Semiconductor device having isolation region

A semiconductor device comprising:

• a substrate including a first region and second region having different active region densities from each other,
• a plurality of first active regions in the first region, the plurality of first active regions including an upper surface having a short axis in a first direction and a long axis in a second direction, being repeatedly disposed along the first direction and along the second direction, and being spaced apart from each other,
• a plurality of second active regions in the second region,
• a first isolation region defining the plurality of first active regions in the first region, and
• a second isolation region defining the plurality of second active regions in the second region,
• wherein the first isolation region comprises a first insulating layer being in contact with side walls of a short axis pair of first active regions which are closest first active regions in the first direction among the plurality of first active regions, and continuously extending along a first shortest distance between the short axis pair of first active regions, and
• the second isolation region comprises a trench insulating layer covering side walls of the plurality of second active regions, an insulating liner covering the trench insulating layer, and a gap-fill insulating layer on the insulating liner.