The present disclosure relates to a transistor device. In some embodiments, the transistor device has an epitaxial layer disposed over a substrate. The epitaxial layer is arranged between a source region and a drain region separated along a first direction. Isolation structures are arranged on opposite sides of the epitaxial layer along a second direction, perpendicular to the first direction. A gate dielectric layer is disposed over the epitaxial layer, and a conductive gate electrode is disposed over the gate dielectric layer. The epitaxial layer overlying the substrate improves the surface roughness of the substrate, thereby improving transistor device performance.