QUANTUM ELECTRONICS --- QUANTUM WELL INFRA RED PHOTODETECTORS
A quantum well infrared photo detector (QWIP) is an infrared photo detector 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.
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, and 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 pm. Fabricated entirely from large
bandgap materials which are easy to grow and process, it is now possible to
obtain large uniform FPAs of QWIPs tuned to detect light at wavelengths from 6
to 25 pm in the GaAs/AlxGal-xAs material system.
Ziyad Alamri
Post #5
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