In this paper, several substitutive and defect reactions have been analyzed to describe the conduction properties in ZnO varistors. The composition of ZnO varistors with added Co and Mn ions of high valence, i.e., Co3+ and Mn4+, was deduced to result in higher donor concentration, trap density, barrier height and leakage current, but a lower nonlinearity coefficient in comparison with that of ZnO varistors with added Co and Mn ions of low valence, i.e., Co2+ and Mn2+. The capacitance-voltage (C-V) and current-voltage (I-V) measurements were employed in the experiment, respectively. Theoretical inference was found to be consistent with the experimental results of electrical characteristics.
At first, Cr-Si (28 wt% Cr, 72 wt% Si) and Ni-Cr (80 wt% Ni, 20 wt% Cr) thin-film materials were deposited by using sputtering method at the same parameters, and their physical and electrical properties were investigated. The resistances of Cr-Si and Ni-Cr thin-film resistors decreased with the increase of deposition time (thickness) and their resistivity had no apparent variations as the deposition time increased. The temperature coefficient of resistance (TCR) of single-layer Cr-Si thin-film resistors was negative and the TCR value of single-layer Ni-Cr thin-film resistors was positive. For that, we used Cr-Si thin films as upper (or lower) layer and Ni-Cr thin films as lower (upper) layer to investigate a bi-layer thin-film structure. The deposition time of Ni-Cr thin films was fixed at 10 min and the deposition time of Cr-Si thin films was changed from 10 min to 60 min. We had found that as Cr-Si thin films were used as upper or lower layers they had similar deposition rates. We had also found that the thickness and stack method of Cr-Si thin films had large effects on the resistance and TCR values of the bi-layer thin-film resistors.
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