A semiconductor device may include a first active pattern, a second active pattern spaced apart at a first distance from the first active pattern, a third active pattern spaced apart at a second distance from the second active pattern, a first device isolation layer between the first and second active patterns, a second device isolation layer between the second and third active patterns, a first channel structure overlapping the first active pattern, a second channel structure overlapping the second active pattern, a third channel structure overlapping the third active pattern, and a separation dielectric layer between the first and second channel structures. The separation dielectric layer may overlap the first device isolation layer. A level of a top surface of the first device isolation layer may be higher than a level of a top surface of the second device isolation layer.