The conduction process and the equivalent barrier height in ZnO-Bi2O3 based varistor ceramics

The relationship between the equivalent barrier height of ZnO-Bi2O3 based varistor ceramics and normalized applied voltage was studied and it was found that the equivalent barrier height is influenced by normalized voltage greatly. With the increase of normalized voltage the equivalent barrier height increases firstly, then changes little and finally decreases quickly. Because the Schottky barrier height at the reverse-biased side is higher than that at the forward-biased side, the equivalent barrier height is determined by the former. The changing trend of the equivalent barrier height with normalized voltage indicates three steps of conduction. Firstly, in low normalized voltage range the velocity of electron injection from the forward-biased barrier area into the amorphous layer at grain-boundary is lower than that of ejection from traps in the amorphous layer into the reverse-biased barrier area. Thus the equivalent barrier height increases with the growth of normalized voltage. Secondly, in medium normalized voltage range, the injection velocity and the ejection velocity of electron is equal and the equivalent barrier height reaches its maximum value. Finally, in high normalized voltage range the injection velocity of electron is quicker than the ejection velocity of electron and the equivalent barrier decreases with the increase of the normalized voltage. In the end the Schottky barrier will break electrically. At the same time the relationship between the equivalent barrier height and the leakage current was analyzed and it was found that the leakage current is determined by an exponential function of the difference between the equivalent barrier heights when normalized voltage is 1 and 0.75.