A semiconductor device includes a first type semiconductor structure, an active structure, and a contact layer. The first type semiconductor structure includes a first lattice constant, a first side and a second side opposite to the first side. The active structure is on the first side of the first type semiconductor structure and emits a radiation, and the radiation has a peak wavelength between 1000 nm and 2000 nm. The contact layer is on the second side of the first type semiconductor structure and includes a second lattice constant. A difference between the first lattice constant and the second lattice constant is at least 0.5%.