Field-effect transistor (FET) devices using thin crystals of FeSe1−xTex (x = 0.9 and 1.0) have been fabricated with an electric-double-layer (EDL) capacitor. Despite the presence of substantial quantities of electron and hole carriers in the bulk due to the semimetallic electronic structure of FeSe1−xTex, we have observed p-channel depletion-type FET characteristics, in contrast to the n-channel normally on FET characteristics of a Bi2Se3 EDL FET. In FeSe1−xTex, the mobile carriers, holes, are depleted in the channel region by accumulating electrons, resulting in a decrease in conductivity. This result is consistent with the experimentally observed positive Hall coefficient at room temperature.

We investigated the temperature dependence of resistivity in thin crystals of FeSe 1−x Te x (x=1.0, 0.95, and 0.9), though bulk crystals with 1.0 × 0.9 are known to be non-superconducting. With decreasing thickness of the crystals, the resistivity of x=0.95 and 0.9 decreases and reaches zero at a low temperature, which indicates a clear superconducting transition. The anomaly of resistivity related to the structural and magnetic transitions completely disappears in 55-to 155-nm-thick crystals of x=0.9, resulting in metallic behavior in the normal state. Microbeam x-ray diffraction measurements were performed on bulk single crystals and thin crystals of FeSe