If one makes a heterostructure with sufficiently thin layers, quantum interference effects begin to appear prominently in the motion of the electrons. The simplest structure in which these may be observed is a quantum well, which simply consists of a thin layer of a narrower-gap semiconductor between thicker layers of a wider-gap material . The band profile then shows a ``rectangular well,'' as illustrated in Fig. 11.
Figure 11: Energy-band profile of a structure containing three quantum wells, showing the confined states in each well. The structure consists of GaAs wells of thickness 11, 8, and 5 nm in AlGaAs barrier layers. The gaps in the lines indicating the confined state energies show the locations of nodes of the corresponding wavefunctions.
The electron wavefunctions in such a well consist of a series of standing waves, such as might be found in a resonant cavity in acoustic, optical or microwave technologies. The energy separation between these stationary states is enhanced by the small effective mass of electrons in the conduction bands of direct-gap semiconductors. With advanced epitaxial techniques, the potential profile of the quantum well need not be rectangular. Because the band-edge energy is usually linear in the composition, will follow the functional form of the composition. The quantum states in two parabolic wells  are illustrated in Fig. 12.
Figure 12: Energy band profile of a structure containing two parabolic quantum wells. The composition is similar to that of Fig. 5, and the overall width of the wells are 20 and 8 nm.
Quantum well heterostructures are key components of many optoelectronic devices, because they can increase the strength of electro-optical interactions by confining the carriers to small regions [1,2].