Conduction mechanism and 1/f noise in ZnO varistors

The conduction mechanism in ZnO varistors has been studied assuming thermionic emission as the only way of charge transport. A model elaborated on that basis has been satisfactorily applied to the approximation of the I–U and C–U characteristics and has made it possible to determine the barrier height Φb(300 K)=0.86 V, the donor concentration ND≂1019 cm−3, the effective number of grains between the electrodes g≂20, the position of the Fermi level EC−EF≂0.07 eV, and the donor level EC−ED≂0.20 eV. The relative power spectral density of 1/f current fluctuations SI/I2 in the varistors in the ohmic region has been calculated, starting from the mobility fluctuation approach of the 1/f noise. The calculated linear relationship between SI/I2 and the varistor resistance in the ohmic region has been confirmed by the experimental results. The calculated 1/f noise density is in agreement with the observed density.