Effect of local electrical properties on the electrostatic discharge withstand capability of multilayered chip ZnO varistors

The local electrical properties at individual grain boundaries of multilayered chip varistors composed of ZnO-Bi(2)O(3) (Bi-ZnO) and ZnO-Pr(6)O(11) (Pr-ZnO) ceramics have been investigated using a scanning probe microscope (SPM) to clarify their effect on the electrostatic discharge (ESD) withstand capabilities. Pr-ZnO varistors exhibit a higher ESD withstand capability compared to Bi-ZnO varistors, although both types of devices exhibit similar electrical nonlinearity and surge current withstand capabilities. Bi-ZnO varistors exhibit asymmetric current-voltage (I-V) characteristics after the application of ESD pulses; their breakdown voltage decreases from 9 to 4 V and the leakage current increases. This indicates that the ESD pulses destroy some electrical potential barriers. Scanning surface potential microscopy (SSPM) measurements of a Bi-ZnO varistor reveal the existence of high electrical potential barriers at grain boundaries and electrode interfaces, and high-resistance secondary phases. In contrast, SSPM measurements of a Pr-ZnO varistor reveal that electrical potential barriers exist only at grain boundaries. This suggests that the difference in ESD withstand capabilities between the Bi-ZnO and Pr-ZnO varistors is reasonably due to the differences in their local electrical properties. (c) 2008 American Institute of Physics.