Distributions of Interface States and Bulk Traps in ZnO Varistors

The distributions for the activation energy E T and the capture cross section σ of deep levels were studied by spectral analysis of deep level transient spectroscopy (SADLTS) in order to characterize precisely the bulk traps and the interface states in the Pr- and Bi-type ZnO varistors. Three bulk traps (B1 ′ , B1 and B2) were observed by SADLTS. The origins of B2 and B1 appear to be the intrinsic defects of ZnO because these traps were not affected by various additives and E T and σ of these traps in both types of ZnO showed almost identical distributions. The distribution of B1' in Bi-type ZnO varistors was different from that of B1 ′ in Pr-type ZnO varistors. This indicates that B1' is caused by the defects induced by either Bi or Pr ions, because the additives can change the forming process. Four interface states in Bi-type ZnO varistors (T0, T1a, T1b and T2) and two interface states in Pr-type ZnO varistors (T0 and T1) were clearly found. T2 was only detected for Bi-type ZnO varistors and had a slower time constant with broadening in the emission rate than the other interface states. The origin of T2 is considered to be the level relating to the spinel particle of Sb. T0, T1a and T1b are the discrete interface states. The distributions (Δ E T and Δσ) yield data which is important in terms of understanding the characteristics of ZnO varistors.