An overview of the steady state electrical properties of tin dioxide varistors is presented. The varistors are manufactured in two ways: (1) a ceramic route by mixing and compaction of tin dioxide powder with various dopants and (2) a sol-gel method, where dopants are dispersed in their aqueous solutions. For a quantitative agreement with the experimental data, the electronic defect states in the bulk of the tin dioxide grains and at the grain boundary interfaces are studied. Scanning electron microscope and x-ray diffraction are employed to study the morphological and structural aspects of tin dioxide varistors. The effect of various sintering temperatures on their nonohmic or nonlinear characteristics was also investigated. The varistors prepared by the sol-gel method showed high nonlinearity, high density, and high threshold voltage properties than the ones produced by the ceramic route. This is probably due to a finer grain structure and a better densification of dopants along the grain boundaries.

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