1. A method of fabricating a semiconductor device including a channel layer, the method comprising: forming a single crystalline silicon layer on a single crystalline silicon substrate, the single crystalline silicon layer having a protrusion extending from a surface thereof; forming a sacrificial layer on the single crystalline silicon layer; removing a portion of the sacrificial layer and a portion of the protrusion of the single crystalline silicon layer in a first polishing process to define a sacrificial layer pattern and a remaining portion of the protrusion; removing the sacrificial layer pattern; and then planarizing the surface of the single crystalline silicon layer in a second polishing process to remove the remaining portion of the protrusion and define a substantially planar silicon channel layer.