A semiconductor device includes a semiconductor substrate and two fin structures. Channels of the fin structures include a second semiconductor material portion over a first semiconductor material portion. Source and drain regions of the first fin structure include a third semiconductor material portion over the first semiconductor material portion. Source and drain regions of the second fin structure include the second semiconductor material portion over the first semiconductor material portion and a fourth semiconductor material portion over the second semiconductor material portion. The first, second, third, and fourth semiconductor material portions are different in composition from each other.