DIFFERENT DEGRADATION PROCESSES IN ZNO VARISTORS

Degradation of ZnO-based varistors has been attributed to ion migratio n in the depletion layers at intergrains. Experiments can then be expl ained as the result of the deformation of Schottky barriers present in the grain boundaries. However, the simple lowering of the barriers is not consistent with the observed results. We found that ion migration can change the dopant profile and then the barriers' shape. We found it necessary to distinguish two processes, permanent degradation and r eversible degradation to account for the observed experimental results .