Semiconductor devices

A semiconductor device, comprising:

• active fins protruding on an upper surface of a substrate;
• a first isolation pattern on the upper surface of the substrate, the first isolation pattern extending on respective lower sidewalls of the active fins;
• a third isolation pattern comprising:
• an upper portion extending into the first isolation pattern;
• and
• a lower portion extending into the upper surface of the substrate, the lower portion having a lower surface comprising a width that is greater than that of an upper surface thereof;
• a second isolation pattern extending in the substrate under the third isolation pattern, contacting the third isolation pattern, and having a rounded lower surface;
• a gate structure on the active fins;
• and
• a source/drain layer on the substrate adjacent to the gate structure,
• wherein the upper portion of the third isolation pattern is directly contacting sidewalls of the first isolation pattern.