Semiconductor device including protrusion type isolation layer

A semiconductor device, comprising:

• a semiconductor layer including an active region;
• an isolation layer surrounding the active region;
• at least two gate structures crossing the active region, wherein an uppermost surface of each of the gate structure is at a level higher than an uppermost surface of the active region;
• a contact plug disposed on the active region between the gate structures;
• and
• a bit line electrically connected to the contact plug.