Abstract: A quantum well device has an active region adapted to pass a tunneling current of charge carriers and comprising layers of material forming alternating potential barriers and potential wells. The structure defines, in use, a first potential well at one end of the active region, said first potential well defining a first quasi-defined energy level, and to further define second and third quasi-defined energy levels. The relative heights and thicknesses of the potential barriers when the device is in use are structured so that the first quasi-defined energy level has a longer lifetime than the second and third quasi-defined energy levels and there is a degree of coupling between the three quasi-defined energy levels which is strongest between the second and third quasi-defined energy levels. The transmission coefficient through the active region shows a resonance peak at each of the energies of the three quasi-defined energy levels.