A semiconductor substrate for use in an integrated circuit, the semiconductor substrate including a channel defined on a surface of the substrate. The channel includes a first wall, a second wall, and a third wall. The first wall is recessed from the surface. The second wall extends from the surface to the first wall. The third wall extends from the surface to the first wall and faces the second wall across the channel. At least one of the second wall and the third wall includes a plurality of structures projecting into the channel from the second wall or the third wall.