Seeing Photons Progress and Limits of Visible and Infrared Sensor Arrays (2010) / Chapter Skim
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2 Fundamentals of Ultraviolet, Visible, and Infrared Detectors
2 Fundamentals of Ultraviolet, Visible, and Infrared Detectors
Pages 23-59
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This electrical signal is processed, usually digitally, transmitted, and/or stored. A two-dimensional array of detectors, called a focal plane array (FPA)
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2003. Extended short wavelength spectral response from InGaAs focal plane arrays.
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Once under the ozone layer, the atmosphere is transparent to wavelengths as short as ~200 nm where oxygen absorption limits the transmission UV 0.28-0.4 Atmosphere is transparent Visible 0.4-0.7 Peak of solar spectrum Near infrared 0.7-1.0 Long-wavelength cutoff defined by silicon detector response SWIR 1.1-2.7 Overlaps with telecommunications wavelengths; large commercial infrastructure available at 1.3 and 1.55 µm MWIR 2.7-6.2 Atmospheric transmission window, molecular vibrational absorptions LWIR 6.2-15.0 Atmospheric transmission window, molecular vibrational absorptions VLWIR 15.0-20.0 Molecular vibrational absorptions NOTE: LWIR = long-wavelength infrared; MWIR = mid-wavelength infrared; VLWIR = very long wavelength infrared.
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Ultraviolet (UV) Visible Near Infrared Short Wavelength Infrared (LWIR)
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1994. High performance infrared detector arrays using thin film microstructures.
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Intensity Intensity, the incident power per unit area, is the most commonly used optical imaging signal. The variation of signal intensity across the focal plane is recorded as a gray-scale image.
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However, it is worthwhile to note that advances in both ultrafast sources and high-speed photon counting detectors will make available in the visible and near-infrared (NIR) spectral regions many of the advanced radar concepts, such as chirped pulses and synthetic imaging concepts, that have been so successful in longer-wavelength spectral regions.
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This is known as an Airy pattern (shown in Figure 2-3) , and the diameter of the central disk is given by ~1.22λ/NA, where λ is the wavelength and NA the numerical aperture of the optical system (the half-angle of the light acceptance cone)
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and, therefore, requires more pixels in the focal plane array if the same area is to 11 J.W. Strutt (III Lord Rayleigh)
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In contrast, the many optical elements in, for example, a standard camera lens are required to faithfully reproduce the image on the flat focal plane of the camera. Our materials technology, which relies on epitaxial crystal growth and the accompanying device fabrication technologies and has largely derived from planar silicon integrated circuit technology, make curved focal plane arrays a difficult option.
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Since these noise sources are in general uncorrelated, the total noise is proportional to the square root of the sum of the squares of the individual noise sources. Photon Statistics and Background-limited Infrared Detection There is noise associated with the signal itself.
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. Generation and Recombination Noise For infrared detectors, the fluctuations in thermally generated carrier densities in the active region also contribute to the noise; this noise source is significant if thermal energies (kT)
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2010. Minority electron unipolar photodetectors based on type II InAs/GaSb/AlSb superlattices for very long wavelength infrared detection.
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, photodetectors and focal plane array imagers operating in this range allow for a number of unique applications -- generally any terrestrial 280 nm radiation can be assumed to arise from man-made sources. Currently, most solar blind imaging is performed with either a photocathode and microchannel plate combination or a UV-enhanced silicon photodiode with a band-pass filter.
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2005. 320x256 solar-blind focal plane arrays based on AlxGa1–xN Applied Physics Letter, 86(1)
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1993. A CCD/CMOS-based imager with integrated focal plane signal processing.
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fundamentals u lt r av i o l e t , v i s i b l e , infrared detectors of and Charge-coupled Device Imagers CCD imagers typically collect and store charge (photoelectrons or holes) generated by incident light under collection electrodes and then sequentially shift the stored charge packets to a readout amplifier to produce a time-dependent output signal containing the image information.
