The disclosure involves a semiconductor device and a manufacturing method thereof. First, a dielectric layer and a stack comprising a Si layer and at least one SiGe layer located on the Si layer are formed in sequence on a substrate. Then the stack and the dielectric layer are patterned to form a dummy gate and a gate dielectric layer, respectively. Next, sidewall spacers are formed on opposite sides of the dummy gate, and source and drain regions with embedded SiGe are formed. Then, the dummy gate is removed to form an opening, in which a gate material such as metal is filled. In RMG techniques, by adopting the stack consisting of Si and SiGe layers as a dummy gate, the method can further increase the compressive stress in the channel of a MOS device and thus improve carrier mobility as compared to traditional polysilicon dummy gate process.