Low voltage ZnO varistor: Device process and defect model

Abstract

A new process is described for achieving the current‐voltage characteristics of a ZnO varistor with a breakdown voltage less than 20 V. The structure has been made by using Ag contacts containing oxides of Bi and Pb which serve both as a diffusion source for grain and/or grain boundary doping, and as metallic contacts on polycrystalline ZnO substrate. The structure was also made by sputtering a layer of various metal oxides such as Bi₂O₃, Sb₂O₃, Co₃O₄, etc. and then evaporating contact metals. The device shows a nonlinear current‐voltage characteristic when annealed at 800–900 °C in air. Annealing in N₂ ambient however, results in linear I‐V characteristics. The presence of certain critical additives such as Bi₂O₃ was found essential for the development of nonlinearity. Based on these observations and the known defect structure of pure ZnO, a defect model which takes into account the role of oxygen in developing the nonlinear behavior of the proposed device is presented.