A field-effect transistor comprising: a gate electrode, a source electrode and a drain electrode, an active layer, which is disposed to be adjacent to the source electrode and the drain electrode and includes a n-type oxide semiconductor; and a gate insulating layer, which is disposed between the gate electrode and the active layer, wherein the n-type oxide semiconductor undergoes substitutional doping with at least one dopant selected from the group consisting of a divalent cation, a trivalent cation, a tetravalent cation, a pentavalent cation, a hexavalent cation, a heptavalent cation, and an octavalent cation, wherein a valence of the dopant is greater than a valence of a metal ion constituting the n-type oxide semiconductor, provided that the dopant is excluded from the metal ion, and wherein the source electrode and the drain electrode include a material selected from the group consisting of metals of Au, Pt, and Pd and alloys including at least any one of Au, Pt, and Pd, in at least contact regions of the source electrode and the drain electrode with the active layer, wherein ohmic contacts are formed between the source electrode and the active layer and between the drain electrode and the active layer; the n-type oxide semiconductor includes at least one element selected from the group consisting of In, Zn, Sn, and Ti; and the n-type oxide semiconductor further includes at least one selected from the group consisting of Si, Ge, Zr, Hf, Al, Ga, Sc, Y, Ln, and alkaline earth metals to suppress oxygen defects. a gate electrode, a source electrode and a drain electrode, an active layer, which is disposed to be adjacent to the source electrode and the drain electrode and includes a n-type oxide semiconductor; and a gate insulating layer, which is disposed between the gate electrode and the active layer, wherein the n-type oxide semiconductor undergoes substitutional doping with at least one dopant selected from the group consisting of a divalent cation, a trivalent cation, a tetravalent cation, a pentavalent cation, a hexavalent cation, a heptavalent cation, and an octavalent cation, wherein a valence of the dopant is greater than a valence of a metal ion constituting the n-type oxide semiconductor, provided that the dopant is excluded from the metal ion, and wherein the source electrode and the drain electrode include a material selected from the group consisting of metals of Au, Pt, and Pd and alloys including at least any one of Au, Pt, and Pd, in at least contact regions of the source electrode and the drain electrode with the active layer, wherein ohmic contacts are formed between the source electrode and the active layer and between the drain electrode and the active layer; the n-type oxide semiconductor includes at least one element selected from the group consisting of In, Zn, Sn, and Ti; and the n-type oxide semiconductor further includes at least one selected from the group consisting of Si, Ge, Zr, Hf, Al, Ga, Sc, Y, Ln, and alkaline earth metals to suppress oxygen defects. wherein ohmic contacts are formed between the source electrode and the active layer and between the drain electrode and the active layer; the n-type oxide semiconductor includes at least one element selected from the group consisting of In, Zn, Sn, and Ti; and the n-type oxide semiconductor further includes at least one selected from the group consisting of Si, Ge, Zr, Hf, Al, Ga, Sc, Y, Ln, and alkaline earth metals to suppress oxygen defects.