The present invention provides a method for manufacturing a semiconductor device, which comprises: providing an SOI substrate, which comprises a base layer, an insulating layer located on the base layer and a active layer located on the insulating layer; forming a gate stack on the SOI substrate; etching the active layer, the insulating layer and a part of the base layer of the SOI substrate with the gate stack as a mask, so as to form trenches on both sides of the gate stack; forming a crystal dielectric layer within the trenches, wherein the upper surface of the crystal dielectric layer is lower than the upper surface of the insulating layer and not lower than the lower surface of the insulating layer; and forming source/drain regions on the crystal dielectric layer. The present invention further provides a semiconductor device. The present invention is capable of eliminating pathway for leakage current between source/drain regions and SOI substrate at the meantime of reducing contact resistance at source/drain regions.