A method for manufacturing a semiconductor device is discussed. The method includes forming a gate electrode on a substrate, forming a gate insulating film over the substrate, depositing an In—Ga—Zn oxide over the gate insulating film while heating the substrate to a temperature of 200 to 300° C., an atomic percent ratio of Zn in the In—Ga—Zn oxide as-deposited being higher than that of In or Ga, heat-treating the deposited In—Ga—Zn oxide at a temperature of 200 to 350° C., thereby forming an active layer crystallized throughout an entire thickness of the active layer, and forming a source electrode and a drain electrode.