Quantum well infrared photodetector

A quantum well infrared photodetector (QWIP), is an infrared photodetector made from semiconductor materials which contain one or more quantum wells. These can be integrated together with electronics and optics to make infrared cameras for thermography. A very common well material is gallium arsenide, used with barrier material aluminium gallium arsenide. There are several companies world-wide manufacturing infrared systems which use QWIPs. It is also considered a game changing technology in the field of air to air missiles (within visual range).

Detection description
QWIPs operate by photoexcitation of electrons between ground and first excited state subbands of multi-quantum wells which are artificially fabricated by placing thin layers of two different, high-bandgap semiconductor materials alternately. The bandgap discontinuity of two materials creates quantized subbands in the potential wells associated with conduction bands or valence bands. The structure parameters are designed so that the photo-excited carriers can escape from the potential wells and be collected as photocurrent.

The lattice matched GaAs/AlGaAs material system is commonly used to create a QWIP structure. Highly uniform and pure crystal layers of such semiconductors can be grown on large substrate wafers, with control of each layer thickness down to a fraction of a molecular layer, using modern crystal-growth methods like molecular beam epitaxy (MBE). Thus, by controlling the quantum well width and the barrier height (which depends on the Al molar ratio of the alloy), this intersubband transition energy can be varied over a wide enough range to enable light detection at any wavelength range between 6-20 µm (infrared).

Spectral responsivity peak wavelength of a QWIP is determined by the energy difference between ground and excited states of the quantum well. Unlike the responsivity spectra of intrinsic infrared detectors, QWIPs spectra are much narrower and sharper due to their resonance intersubband absorption. Typically, responsivity spectra of the bound and quasibound excited state QWIPs are much narrower than the continuum QWIPs. This is because when the excited state is placed in the continuum band above the barrier the energy width associated with the state becomes wide. Spectral band width of these QWIPs can be further increased by replacing single quantum wells with small superlattice structures (several quantum wells separated by thin barriers) in the multi-quantum well structure. Such a scheme creates an excited state miniband due to overlap of the excited state wavefunctions of quantum wells.

The dominant noise in QWIP devices is due to the shot noise resulting from the total current in the device. Unlike conventional detectors, there is no significant thermal noise in QWIPs. In addition QWIP have good noise stability (low 1/f noise), enabling long integration times and scanning strategies commonly required in focal plane array imaging applications.

References and external links

 * NASA qwip research
 * Acreo