Abstract:
An infrared detector array includes a plurality of detector pixel structures, each having a plurality of coplanar sections responsive to different bands of infrared radiations. Each section of a pixel structure comprises a plurality of elongate quantum well infrared radiation absorbing photoconductor (QWIP) elements. The group of QWIP elements are spaced such that they comprise a diffraction grating for the received infrared radiation. Top and bottom longitudinal contacts are provided on opposite surfaces of the QWIP elongate elements to provide current flow transverse to the axis of the element to provide the required bias voltage. An infrared radiation reflector is provided to form an optical cavity for receiving infrared radiation. A plurality of detector pixel structures are combined to form a focal plane array. Each pixel structure section produces a signal that is transmitted through a conductive bump to a terminal of a read out integrated circuit. The group of signals from the detector pixel structures produces a multi-band (color) image corresponding to the received infrared radiation.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention pertains in general to infrared detectors and in particular to such detectors which are responsive to multiple bands of infrared radiation. 
     BACKGROUND OF THE INVENTION 
     Infrared detectors are used for collecting image information under conditions which do not allow generally conventional optical observation, such as at night or through clouds, haze or dust. The information gathered within infrared imaging can be enhanced if multiple bands of infrared radiation can be collected concurrently. This is much like color in an optical image. Infrared radiation in different bands can be indicative of different elements in a scene, such as different materials, reflectivity, temperatures and so forth. Therefore, for optimum viewing through use of infrared radiation, it is desired to have a sensor capable of concurrently detecting multiple bands of infrared radiation. 
     Multi-band infrared sensing has been performed with detectors of different types. Semiconductor infrared detectors are becoming among the most widely used, and for observing scenes, a well known type is referred to as a “staring array.” One configuration for a multi-band staring array detects each of the different bands of radiation with a planar layer of photosensitive material with the multiple layers in a stack wherein all of the upper layers are transparent to the radiation absorbed by the lower layers. 
     The present invention is directed to a configuration for an infrared detector which has a unique configuration that is particularly adaptable for use with multi-quantum well (MQW) photosensitive detection material. 
     SUMMARY OF THE INVENTION 
     A selected embodiment of the present invention is a multi-band quantum well infrared photodetector which has a pixel structure that includes a plurality of co-planar sections, with each section being responsive to a respective, different band of infrared radiation. Each section of the pixel structure has a plurality of elongate, multiple quantum well infrared radiation absorbing elements. Each of the elements has first and second opposite longitudinal surfaces. The elements in each section have a physical configuration which includes a periodic spacing dimension for the elements and a width dimension for the elements. Each of the sections has a respective, different configuration. The multiple quantum well elements comprise a diffraction grating for the infrared radiation. A first contact includes a plurality of planar, electrically interconnected strips respectively in contact with and extending along the first surfaces of the multiple quantum well elements. A plurality of second contacts are respectively located in each of the sections with the second contact in each section electrically connected to the second surfaces of the multiple quantum well elements in the corresponding one of the sections. The first and second contacts are positioned on opposite longitudinal sides of each of said multiple quantum well elements to provide current flow through said elements in a direction substantially transverse to the axis of the elements. A planar reflector is provided for the infrared radiation. The reflector is positioned on an opposite side of the second contacts from the multiple quantum well elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a section, elevation view of a prior art QWIP having a metal surface relief diffraction grating and GaAs substrate; 
     FIG. 2 is a prior art QWIP having a metal surface relief diffraction grating; 
     FIGS. 3A,  3 B,  3 C and  3 D are section, elevation views illustrating a sequence of fabrication steps for producing a structure for the present invention; 
     FIGS. 4A,  4 B and  4 C are section, elevation views showing multiple embodiments of enhanced QWIPs (EQWIP) each consisting of a group of multiple quantum well elements forming a diffraction grating for infrared radiation; 
     FIG. 5 is a perspective, partially sectioned, view of a multi-pixel, focal plane array with unpolarized two dimensional grating EQWIP elements hybridized to a silicon readout integrated circuit (ROIC) in accordance with the present invention; 
     FIG. 6 is a perspective, partially sectioned view of a further embodiment of the present invention, similar to that shown in FIG. 5, but having polarized one-dimension grating EQWIP elements; 
     FIG. 7 is a planar view of a further pixel structure configuration in accordance with the present invention; 
     FIG. 8 is a planar view of a still further pixel structure configuration (elongate) in accordance with the present invention; 
     FIG. 9 is a still further pixel structure configuration in accordance with the patent invention. 
