A semiconductor device is provided, including a substrate with an isolation layer formed thereon, wherein the substrate has a fin protruding up through the isolation layer to form a top surface and a pair of lateral sidewalls of the fin above the isolation layer; a silicon-germanium (SiGe) layer epitaxially grown on the top surface and the lateral sidewalls of the fin; and a gate stack formed on the isolation layer and across the fin, wherein the fin and the gate stack respectively extend along a first direction and a second direction. The SiGe layer formed on the top surface has a first thickness, the SiGe layer formed on said lateral sidewall has a second thickness, and a ratio of the first thickness to the second thickness is in a range of 1:10 to 1:30.