Integrated circuit devices including device isolation structures and methods of fabricating the same

A semiconductor device comprising:

• a semiconductor substrate including a low voltage region, a middle voltage region, and a high voltage region;
• a low voltage field effect transistor (FET) including a low voltage gate insulating layer on the low voltage region;
• a middle voltage FET including a middle voltage gate insulating layer on the middle voltage region;
• a high voltage FET including a high voltage gate insulating layer on the high voltage region;
• and
• at least one device isolation layer structure that electrically separates two adjacent regions among the low voltage region, the middle voltage region, and the high voltage region,
• wherein the at least one device isolation layer structure includes first and second trenches separated by a predetermined distance in the semiconductor substrate, electrically insulating layers respectively provided in the first and second trenches, and a guard ring region extending between the first and second trenches.

A semiconductor device comprising:

• a semiconductor substrate having a first voltage region configured to be driven at a first voltage and a second voltage region disposed adjacent to the first voltage region and configured to be driven at a second voltage;
• a first field effect transistor (FET) on the first voltage region and including a first gate insulating layer having a first thickness;
• a second FET on the second voltage region and including a second gate insulating layer having a second thickness;
• and
• a device isolation layer structure that electrically separates the first and second voltage regions,
• wherein the device isolation layer structure includes first and second trenches separated by a predetermined distance in the semiconductor substrate, electrically insulating layers respectively provided in the first and second trenches, and a guard ring region extending between the first and second trenches, wherein the guard ring region has a bottommost surface located on a same or lower level than at least one lowest surface of the first and second trenches.