What is claimed is:1. A semiconductor device comprising:a substrate;a fin protruding from the substrate and extending in a first direction;a recess in the fin and extending in a second direction that is substantially perpendicular to the first direction;a first gate structure crossing over the fin on a first side of the recess;a second gate structure crossing over the fin on a second side of the recess;a first source/drain region between the first gate structure and the recess; anda second source/drain region between the second gate structure and the recess,wherein the recess includes a portion in which a width increases as a depth of the recess in the fin increases.2. The semiconductor device of claim 1, wherein a widest width of the recess is at a bottom portion of the recess.3. The semiconductor device of claim 2, wherein the bottom portion of the recess is lower than a bottom portion of the first source/drain region in a depth direction.4. The semiconductor device of claim 1, wherein a first width of the recess at a level corresponding to a bottom portion of the first source/drain region is smaller than a second width of the recess at a bottom portion of the recess.5. The semiconductor device of claim 1,wherein a first width of the recess adjacent a top surface of the fin is smaller than a second width of the recess at a level corresponding to a bottom portion of the first source/drain region, andwherein a third width of the recess at a bottom portion of the recess is greater than the second width.6. The semiconductor device of claim 1, wherein a height of the recess is smaller than that of the fin.7. The semiconductor device of claim 1, further comprising a device isolation layer in the recess.8. The semiconductor device of claim 7, further comprising a capping layer that conforms to an inner surface of the recess, wherein the device isolation layer is on the capping layer.9. The semiconductor device of claim 1, further comprising a dummy gate structure crossing over the recess.10. The semiconductor device of claim 9, further comprising first and second spacers at opposite sides of the dummy gate structure.11. The semiconductor device of claim 10,wherein the dummy gate structure includes a gate electrode and a gate insulation layer, andwherein a widest width of the recess is wider than a widest width of the dummy gate structure.12. The semiconductor device of claim 1,wherein the first gate structure comprises a first gate electrode on a first gate insulation layer, andwherein the second gate structure comprises a second gate electrode on a second gate insulation layer.13. A semiconductor device comprising:a fin protruding from a substrate and extending in a first direction;a recess extending in a second direction different from the first direction in the fin;a dummy gate structure overlapping the recess extending in the second direction;a spacer on a sidewall of the dummy gate structure on the fin;an inner spacer on an inner sidewall of the spacer; anda source/drain region at opposite sides of the recess,wherein a lowermost portion of the recess comprises a first portion,wherein the recess further comprises a second portion above the first portion, andwherein a width of the second portion in the first direction is smaller than a width of the first portion in the first direction.14. The semiconductor device of claim 13, wherein the second portion is coplanar with a lowermost portion of the source/drain region.15. The semiconductor device of claim 14, wherein the recess comprises a sidewall extending vertically from an upper surface of the substrate to the second portion.16. The semiconductor device of claim 14,wherein the recess comprises a third portion between the first portion and the second portion, andwherein a width of the third portion in the first direction is smaller than the width of the first portion in the first direction, and is greater than the width of the second portion in the first direction.