Various embodiments provide semiconductor devices and methods for forming the same. A first fin and a second fin are formed on a semiconductor substrate. A first dielectric layer is formed on the semiconductor substrate and has a top surface lower than a top surface of both of the first fin and the second fin. A gate structure is formed on the first dielectric layer and covering across a first portion of each of the first fin and the second fin. A second portion of the first fin on both sides of the gate structure is removed, forming a first recess. A first semiconductor layer is formed in the first recess. A second dielectric layer is formed on the first dielectric layer and the first semiconductor layer, and exposes a top surface of the second fin. A second semiconductor layer is formed on the exposed top surface of the second fin.