Infrared Detectors
Practical single element infrared detectors were developed
during World War II by the German military from a lead salt compound (PbS). This
basic technology, much refined, is still in use today. During the ensuing years,
the major application of infrared detectors has continued to be primarily
military, for applications in weapon guidance and surveillance systems. The
requirements of these specialized applications have pushed the development of
improved detectors with high responsivity and detectivity. 
Atmospheric absorption of infrared
energy generally limits the useful band of infrared detectors to either the 3 to
5µm band or the 8 to 12 µm bands. Therefore, most research has been in
optimizing the detector response in these two bands.
Over the past 25 years, the availability of high performance infrared
detectors has spurred commercial applications in spectrometry, protein analysis,
fire detection systems, preventative maintenance, process control, and
astrophysical studies. Much of the recent growth is centered around
environmental applications, such as pollution detection and medical
applications, such as blood analysis.
Today's detectors range in format from single element, uncooled
detectors to specialized multi-spectral, staring array detectors. There
are two main classes of infrared detectors with several types within each class.
Selection of a specific detector depends on the wave band of interest, the sensitivity
required and cost constraints. EOI can provide
testing equipment to test and characterize all infrared detectors.
Thermal Type
The main features of thermal type infrared detectors include
responsivity with little dependence on wavelength and operation at room
temperature. However, the response speed and detectivity are lower than
the quantum type.
Thermopiles are the oldest type of infrared detectors and utilize
thermo-electromotive force generated between two different types of
conductors. Thermopiles are made from both metals and semiconductors.
These are resistors which change in resistance with incident
infrared radiation. Bolometer arrays have become the
focus of most uncooled detector development.
There are two detectors: Golay cells and capacitor microphones.
In Golay cells, the sealed xenon gas expands when it is warmed by
incident infrared radiation. The resultant variation of pressure shifts
a mirror located between a light source and a photocell, varying the
amount of light entering the photocell and thus changing the output of
the photocell.
In capacitor microphones, the varying expansion of the gas affects the
capacitor film, which in turn produces the variation in the electrostatic
capacity.
When infrared radiation is incident, temperature changes are
generated in the crystal. An electric charge is then generated on the
surface of the crystal in accordance with the amount of temperature
variation.
Pyroelectric detectors are current sources with an output proportional
to the rate of change of its temperature. Capable of extremely rapid
response and insensitive to DC effects, they are frequently used in
industrial radiometric systems and in the analysis of infrared lasers.
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Quantum Type
Quantum type detectors feature high detectivity and fast response
speed. Responsivity is wavelength dependent and, except for detectors in the
near infrared range, cooling is normally used with these detectors. Quantum
type detectors are further classified into intrinsic types and extrinsic
types.
Intrinsic type detectors have detection wavelength limits
determined by their inherent energy gap and responsivity drops drastically
when the wavelength limit is exceeded. Among them. the photoconductive
detectors, which change their conductivity when infrared radiation is
incident, have high responsivity and allow simple signal processing. The
photovoltaic detectors generate an electric current when infrared
radiation is incident and have high responsivity and a fast response
speed. 
HgCdTe or PbSnTe detectors are also included in the intrinsic type
detectors. The wavelength of peak responsivity of these detectors can be
changed by controlling the composition of the ternary mixture.
In particular, the HgCdTe detectors are useful since they respond to
wavelengths in the 3 to 5 µm and 7 to 13 µm ranges.
These are photoconductive detectors whose
wavelength limits are determined by the level of impurities doped in high
concentrations to the Ge or Si semiconductors. The biggest difference
between intrinsic type detectors and extrinsic type detectors is the
operating temperatures. Extrinsic type detectors must be cooled down to
the temperature of liquid helium.
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Typical Detector
Responsivity
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