is operated in the low conduction regime [60].Afurther improvement of the depletion
layer model is achieved by introducing a process of creation, by hot electrons [56]
and impact ionisation [58, 59, 61], of holes [62, 63] in the depletion layers which are
inversely polarised compared with the double Schottky barrier. Under the effect of
the field, the created holes are collected at the interface and narrow the potential hill
at the intergranular interface and favour tunnelling [53]. However, tunnelling was
rejected and the presence of holes was associated at the interface with lowering of
the potential barrier height [60, 64]. As a result, a sharp increase of current for a very
small increase in applied voltage is obtained. A further step in the formulation of the
conduction mechanism takes into account recombination of the created holes [58].
5.3.4.2 Description of the high conduction model
Consider the situation where two ZnO grains meet to form a grain boundary. The
thickness of the intergranular layer is assumed very thin, so that it can be neglected.
Gap states are then formed at the interface and trap a negative charge. This charge
is screened by the ionised donors’ ZnO. As a result, depletion layers appear on both