of Aluminum Doping on the Electrical Properties of ZnO Varistors

Analysis and calculations have been made of the concentration and state of various point defects of ZnO varistors before and after donor doping at high and room temperatures using the point defect model. The results obtained show that donor doping (e.g., the doping of AI) appreciably increases the concentration of the carriers in ZnO varistors, reduces the upturn potential, and significantly lessens the dependence of the conducting behavior for large currents on sintering conditions. In particular, when the effective concentration of doped donor approaches 5 x 10¹⁷cm^-3; the concentration of carriers at room temperature will in the main not vary with the change in sintering conditions and is very close to the concentration of the carriers at high temperatures. An analysis based on the energy band model shows that, owing to the effect of segregation, excessive AI doping will deteriorate the nonlinearity characteristics in the region of small current. This can be improved by quickly dropping the temperature.