Degradation mechanism of zinc oxide varistors under dc bias

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Abstract
The degradation mechanism of ZnO varistors under dc bias is investigated from the viewpoint of the time dependence of leakage current under dc biasing, the voltage-current characteristics, and the thermally stimulated current and dielectric properties before and after dc biasing test. The experimental results are analyzed on the basis of the Schottky barriers which control current transport in varistors at the grain boundary regions. The increase in leakage current is expressed as J = A exp (Bt0.3) ( J is the leakage current; A and B are constants, and t the biasing time) and is attributed to the deformation of the Schottky barriers. The deformation corresponds to the lowering of the barrier height and decrease in the depletion width. This is due to the uneven distribution of positive charges caused by the migration of donor ions. In case of the degradation of varistors under long term dc bias, the deformation is mainly due to decrease in the depletion width. Taking into account the electric field due to positive charges and the skew of Fermi level, etc., the asymmetrical deformation can be also explained as a phenomenon due to surplus positive charges, formed in the reverse-biased depletion layer.

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