Detector materials

E. E. HaUer, H. W. Kraner, and W. A. Higinbotham, eds. MuclearRadiation Detector Materials, North-HoUand, New York, 1983. 

Dark current comes from the thermal excitation of electrons in the detector material - thermally generated electrons can not be distinguished from photoelectrons. 

Figure 3. Optical and infrared detector zoology . The wavelength region is stated on the first row, with corresponding wavelength (in /rm) shown on the second row. The type of detector material and associated manufacturers are shown in the boxes below, which also depict the wavelength coverage possible with each kind of detector material.

Since IR detector materials are direct bandgap materials (with no change in electron momentum required), they are very efficient absorbers (and emitters) of light - all IR photons are absorbed within the first few /rm of material. The reason that infrared detectors are 10 to 15 ptm thick is for structural and fabrication reasons, not for light absorption reasons. 

M. Hage-Ali and P. Siffert, Growth Methods of CdTe Nuclear Detector Materials M. Hage-Ali and P Siffert, Characterization of CdTe Nuclear Detector Materials M. Hage-Ali and P. Siffert, CdTe Nuclear Detectors and Applications... 

A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

Deuterated triglycine sulfate (abbreviated DTGS) is a common ferroelectric infrared detector material. Explain how it works. 

Ionization in a Solid (Semiconductor Detectors) In a semiconductor radiation detector, incident radiation interacts with the detector material, a semiconductor such as Si or Ge, to create hole-electron pairs. These hole-electron pairs are collected by charged electrodes with the electrons migrating to the positive electrode... 

Figure 3.15 Response curves of various detector materials. Reprinted with kind permission of Prentice-Hall International.

The output bias contact 107 is shaped to concentrate an electric field bias in the immediate vicinity of the contact 107. This concentrated field sweeps away minority carriers which otherwise accumulate near the output contact. It is proposed that the contact 107 is shaped by extending it towards input bias contact 105, or that the detector material 103 near this contact 107 is configured by slotting or tapering, or that an annular ring input bias contact surrounds a circular disc output contact. 

The length of the detector element 3 or the magnitudes of bias and scan velocity are selected so that the time taken to scan the detector element 3 from one end 5 to a read-out region 9 of the detector element 3 is greater than the lifetime of the photocarriers generated in the element 3. In order to avoid loss of resolution by photocarrier diffusion the photocarrier lifetime of the detector material has of necessity a relatively low value. 

Detector elements having three layer bonding pads are disclosed in JP-A-63148677. This structure allows bonding of the elements without damaging the HgCdTe detector material. 

Parallel elongate cavities 13 and a layer of optical isolating material 18, surrounding each of the cavities, are formed within a substrate 10 of CdTe. The cavities each have a vertical wall 15 and a pyramidal floor 16. The wall and floor of each cavity has a body of detector material, HgCdTe, formed as a layer thereon. The body of detector material is comprised of a layer of a first type 22 and a layer of a second type 24. Individual electrical contacts 28 and a common electrical contact 29 are provided. An insulating layer 32 insulates the common contact from the substrate. A diffusion layer 34 of semiconductor material of the second type provides electrical communication between the common contact and the material of the second type formed in the cavity. The cavities may have cylindrical walls and a round floor. 

Placing detectors on a focal plane face of a supporting module is very demanding. The goal is to end up with thin diode chips attached to the end of the module and in electric contact with lead points on the module. In US-A-4290844 a wafer of a detector material is secured... 

One problem which arises when a detector array is attached to the face of a multi-layer module is the inability of the detector material to absorb forces generated by a mismatch of coefficient of thermal expansion between the detector array material and the module. Furthermore, it is difficult to isolate a fault that may be attributable to either the detector elements, module wiring or processing elements. 

The problem stated above is solved in FR-A-248470S by removing the detector material except for islands which form the detector elements. 

The difference of thermal expansion coefficient between an HgCdTe detector material and a silicon read-out material is dealt with in FR-A-2494910 where detector elements in the shape of islands are connected on one side to a silicon read-out chip and on the other side to a substrate having a similar thermal expansion coefficient as the silicon read-out chip. 

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