Much of the research on ZnO varistors has concentrated on the explanation of their dc current-voltage characteristics. However, varistors also have unusual ac properties which can be technologically important, and must be described by any comprehensive model. In an ideal varistor with identical grain boundaries throughout, there should be no dispersive capacitance at zero bias. In real varistors this capacitance varies considerably with frequency. This dispersion has two causes, charge trapping in the depletion regions and differing grain boundary barriers. Calculations for each process are given. As the voltage across the varistor is increased, the low frequency capacitance is observed to increase well below the breakdown voltage. At even higher voltages the capacitance turns over and becomes negative. All of these effects can be described with a double depletion layer/thermionic emission model.
NASA STI/Recon Technical Report N
- Pub Date:
- Dielectric Properties;
- Electric Potential;
- Grain Boundaries;
- Thermionic Emission;
- Electronics and Electrical Engineering