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This feature is typically exploited for time delay-and-integrate (TDI) imagers that move the charge packets at a steady rate in one direction to precisely match the motion of an image slewing across the focal plane.
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Technical Digest for the Gallium Arsenide Integrated Circuit (GaAs IC) Symposium 301-304.
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:181-186. 43 In Ggeiger mode operation of an avalanche photodiode, the bias is sufficiently large that a single incident photon causes an uncontrolled discharge that is not self-limiting.
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2009. Noise analysis and comparison of analog and digital readout integrated circuits for infrared focal plane arrays.
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The presence of other electronics outside the pixel, such as banks of analog-to-digital converters, can also be an issue if the chip is going to be used in a four-side abutted (tiled) arrangement as part of a large mosaic focal plane.
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Avalanche Photodiodes Avalanche photodiodes, mentioned briefly above, are highly sensitive semiconductor electronic devices that use amplification by avalanche processes to enhance the sensitivity for low light levels (see Figure 2-5)
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2009. Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain–bandwidth product.
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2006. Scaling of dark count rate with active area in 1.06 µm photon-counting InGaAsP/InP avalanche photodiodes.
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. Knowledge of these parameters is important to fully characterize and efficiently operate avalanche photodiodes in Geiger mode.64 Thus, it is necessary to quantify the dark count rate, the excess reverse bias voltage, the optimum operating temperature, the photon detection probability, the after-pulsing probability, and the hold-off time.
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These are electron optic systems that are made up of an input phosphor-photocathode screen that converts incoming radiation into a beam of electrons, electrodes to control the movement of electrons, and an output phosphor screen that produces the output image.66 They convert spectral radiation to a visible light image, which after additional processing can be displayed on a monitor. Most commercially available image intensifiers have axial symmetry; however, some nonaxisymmetrial intensifiers have recently been designed.67 Intensifiers work utilizing an avalanche or Geiger mode gain in back of a photocathode.
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Mid-, Long-, and Very Long Wavelength Infrared Brief History of Infrared Detection Thallium sulfide and lead sulfide (or galena) were among the first infrared detector materials, developed during the 1930s.
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2010. "Rule 07" revisited: still a good heuristic predictor of p/n HgCdTe photodiode performance?
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"Rule 07" revisited: still a good per formance heuristic predictor of p/n HgCdTe photodiode performance. Journal of Electronic Materials.
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FIGURE 2-7 HgCdTe dark current from Rule 07 relative to the external radiative limit, corresponding to a cold shield at the device temperature. At MWIR there is relatively little room for improvement at the lowest temperatures; however at other wavelengths there is substantial excess dark current, which limits the 2-7 w-replacement type.eps detector performance for low-background situations.
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2007. Dark current suppression in type II InAs/GaSb superlattice long wavelength infrared photodiodes with M-structure barrier.
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and quantum-dot infrared photodetectors (QDIPs) are unipolar photoconductive devices based on intraband absorption between electronic levels defined by quantum confinement in traditional III-V semiconductors, principally GaAs and InP.
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FABRICATION OF DETECTORS AND FOCAL PLANE ARRAYS Detectors Each material system brings its own unique set of fabrication issues to main tain high performance. Overall the dimensional scale of even visible pixels is large 75A.
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. Focal Plane Arrays A focal plane array is created by arranging individual detector elements in a lattice-like array.
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One simple example is that the silicon industry has standardized on a field size of 22 × 33 mm2 for its lithography tools. The drive to larger pixel counts for FPAs often requires much larger overall FPA sizes, which can only be accomplished by abutting multiple fields, requiring special considerations in the design of the focal plane arrays.
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fundamentals u lt r av i o l e t , v i s i b l e , infrared detectors of and FIGURE 2-9 Material systems for UV-visible-infrared detection. Except for the bottom two entries, these material systems have been known and developed for decades.
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