A barrier model for ZnO varistors

A model of current transport in ZnO varistors is proposed. The model is a modification of an earlier one suggested by Levine, with the major difference being the inclusion of a thin (?10 Å) ’’disordered’’ layer which separates the single-crystal ZnO grains. The proposed model is shown to be consistent with electrical and microstructural observations and can satisfactorily account for α’s in excess of 60. A technique for extracting the surface-state density Ns(E) and doping profiles from C-V and I-V data is described. It is also shown from the theory that the maximum α is approximately proportional to the grain voltage, a result that is in agreement with measurements on varistors made under a wide range of processing conditions. By calculating the I-V characteristic for different Ns(E) profiles, it is shown that Ns(E) largely determines this characteristic and that certain profiles are to be favored over others when processing conditions are considered.