A semiconductor device includes a semiconductor substrate, and a first transistor disposed on the semiconductor substrate. The first transistor includes an insulation structure disposed on the semiconductor substrate, a channel region disposed on the insulation structure and including a first semiconductor layer, and extending in a direction crossing the semiconductor substrate, first source and drain regions electrically connected to the channel region, a first gate insulating layer disposed on the channel region, and a first gate electrode disposed on the first gate insulating layer. A first region that is one of the first source and drain regions and a second region that is another one of the first source and drain regions include different materials or have different crystal structures.