The semiconducting properties of ZnO were very well known [33] well before Matsuokaet al. [34] investigated the non-linear voltage–current characteristic of ZnOdoped with alkali earth metal oxides. It was found that with appropriate amounts(0.5 mol%) of additives having an ionic radius larger than that of Zn+2 (which wouldnot dissolve in the ZnO lattice and would thus form a segregation layer at the ZnO grain boundary), the ceramic exhibits a high coefficient of non-linearity α on its V –I characteristic. Furthermore, it was shown that the new material was exceptionally superior compared with conventionally used SiC varistors [35]. The non-linearity coefficient, α, of early ZnO varistors was in the range 25–50. In contrast, that of SiC varistors was between 2 and 7 [36].