A semiconductor device includes a substrate including an active pattern, a channel pattern on the active pattern, and a source/drain pattern on a side surface of the channel pattern, the source/drain pattern including a first section between a first level and a second level that is higher than the first level, a first variation section between the second level and a third level that is higher than the second level, and a second section between the third level and a fourth level that is higher than the third level, where a rate of change in germanium concentration in the first variation section in a first direction is greater than a rate of change in germanium concentration in each of the first section and the second section in the first direction, and a germanium concentration at each of the first level and the second level is greater than 0 at % and equal to or less than 10 at %.