Abstract: A semiconductor heterostructure useful as a photodetector in the far infrared. The barrier layers of the heterostructure are doped os that charge carriers migrate from the energy bands of the barrier layers towards the energy bands of the quantum wells, but remain weakly bound to the doping impurities in the barrier layers. Because of weak residual bonding, the energy necessary to raise these electrons fully into the quantum wells' energy band is significantly reduced, extending the lower frequency range at which such devices are useful as photodetectors. Selection of several of the heterostructure's dimensions determine the impurities' resonant absorption frequency, and application of an electric or magnetic field shifts the well's resonant absorption frequency, in effect frequency fine tuning the heterostructure.