     FIG. 10 is a planar view of a focal plane array utilizing a large number of detector pixel structures in accordance with the present invention; and 
     FIG. 11 is a block and schematic illustration of an electrical circuit for each EQWIP pixel structure in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a photodetector in which a single pixel has a structure that is responsive to multiple bands of infrared radiation. This pixel structure utilizes photodetector material and a preferred type of such material is termed multiple quantum well (MQW). A structure which utilizes this is often termed a Quantum-Well Infrared Photodetector (QWIP). 
     Designs for QWIPs are presented in “Quantum-Well Infrared Photodetectors,” J. Appl. Phys. 74 (8), Oct. 15, 1993, by B. F. Levine. Detector structures with metal gratings are shown in FIG. 142 of this article. These are shown in FIGS. 1 and 2 herein. 
     Referring to FIG. 1, a QWIP  20  receives incident mode infrared radiation as indicated by arrow  22 . Reflected zero order diffractive mode radiation is indicated by arrows  24 . Trapped higher order diffractive mode radiation is indicated by arrows  26 . The QWIP  20  includes a GaAs substrate  30  which is joined to an AlAs reflector  32 . An n + GaAs contact  34  is joined to the reflector  32  and to an MQW region  36  comprising a group of layers. A metal diffractive grating  38  is joined to the MQW region. Because of quantum mechanical selection rules an MQW only absorbs radiation through modes in which a component of the electric field is perpendicular to the MQW layers in the MQW region  36 . The purpose of the grating  38  is to increase the absorption of radiation by diffracting that radiation, as indicated by arrow  22 , by the grating  38  to produce trapped diffracting radiation modes shown by the arrows  26  with electric field component perpendicular to the MQW region. The arrows  26  represent trapped radiation. Only the radiation reflected normal to the surface of the QWIP  20 , as shown by arrows  24 , is lost by the QWIP  20 . Thus, the addition of the metal grating  38  increases the absorption of infrared radiation by the QWIP  20 . 
     A further prior art QWIP configuration is shown in FIG. 2. A QWIP  40 , similar to that shown in FIG. 1, has incident mode radiation indicated by arrow  22 , reflected zero order diffractive mode radiation indicated by arrows  24  and trapped higher order diffractive mode radiation indicated by arrows  26 . The QWIP  40  includes a thin GaAs substrate  42 , an n +  contact  44 , MQW region  46  and a metal grating  48 . 
     For LWIR applications, QWIPs  36  and  46  shown in FIGS. 1 and 2 each comprise approximately a 50 period GaAs/Al x Ga 1−x As MQW with 40-50 A GaAs wells doped n-type with a doping density of N D ≈(0.7-1.5)×10 18  cm −3 , and 300-500 A undoped Al x Ga 1−x As barrier. The Aluminum to Gallium alloy ratio, x, is typically 0.26-0.29 for LWIR application. The MQW region  36  and  46  includes an MQW sandwiched between two highly doped n-type (N D ≈2×10 18  cm −3 ) n + GaAs contact layers with typical thicknesses of 0.5-1 μm. The QWIP structure is epitaxially grown on a lattice matched GaAs substrate. 
     A sequence of processing steps for fabricating a detector in accordance with the present invention is shown in FIGS. 3A,  3 B,  3 C and  3 D. Referring to FIG. 3A, a detector  60 , in accordance with the present invention, is fabricated on a GaAs substrate  62  with a thickness of approximately 635 μm. An Al .6 Ga .4 As etch stop layer  64  is formed on the surface of the substrate  62 . The layer  64  has a preferable thickness of 1,000 Å. 
     A conductive n + GaAs contact  66  is formed on the etch stop layer  64 . The contact  66  has a preferable thickness of 0.5 μm. A multiple quantum well (MQW) structure  68  is formed over the contact  66 . Structure  68  comprises a plurality of material layers which make up the MQW. The detailed characteristics of structure  68  are shown in Table I below. An n + GaAs contact  70  is formed on the upper surface of the structure  68 . Contact  70  has a preferred thickness of 0.4 μm. 
