A barrier model for ZnO varistors

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Abstract
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.

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