A semiconductor device includes a silicon substrate, a fin shaped structure and a shallow trench isolation. The fin shaped structure includes a top portion which protrudes from a bottom surface of the fin shaped structure and the fin shaped structure is directly disposed on the silicon substrate. The bottom surface of the fin shaped structure covers an entire top surface of the silicon substrate. The fin shaped structure further includes a silicon germanium (SiGe) layer extending within the fin shaped structure and occupying the whole top portion of the shaped structure. The fin shaped structure is a semiconductor fin shaped structure, and the material of the silicon substrate is different from the material of the silicon germanium layer The shallow trench isolation is disposed on the top portion and the bottom surface of the fin shaped structure.