     The doping for contacts  66  and  70  is shown in Table I. 
     Referring to FIG. 3B, openings  76  and  78  are formed by etching through the contact  70  and MQW structure  68 . 
     Referring to FIG. 3B, the detector  60  has an etched, planar Au/Ge ohmic contact  74  formed on the surface of the contact  70 . An Au contact/reflector  72 , which is a reflector for infrared radiation, is formed on the surface of contact  70  and contact  74 . 
     Referring to FIG. 3C, an In (indium) bump  80  is bonded to the reflector  72  and to a contact terminal (not shown) of a silicon readout integrated circuit (ROIC)  81 . The bump  80  is formed by a photoresist image reversal process followed by In layer deposition and liftoff. A wicked epoxy  83  provides a physical bond between the ROIC  81 , the bump  80  and the remaining exposed structure of the detector  60 . 
     A representative direct injection ROIC structure for use with the present invention is described in “The Infrared &amp; Electro-Optical Systems Handbook,” Volume 3, Electro-Optical Components, William D. Rogatto, Editor, Chapter 5, “Readout Electronics for Infrared Sensors,” John L. Vampole, ERIM/SPIE, 1993 (see section 5.6.6). 
     Further processing of the detector  60  is shown in FIG.  3 C. The structure shown in FIG. 3C is inverted from that shown in FIG.  3 B. The bulk of the substrate  62  is removed by lapping and the remainder is removed by etching down to the etch stop layer  64 . The layer  64  itself is then removed. Contact  66  is thinned from 0.5 μm to 0.4 μm. 
     Referring to FIG. 3D, a conventional resist mask is applied to the surface of the layer  66  and an etch operation is performed. The etch extends through the layer  66 , the MQW structure  68  down to the contact  70 . This produces elongate structures  82 ,  84 ,  86 ,  88 ,  90  and  92 . Elongate structure  84  comprises a contact  66 A over an elongate MQW element  68 A. In a similar fashion, elongate structure  86  comprises a contact  66 B which is in physical and electrical contact with an elongate MQW element  68 B, structure  88  comprises a contact  66 C over an MQW element  68 C and structure  90  comprises a contact  66 D over an MQW element  68 D. The elements  84 ,  86 ,  88  and  90  are spaced in such a manner as to form a diffraction grating for the infrared radiation to be received by the detector  60 . The MQW elements  68 A-D are each infrared radiation absorbers. 
     The etching process shown in FIG. 3D also produces structures  82  and  92 . The structure  82  comprises an n + GaAs conductor  66 E above a region of the epoxy  83 . The structure  92  comprises an n + GaAs conductor  66 F which is supported by a region of the epoxy  83 . 
     The structures  84 ,  86 ,  88  and  90  each comprise a quadrant of one pixel in accordance with the present invention. Variable parameters in this configuration include the center-to-center spacing between the photodetector structures as well as the width of each individual one of the structures. A different configuration is produced by varying one or both of these parameters. Changes in the configuration change the band of infrared radiation which is absorbed by the structure. In the present invention, one pixel comprises four sections, each having a different configuration of dimensions. These dimensions are selected such that the pixel is responsive to the desired bands of infrared radiation. 
     The Figures are not necessarily drawn to scale, but are illustrated for best understanding. 
     Referring to FIG. 4A, there is shown a dimensioned layout for a detector  100  in accordance with the present invention. This represents one section of a pixel structure within an array of such pixel structures. The detector  100  includes elements  102 ,  104 ,  106 ,  108  and  110  which are formed in the same manner as the elements  84 ,  86 ,  88  and  90  shown in FIG.  3 D. Each of the elements includes a top elongate n + GaAs contact, such as  102 A, an elongate MQW element  102 B and a flat planar n + GaAs contact  112  which is connected to the bottom surface of MQW elements for each of the elements  102 - 110 . A metal contact/reflector  114  made of Au and having a thickness of 2,000 Å is formed on the opposite side of the contact  112  from the MQW elements such as  102 B. 
     An ohmic contact  113  is a rectangular, planar structure comprising Au/Ge and having a thickness of approximately 900 Å and dimensions of 5 μm by 10 μm. The ohmic contact  113  is fabricated on the surface of contact  112  before the formation of reflector  114 . The ohmic contact  113  provides a good ohmic connection between the contact  112  and the reflector  114 . 
