A semiconductor device includes a first semiconductor structure and a second semiconductor structure. The first semiconductor structure includes silicon. The second semiconductor structure is embedded in the first semiconductor structure, in which the second semiconductor structure has at least one convex portion and at least one concave portion. The convex portion and the concave portion are on at least one edge of the second semiconductor structure, and a shape of the concave portion includes rectangle, trapezoid, inverted trapezoid, or parallelogram. The second semiconductor structure includes germanium, elements of group III or group V, or combinations thereof.