Embodiments herein describe techniques, systems, and method for a semiconductor device. A semiconductor device may include isolation areas above a substrate to form a trench between the isolation areas. A first buffer layer is over the substrate, in contact with the substrate, and within the trench. A second buffer layer is within the trench over the first buffer layer, and in contact with the first buffer layer. A channel area is above the first buffer layer, above a portion of the second buffer layer that are below a source area or a drain area, and without being vertically above a portion of the second buffer layer. In addition, the source area or the drain area is above the second buffer layer, in contact with the second buffer layer, and adjacent to the channel area. Other embodiments may be described and/or claimed.