| abstract |
Provided is a transistor having stable electrical properties. Alternatively, provided is a transistor having normally-off electrical properties. An oxide semiconductor layer having crystallinity is formed on a substrate, oxygen is added by ion implantation to the oxide semiconductor layer while heating the substrate, and a semiconductor element having an oxide semiconductor layer is formed, wherein the crystallinity is almost unchanged before and after the oxygen is added. Furthermore, oxygen or fluorine is added by ion implantation to an insulation layer in contact with the oxide semiconductor layer while heating the substrate, thus improving the transistor breakdown voltage. |