A semiconductor device structure includes a first MOSFET device disposed at a first region of a semiconductor substrate, the first MOSFET device comprises a bulk semiconductor layer contacting the semiconductor substrate, and the bulk semiconductor layer has a first height, a first gate structure over the bulk semiconductor layer, and first S/D regions disposed in the bulk semiconductor layer on opposite sides of the first gate structure; a second MOSFET device disposed at a second region of the semiconductor substrate, the second MOSFET device comprises a semiconductor layer over the semiconductor substrate, and the semiconductor layer has a second height different than the first height, a second gate structure over the semiconductor layer, and second S/D regions disposed in the semiconductor layer on opposite sides of the second gate structure; an insulator disposed between and in contact with the semiconductor substrate and semiconductor layer; and a spacer layer isolating first and second MOSFET devices, and a portion of the spacer layer is disposed between and in contact with the insulator layer and bulk semiconductor layer.