To provide a field-effect transistor improved in transparency, electrical properties, stability, uniformity, reproducibility, heat resistance and durability, and as a reduced overlap capacity between electrodes. A field-effect thin film transistor 1001 includes a gate electrode 1025, an active layer, a source electrode 1022 and a drain electrode 1023, wherein a crystalline oxide 1021 containing indium and having an electron carrier concentration of less than 10.sup.18/cm.sup.3 is used as the active layer, and the gate electrode 1025 is in self-alignment with the source electrode 1022 and the drain electrode 1023. The crystalline oxide 1021 contains a positive trivalent element different from a positive divalent element or indium.