A semiconductor device includes a substrate, in which a lower semiconductor layer, an insulating gapfill layer, and an upper semiconductor layer are sequentially stacked. A gate structure is disposed on the upper semiconductor layer. A source/drain electrode is disposed on a sidewall of the gate structure. A semiconductor pattern is disposed between the source/drain electrode and the upper semiconductor layer. The gate structure includes a gate electrode and a spacer structure. The spacer structure includes a first spacer pattern, a second spacer pattern, and a third spacer pattern, sequentially disposed on a sidewall of the gate electrode. The semiconductor pattern is extended to a region below a bottom surface of the third spacer pattern and is connected to the second spacer pattern.