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Ceramic samples of CaCu3Ti4O12 (CCTO) were prepared and the voltage–current characteristics were investigated. Extremely strong nonlinear behavior, similar to the reported results in literature [Nature Materials 3[11] 774–8 (2004); Appl. Phys. Lett., 89[19] 191907 (2006); Appl. Phys. Lett., 89[21] 212102 (2006); Appl. Phys. Lett., 91[9] 091912 (2007)] was observed. However, we found that this is a measurement artifact due to a thermionic emission current feedback amplification effect. Compared to that of ZnO varistor, the thermionic emission current of CCTO sample is about six orders of magnitude higher and it is feedback amplified due to strong negative temperature coefficient resistance behavior when the CCTO sample is tested with a voltage (or current) source. Eliminating the thermionic emission current feedback amplification effect, the true value of nonlinear coefficient of the CCTO sample is much lower.

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Nonpolar a-plane GaN-based metal-semiconductor–metal UV PDs, with high responsivity and low dark current, were made from a-plane GaN epitaxial films grown on r-plane sapphire by controlling the dislocation density.

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The improved dielectric properties and voltage‐current nonlinearity of nickel‐doped CaCu3Ti4O12 (CCNTO) ceramics prepared by solid‐state reaction were investigated. The approach of A′‐site Ni doping resulted in improved dielectric properties in the CaCu3Ti4O12 (CCTO) system, with a dielectric constant ε′≈1.51×105 and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X‐ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co‐existence of Cu+/Cu2+ and Ti3+/Ti4+. A steady increase in ε′(f) and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu‐rich phase in the intergranular phase caused by the Ni substitution in the grains. The low‐frequency relaxation leading to a distinct enhancement in ε′(f) beginning with CCNTO25 was confirmed to be a Maxwell‐Wagner‐type relaxation strongly affected by the Ni‐related phase with the formation of a core‐shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current‐voltage characteristics of the CCTO ceramics. The present A′‐site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.