Low-Threshold Varistors Based On Oxide Thin Films

Varistors are one kind of devices whose conductivity increases abruptly when the voltage reaches a threshold. At present, varistors based on ZnO are most widely used commercially. ZnO is one wide-bandgap semiconductor material. It can be used in many areas due to its piezoelectric, thermoelectric, gas sensing and photoconducting multiple properties. In the 1970s, Zinc oxide varistors are widely used as transient surge suppression device for the protection of electronic circuits against high abnormal voltage surges. As the development of communication technology and VLSI electronics, transient surge suppression for low-voltage electronic circuits, i.e., low threshold varistors has attracted attention.In this paper, properties and applications of zinc oxide, the present situation and development trend of varistors and methods of lowering threshold voltages are introduced. Thin films varistors of Al-ZnO-Al and Al-CuO-Al sandwich structures have been fabricated by DC magnetron sputtering. Varistors with good properties are prepared with adjusting gas fluxes and sputtering times. Thin film properties such as thinness, crystalline structure and grain size are analyzed by X-ray diffraction and the I-V curve characteristics of thin film ZnO varistors are tested.The results show that layer-by-layer ZnO thin films of Al-ZnO-Al sandwich structures have been fabricated by DC magnetron sputtering and varistors with threshold voltage 3.2V are obtained.Threshold voltage were lowered by the decrease of the thickness of ZnO film in the varistors. Layer-by-layer CuO thin films of Al-CuO-Al sandwich structures have also been fabricated by DC magnetron sputtering, and CuO thin films varistors with threshold voltage 5.5V are fabricated. With the superposition of such cells, multicell thin films varistors with adjustable thresholds can be conveniently realized, and their threshold voltages have a linear relationship with cell numbers.