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.
Most of the accepted models assume a common description of the band structure in
the vicinity of the intergranular layer region. The potential barrier is assimilated to an
assembly of two back-to-back Schottky junctions. The Schottky emission model [27]
was extended to include thin intergranular layers, electron traps in the intergranular
layer [41] and to consider the depletion regions formed in the ZnO grains [58, 59]. The
parallel currents in the bismuth rich intergranular layer are important when the varistor