An oxide semiconductor field effect transistor (OS FET) device includes a first dielectric layer formed on a substrate, an oxide semiconductor (OS) island formed on the first dielectric layer, a first gate electrode formed on the OS island, a gate dielectric layer formed in between the first gate electrode and the OS island, a patterned hard mask layer formed on a top surface of the first gate electrode, an etch stop layer covering a top surface of the patterned hard mask layer and sidewalls of the first gate electrode, and a source electrode and a drain electrode formed on the OS island. At least one of the source electrode and the drain electrode partially overlaps the etching stop layer on the sidewalls of the first gate electrode.