A semiconductor device includes a substrate, a first well formed in the substrate, a second well formed in the substrate, a first fin formed on the first well, and a second fin formed on the second well. The first well includes a first conductivity type, the second well includes a second conductivity type, and the first conductivity type and the second conductivity type are complementary to each other. The substrate includes a first semiconductor material. The first fin and the second fin include the first semiconductor material and a second semiconductor material. A lattice constant of the second semiconductor material is larger than a lattice constant of the first semiconductor material. The first semiconductor material in the first fin includes a first concentration, the first semiconductor material in the second fin includes a second concentration, and the second concentration is larger than the first concentration.