A semiconductor device employing a transistor having a recessed channel region and a method of fabricating the same is disclosed. A semiconductor substrate has an active region. A trench structure is defined within the active region. The trench structure includes an upper trench region adjacent to a surface of the active region, a lower trench region and a buffer trench region interposed between the upper trench region and the lower trench region. A width of the lower trench region may be greater than a width of the upper trench region. An inner wall of the trench structure may include a convex region interposed between the upper trench region and the buffer trench region and another convex region interposed between the buffer trench region and the lower trench region. A gate electrode is disposed in the trench structure. A gate dielectric layer is interposed between the gate electrode and the trench structure.