Improved varistor nonlinearity via donor impurity doping

Abstract

Zinc oxide (ZnO) varistors exhibit a voltage upturn at higher current densities which reduces their effectiveness as over-voltage protection devices. This effect has been attributed to a limiting series impedance offered by the ZnO grains. Based on literature data, the varistors were made for this investigation wherein the grain impedance has been reduced by doping with Al2O3 or Ga2O3. The high current surge and high frequency impedance data combined with microstructural investigation indicated an improved varistor nonlinearity via donor doping. A simple defect model developed for a pure ZnO crystal has been used to explain the doping behavior of the varistor grain.