A semiconductor device is provided, comprising a substrate with a first insulating film formed thereon, and a transistor formed on the first insulating film. The transistor at least comprises an oxide semiconductor layer formed on the first insulating film, a first gate insulation film formed on the oxide semiconductor layer, a gate electrode formed above the first gate insulation film, and spacers formed on the oxide semiconductor layer. The spacers at least cover the sidewalls of the first gate insulation film and the sidewalls of the gate electrode. The gate electrode has a gate width and the first gate insulation film has a first width, wherein the gate width is different from the first width.