A method includes forming a gate structure, a first edge structure and a second edge structure on a semiconductor strip. The method includes forming a first source/drain feature between the gate structure and the first edge structure; and a second source/drain feature between the gate structure and the second edge structure. A distance between the gate structure and the first source/drain feature is from about 1.5 to about 4.5 times greater than a distance between the gate structure and the second source/drain feature. The method includes implanting a buried channel in the semiconductor strip. A top surface of the buried channel is spaced from a top surface of the semiconductor strip. A bottom surface of the buried channel is closer to the top surface of the semiconductor strip than a bottom surface of the first source/drain feature. A dopant concentration of the buried channel is highest under the gate structure.