By improving the structure of an oxide semiconductor field effect transistor (OSFET), an OSFET suitable for higher integration was fabricated while maintaining low off-state leakage current. Such an OSFET is called a vertical-channel OSFET (VFET), in which the direction of current flowing through a channel of the OSFET is vertical to the substrate. The VFET can reduce the footprint to as small as one channel hole, which is smaller than the conventional trench-gate self-aligned (TGSA) structure. In addition, the channel length of the VFET can be precisely controlled even when the channel length is short.