DEGRADATION PHENOMENA OF MULTILAYER ZNO-GLASS VARISTORS STUDIED BY DEEP-LEVEL TRANSIENT SPECTROSCOPY

The degradation of non-Ohmic electrical characteristics of ZnO-glass c hip varistors due to high-intensity impulse currents was correlated wi th the deep trap levels investigated by means of deep level transient spectroscopy. Three electron traps located at 0.11, 0.27, and 0.94 eV below the conduction band were observed for chip varistors before curr ent impulse testing. These trap energy depths of the chip varistors af ter current impulse testing were found to be almost unchanged, but the ir trap densities and capture cross sections both decreased. The conce ntration of the trap at 0.94 eV was decreased and ascribed to oxygen v acancies existing at the grain boundaries of the varistor. The electri cal degradation phenomenon of the chip varistors is closely related to the reaction between the trap at 0.94 eV and adsorbed oxygen ions at the grain boundaries. (C) 1996 American Institute of Physics.