Provided are semiconductor devices that include an active pattern on a substrate, first and second gate electrodes on the active pattern and arranged in a first direction relative to one another and a first source/drain region in a first trench that extends into the active pattern between the first and second gate electrodes. The first source/drain region includes a first epitaxial layer that is configured to fill the first trench and that includes at least one plane defect that originates at a top portion of the first epitaxial layer and extends towards a bottom portion of the first epitaxial layer.