A field-effect transistor, which contains: a gate electrode configured to apply gate voltage; a source electrode and a drain electrode, which are configured to extract electric current; an active layer formed of a n-type oxide semiconductor, provided in contact with the source electrode and the drain electrode; and a gate insulating layer provided between the gate electrode and the active layer, wherein the n-type oxide semiconductor is a triclinic crystal compound, a monoclinic crystal compound, or a trigonal crystal compound, each of which is substitutionally doped with at least one dopant selected from the group consisting of a divalent cation, a trivalent cation, a tetravalent cation, a pentavalent cation, and a hexavalent cation, and wherein a valence of the dopant is greater than a valence of a metal ion constituting the n-type oxide semiconductor, excluding the dopant.