Off current of a bottom gate thin film transistor in which a semiconductor layer is shielded from light by a gate electrode is reduced. A thin film transistor includes a gate electrode layer; a first semiconductor layer; a second semiconductor layer, provided on and in contact with the first semiconductor layer; a gate insulating layer between and in contact with the gate electrode layer and the first semiconductor layer; impurity semiconductor layers in contact with the second semiconductor layer; and source and drain electrode layers partially in contact with the impurity semiconductor layers and the first and second semiconductor layers. The entire surface of the first semiconductor layer on the gate electrode layer side is covered by the gate electrode layer; and a potential barrier at a portion where the first semiconductor layer is in contact with the source or drain electrode layer is 0.5 eV or more.