QWIPs belong to the category of so called photon detectors; the absorption of an infrared photon results directly in some specific quantum event, such as the photoelectric emission of electrons from a surface, or electronic inter band transitions in semiconductor materials. Therefore, the output of photon detectors is governed by the rate of absorption of photons and not directly by the photon energy. Photon detectors typically require cooling down to cryogenic temperatures in order to get rid of excessive dark current, but in return their general performance is high. QWIP’s are most often used as photo-conductive detectors. In this type of detectors photo-generated charge carriers increase the conductivity of the device material.
In QWIP’s the quantum wells (QW) are formed by layers of different materials with different band gap. This gives rise to potential-wells for charge carriers in the conduction band as well as in the valence band.
Band diagram of a quantum well
When these layers are sufficiently thin, the energy levels will show confinement; the continuous energy levels found in bulk material become discrete energy levels in these layers. In the following part of the discussion we will only consider the conduction band.
By choosing the right barrier and well materials and layer thicknesses, two energy levels will form in the well. With bias voltage applied the higher level aligns to the edge of the conduction band. This structure can then act as a photon detector: when a photon with the right energy arrives, it will excite an electron from the ground state to the higher state. This electron can freely participate in the charge transport in the conduction band. This will result in a photo current in the detector.
Quantum well with applied bias
However not only photons can excite an electron, phonons can as well. Phonons are quantized vibration modes of the atomic lattice and are generally generated by the temperature of the lattice. For all temperatures above absolute zero, electrons will be excited and contribute to the so called dark current. For a detector responding in the range 7.5-9.0 µm, cooling to temperatures near 70 K is necessary in order to reduce dark currents to a sufficiently low level compared to the photon generated current.
By an appropriate choice of material and design of the quantum wells, the energy levels can be tailored to absorb radiation in the infrared region from 3 to 20 µm. An excellent material combination in this respect is the aluminium gallium arsenide/gallium arsenide (AlGaAs/GaAs) material system. This material system is well known and high quality detectors can be mass produced which is an advantage compared to other more difficult material systems commonly used in manufacturing of cooled photon detectors.