Memory device and method for forming the same

A memory device, comprising:

• a substrate;
• an active region protruding from a top surface of the substrate, the active region having at least one ring structure, wherein a sidewall of the active region includes a (010) crystal plane, a (110) crystal plane, and a (100) crystal plane;
• a first gate structure and a second gate structure over the substrate and crossing the active region, wherein the first gate structure is in contact with the (010) crystal plane, the (110) crystal plane, and the (100) crystal plane of the sidewall of the active region;
• and
• a first word line and a second word line electrically connected to the first gate structure and the second gate structure, respectively.

A method, comprising:

• forming a patterned mask having an opening over a substrate, wherein the opening of the patterned mask has a ring shape;
• etching the substrate by using the patterned mask as an etch mask to form an active region having a ring shape protruding from the substrate;
• forming a first dummy gate structure and a second dummy gate structure over the active region;
• replacing the first dummy gate structure and the second dummy gate structure with a first metal gate structure and a second metal gate structure;
• and
• forming a first word line and a second word line electrically connected to the first metal gate structure and the second metal gate structure, respectively.