The microstructure and conduction mechanism of the nonlinear ZnO varistor with Al/sub 2/O/sub 3/ additions

The microstructure and electrical properties of nonlinear ZnO varistors with Al/sub 2/O/sub 3/ additions are investigated. The variation of nonlinear behavior with Al/sub 2/O/sub 3/ additions is indicated from J-E and C-V measurement to be a result of the change of the interface defects density N/sub t/ at the grain boundaries and the donor concentration N/sub d/ in the ZnO grains. The optimum composition which has a nonlinear coefficient /spl alpha/ of /spl sim/57 was observed in the sample with 0.005 wt.% Al/sub 2/O/sub 3/ additions. The conduction mechanism in the pre-breakdown region is consistent with a Schottky thermal emission process obeying a relation given by J/spl prop/exp[-(/spl phi/-/spl beta/E/sup 1/2/)/kT] and the conduction process at the breakdown region follows Fowler-Nordheim tunneling.