Pinning effect by interface states in Pr-type ZnO varistors

The interface states in Pr-type ZnO varistors, which consist of two adjacen t levels, TI and TO, were studied using spectral analysis of deep-level tra nsient spectroscopy (SADLTS) to characterize the pinning effect of the Ferm i level by two interface states and the bias dependence on the emission pro cess of the interface states. The measurements were carried out by changing the applied steady-bias voltage for the injection pulse. When the steady-b ias voltage is below 60% of the breakdown voltage, the obtained values of t he activation energy E-T and the capture cross section sigma of the TI leve l are almost constant. The Fermi level is perfectly pinned by the T I level . In contrast, the Fermi level is never pinned by the TO level since E-T an d sigma of the TO level vary with increasing steady-bias voltage and the TO level has a faster emission process.