Effect of trap density on the dielectric response of varistor ceramics

Abstract

A careful analysis of the dielectric response of ZnO-based commercial varistors in the frequency range between 100 Hz and 15 MHz and temperature range from 290 to 400 K enables a complete characterization of deep level electronic transitions in space-charge regions. In the investigated systems, oxygen vacancies are the origin of trap states situated ∼0.30 eV below the conduction band. However, samples exhibit a contrasting dielectric response concerning the power law exponent of the loss peak. First we present the case of near-Debye behaviour modeled by a Cole-Cole response with slight deviations α∼0.1 (it is ascribed to a distribution of activation energies) and next the case of very broad loss peaks (nearly constant-loss dielectric response). In this last case, the relative increment in the value of the real part of the capacitance ( C 0- C ∞)/ C ∞ (which is commonly used as a measure of the density of trapping states in semiconductor devices) reaches higher values than those obtained for the former. This suggests that dielectric loss broadness can be regarded as an indication of high density or a disordered distribution of traps.