The present invention provides a semiconductor device and a method of forming the same, and the semiconductor device including a substrate, an oxide semiconductor layer, two source/drain regions, a high-k dielectric layer and a bottom oxide layer. The oxide semiconductor layer is disposed on a first insulating layer disposed on the substrate. The source/drain regions are disposed on the oxide semiconductor layer. The high-k dielectric layer covers the oxide semiconductor layer and the source structure and the drain regions. The bottom oxide layer is disposed between the high-k dielectric layer and the source/drain regions, wherein the bottom oxide layer covers the source/drain regions and the oxide semiconductor layer.