A quantum well designed to detect infrared (IR) light is called a quantum well infrared photodetector (QWIP). An elegant candidate for QWIP is the square quantum well of basic quantum mechanics. When the quantum well is sufficiently deep and narrow, its energy states are quantized (discrete). The potential depth and width of the well can be adjusted so that it holds only two energy states: a ground state near the well bottom, and a first excited state near the well top. A photon striking the well will excite an electron in the ground state to the first excited state, then an externally-applied voltage sweeps it out producing a photocurrent. Only photons having energies corresponding to the energy separation between the two states are absorbed, resulting in a detector with a sharp absorption spectrum.
Designing a quantum well to detect light of a particular wavelength becomes a simple matter of tailoring the potential depth and width of the well to produce two states separated by the desired photon energy. The GaAs/AlxGal-xAs material system allows the quantum well shape to be tweaked over a range wide enough to enable light detection at wavelengths longer than 6 μm. It is now possible to obtain large uniform QWIPs tuned to detect light at wavelengths from 6 to 25 μm fabricated entirely from large bandgap materials, which are easy to grow and process.