Pinning Effect by Interface States in Pr-type ZnO Varistors

Abstract

The interface states in Pr-type ZnO varistors, which consist of two adjacent levels, T1 and T0, were studied using spectral analysis of deep-level transient spectroscopy (SADLTS) to characterize the pinning effect of the Fermi level by two interface states and the bias dependence on the emission process of the interface states. The measurements were carried out by changing the applied steady-bias voltage for the injection pulse. When the steady-bias voltage is below 60% of the breakdown voltage, the obtained values of the activation energy ET and the capture cross section σ of the T1 level are almost constant. The Fermi level is perfectly pinned by the T1 level. In contrast, the Fermi level is never pinned by the T0 level since ET and σ of the T0 level vary with increasing steady-bias voltage and the T0 level has a faster emission process.