Anisotropic resistivities of Bi_2Sr_2Ca_{1-x}Er_xCu_2O_8 single crystals were measured and analyzed from 4.2 to 500 K with special interest in the parent antiferromagnetic insulator of x=1.0. Although the resistivity is semiconducting along both the in- and out-of-plane directions, the temperature dependence is found to be significantly different. As a result, the resistivity ratio for x=1.0 takes a broad maximum near room temperature. The electric conduction in parent antiferromagnetic insulators is different from other semiconductors, and is as unconventional as that in high-temperature superconductors.
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