A semiconductor device includes a substrate having a first region and a second region, a first MOS transistor including a first fin structure and a first gate electrode in the first region, the first fin structure having a first buffer pattern, a second buffer pattern, and a first channel pattern which are sequentially stacked on the substrate, and a second MOS transistor including a second fin structure and a second gate electrode in the second region, the second fin structure having a third buffer pattern and a second channel pattern which are sequentially stacked on the substrate. Related fabrication methods are also discussed.