Integrated circuit devices including strained channel regions and methods of forming the same are provided. The integrated circuit devices may include enhancement-mode field effect transistors. The enhancement-mode field effect transistors may include a quantum well channel region having a well thickness TW sufficient to yield a strain-induced splitting of a plurality of equivalent-type electron conduction states therein to respective unequal energy levels including a lowermost energy level associated with a lowermost surface roughness scattering adjacent a surface of the channel region when, the surface is biased into a state of inversion.