A structure and method for a semiconductor device includes a silicon device layer and a gallium nitride (GaN) device layer. In an embodiment, the silicon device layer and the GaN device layer have upper surfaces which are coplanar with each other. In another embodiment, the GaN device layer does not directly underlie the silicon device layer, and the silicon device layer does not directly underlie the GaN device layer. The semiconductor device can further include a silicon-based semiconductor device formed on and/or within the silicon device layer, and a nitride-based semiconductor device formed on and/or within the GaN device layer. The GaN device layer can include a plurality of layers which can be formed as conformal blanket layers and then planarized, or which can be selectively formed then planarized.