Conduction mechanism and flicker noise in ZnO varistors

Measurements have been performed on the power spectral density S _v of the flicker noise in ZnO varistors as a function of temperature (from 110 to 370 K), bias voltage (from 10 to 90% of the varistor voltage), and frequency (from 0.1 Hz to 2 kHz). Capacitance-voltage (C-V) curves and current-voltage (I-V) characteristics as a function of temperature have also been measured. The picture resulting from these measurements is very self-consistent when the data are interpreted on the basis of a Schottky emission model and the flicker island models. In particular the temperature and frequency dependence of S_v have been found to provide direct experimental evidence for the thermally activated emission of electrons from localized states, distributed over a large spectrum of energy levels, at the ZnO grain boundaries. It is shown that both I-V, C-V curves and S_v data enable one to obtain reliable information on the intergranular barrier height, its dependence on temperature, and bias voltage. The degradation of I-V and noise characteristics observed in some samples under dc and temperature stress suggests that flicker noise can be used as a technique for the study of degradation effects.