A semiconductor device is provided in the present invention, which includes a substrate, an oxide-semiconductor layer, source/drain regions, a dielectric layer, a first gate electrode, a second gate electrode and a charge storage structure. The oxide-semiconductor layer is disposed on the first gate electrode on the substrate. The source/drain regions are disposed on the oxide-semiconductor layer. The first dielectric layer covers on the oxide-semiconductor layer and source/drain regions. A second gate electrode is disposed between source/drain regions and partially covers the oxide-semiconductor layer. The oxide-semiconductor layer may be optionally disposed between the first gate electrode and the oxide-semiconductor layer or be disposed on the second gate electrode.