Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-03-23
2002-07-16
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S225000
Reexamination Certificate
active
06420728
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to semiconductor III-V alloy compounds, and more specifically to multispectral QWIPS.
BACKGROUND OF THE INVENTION
The importance of semiconductor detectors is rapidly increasing along with progress in other opto-electronic fields, such as optical fiber communication, charge-coupled devices, and solid state lasers.
Thermal detectors and thermal imaging detector arrays are based on the local detection of infrared radiation emitted by a scene, and in the case of an imaging detector array allows the representation of the scene's objects by the thermal gradients. This is because the infrared energy radiated by an object is proportional to its absolute temperature T
0
and its emissivity &egr;
0
. On the other hand, sources of stray infrared radiation (background, sky, Sun, other objects, etc.) having temperatures T
1
, T
2
, T
3
, etc. and emissivities &egr;
1
,&egr;
2
, &egr;
3
, and so on, can add, after reflection, a disturbing radiative contribution to the intrinsic emission and reflectance of the object being imaged. The infrared detector or imager is sensitive to the sum of these reflected and emitted energies.
A multi-spectral infrared deflector is defined as a detector which is sensitive to more than one band of wavelengths of infrared radiation (usually infrared radiation is defined as wavelengths &lgr; in the range between &lgr;=1 &mgr;m and &lgr;=50 &mgr;m). Each band of wavelengths has a cut-on and cut-off wavelength, but is most commonly defined by a peak wavelength &lgr;
p
and a bandwidth &Dgr;&lgr;.
In the most favorable measurement case there are no stray reflections and the object is assumed to have a constant emissivity within the spectral intervals &Dgr;&lgr;1 and &Dgr;&lgr;2 (gray body). Then the energy emitted by the object itself depends only on the two quantities &egr;
0
and T
0
. However, a measurement in a single spectral band gives only one relationship and it is not possible to solve one equation for the two unknown variables. In this case the emissivity must be estimated or calibrated externally to the measurement. On the other hand, a thermal measurement system operating in two wavelength bands allows the setting up of two relationships (for the two unknowns) which ten determine the temperature T
0
and emissivity &egr;
0
of the object. Thus only a bispectral measurement can give fast and accurate remote access to the thermal characteristics of an unknown object when the latter radiates as a gray body. Multi-spectral imaging (more then two wavelength bands detected) can be used to eliminate the effects of stray reflections.
Bispectral or multi-spectral imaging can provide information about the relative effects of emissivity and temperature when &egr;
0
is not constant, but is instead a function of wavelength. It is also possible to image or measure the temperature of two thermally different objects that would be indistinguishable ins single-band observation. In this case, if the two objects produce the same radiance in a spectral band &Dgr;&lgr;
2
. Thus, the thermal constant between the two objects in the band &Dgr;&lgr;
2
can be used to increase the contrast of the overall image.
Practical applications of multi-spectral detectors include infrared “heat seeking” missiles which use infrared imaging arrays to locate and track the movement of the missile's target. Often these targets are equipped with infrared countermeasures. These can include small decoys, such as flares or chafe, or powered decoy vehicles that can be deployed by an aircraft or warship to lure away the heat seeking missile. Countermeasures can also include infrared lamps and lasers intended to overload (blind) or burn the missile's infrared detector. Decoys usually lure away the missile by providing a brighter (stronger) infrared signal at the wavelength to which the missile's detector is most sensitive. In either case, a bispectral or multi-spectral detector is less likely to fail when opposed by such countermeasures. It can continue to track the target even if one waveband has been blinded/burned by a laser. And since most decoys cannot match the temperature and emissivity signature of the target across several wavebands, the spectral “fingerprint” of the target can be used to ignore decoy countermeasures.
When analyzing infrared images taken by a multi-spectral detector, one image is first generated for each waveband. In order to combine the images to improve the contrast, or to compute the temperature and emissivity, the signal for each pixel in the images must have come from the same source. If more than one detector is used, each detector will view a slightly different scene (parallax). This makes the pixel-to-pixel registration of the images computationally difficult and time consuming. For this reason monolithically integrated detectors, wherein the detectors are stacked on top of each other, are desirable.
SUMMARY OF THE INVENTION
An object, therefore, of the invention is a QWIP (Quantum Well Infrared Photodetector) consisting of a multiple quantum well structure grown on laP substrate for use as a sensor having multi-spectral detection.
A further object of the subject invention is a two-color QWIP structure for detection of two wavelengths in the range 3<&lgr;<5 microns.
A further object of the subject invention is a two-color QWIP structure for detection of two wavelengths, one in the range 3<&lgr;<5 and one in the range 8<&lgr;<12 microns.
A further object of the subject invention is a multi-spectral QWIP structure for simultaneous three-color detection of wavelengths in the ranges 3<&lgr;
1
<5 microns, 8<&lgr;
2
<12 microns.
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K.L. Tsai, et al., “Two-Dimensional Bi-Periodic Grating Coupled One- and Two-Color Quantum Well Infrared Photodetectors”, IEEE Electron Device Letters, vol. 16, No. 2, Feb. 1995, pp. 49-51.
Le Bau T
Nelms David
Welsh & Katz Ltd.
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