Detection and Characterization of Trap Levels in Semiconducting Ceramics

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The degradation phenomenon in a ZnO varistor was investigated by means of isothermal capacitance transient spectroscopy (ICTS), paying special attention to the trap levels. From the analysis of ICTS signals before and after degradation, the interface trap levels were found to shift toward the conduction band, and their energetical distribution was broadened. The degradation phenomenon was related to the interface states of the grain boundaries.The PTCR effect in BaTiO_3 is a grain boundary effect caused by a two dimensional layr of acceptor states which attract electrons from the grain interior. The acceptor state density and the energy gap, Es, between the conduction band and the energy level of the acceptor states govern the PTCR-properties. The samples annealed at high Po_2 (10 MPa) had a higher Es than the samples annealed at lower Po_2 (0.1 MPa), leading to improved PTCR-properties. Based on this finding and results from the literature, a phenomenological PTCR model and an accompanying PTCR chart for acceptor-donor-codoped BaTiO_3 were proposed. The PTCR chart clarifies that acceptor dopant concentrations, oxidation time, and oxygen partial pressure during oxidation or cooling can be optimized simultaneously to obtain the best PTCR ceramics.