Semiconductor device including fin structures and manufacturing method thereof

A method for manufacturing a semiconductor device includes forming a fin structure over a substrate. The fin structure has a top surface and side surfaces and the top surface is located at a height H0 measured from the substrate. An insulating layer is formed over the fin structure and the substrate. In the first recessing, the insulating layer is recessed to a height T1 from the substrate, so that an upper portion of the fin structure is exposed from the insulating layer. A semiconductor layer is formed over the exposed upper portion. After forming the semiconductor layer, in the second recessing, the insulating layer is recessed to a height T2 from the substrate, so that a middle portion of the fin structure is exposed from the insulating layer. A gate structure is formed over the upper portion with the semiconductor layer and the exposed middle portion of the fin structure.