The first model of the ZnO conduction mechanism was based on space charge limited current (SCLC) [55]. Although yielding high coefficients of non-linearity α [56], this model is inappropriate to ZnO varistors because of the passive role of
the intergranular layer in determining the breakdown voltage [39, 53]. Subsequently, a simple tunnelling process [39, 47] and a two-step transport mechanism [52] that consisted of steps of electron hopping [57] and electron tunnelling were suggested. None of these models could account for the high α observed in ZnO varistors.