     A further embodiment of a detector  120  in accordance with the present invention is shown in FIG.  4 B. This is essentially the same as the detector  100  shown in FIG. 4A, but the etching operation which formed the elements  102 - 110 , is continued until the etching extends through a portion of the lower contact  112 , but not the entire thickness of the contact  112 . This etching operation produces elements  122 ,  124 ,  126 ,  128  and  130 . The width and spacing of these elements is the same as the width and spacing of the elements  102 - 110  shown in FIG.  4 A. The element  122 , as an example for all of the elements in detector  120 , includes an elongate n + GaAs contact  122 A which is in physical and electrical contact with the upper surface of the elongate MQW element  122 B and a lower elongate n + GaAs portion of the contact  112 . The patterned contact  112  is positioned on the surface of a metal reflector  132 , which is essentially the same as the reflector  114  shown in FIG. 4A. A planar ohmic contact  134 , corresponding to the ohmic contact  113  shown in FIG. 4A, is formed on the n + GaAs contact  112  at a position to the lower right of the element  130 . The contact/reflector  132  is then formed. The ohmic contact  134  provides a good ohmic connection between the contact  112  and the reflector  132 . 
     A further embodiment of a detector  120  in accordance with the present invention is shown in FIG.  4 C. This is the same as the detector  100  shown in FIG. 4A but the etching operation which formed the elements  102 - 110  is continued until the etching extends through the lower contact  112  except for one region. A region  112 A is not removed. This etching produces elements  138 ,  139 ,  140 ,  141  and  142 . The width and spacing of these elements is approximately the same as the width and spacing of the elements  102 - 110  shown in FIG.  4 A. The sensitivity of a detector to a given wavelength of infrared radiation is related to the width and spacing of the MQW elements. The element  138 , as an example of all of the elements, includes an elongate upper n + GaAs contact  138 A in physical and electrical contact with the upper surface of an elongate MQW element  138 B and a lower elongate n + GaAs contact  138 C is connected to the lower surface of the MQW element  138 B. The contact  138 C and the corresponding contacts on the other elements, are positioned on the surface of a metal contact/reflector  137 , which is essentially the same as the reflector  114  shown in FIG. 4A. A planar ohmic contact  143 , corresponding to the ohmic contact  113  shown in FIG. 4A, is formed on the n + GaAs contact  112  at the position of the region  112 A. The contact/reflector  137  is then formed. The ohmic contact  143  provides a good ohmic connection between the region  112 A, which is connected electrically to all of the other lower contacts, such as  138 C, and the reflector  137 . 
     FIGS. 4A,  4 B and  4 C represent section views of the elongate elements  102 - 110 ,  122 - 130  and  138 - 142 . 
     Referring now to FIG. 5, there is shown a focal plane detector array  150  which includes a pixel structure  153 . The array  150  has a plurality of such pixel structures, arranged in an array. A view of the detector array  150  as indicated by the arrows  152  corresponds substantially to the views shown in FIGS. 3D and 4A. The pixel structure  153  includes detector pixel structure sections  154 ,  156 ,  158  and  160 . Pixel structure sections  162  and  164  are in adjacent pixel structures. Each pixel structure section  154 - 164  has a corresponding In bump such as bump  166  for pixel structure section  154 , bump  168  for pixel structure section  158 , bump  170  for pixel structure section  162  and bump  172  for pixel structure section  164 . Each of the bumps is connected to a corresponding terminal (not shown) of a ROIC  180 . A wicked epoxy  182  bonds the pixel structure sections  154 - 164  to the ROIC  180 . 
     The pixel structure section  158  is described in detail. The remaining pixel structure sections have a similar configuration. The entire top surface of the detector array  150  comprises an upper array n + GaAs contact  186  which has been etched to have elongate (strip) segments including  186 A and  186 B which extend between wider elongate n + GaAs conductors  186 C and  186 D. Elongate segments in pixel structure section  158  include segments  186 E,  186 F,  186 G and  186 H. The segments  186 E- 186 H are each connected at a common end to a wide n + GaAs conductor  186 J and a similar conductor (not shown) at the opposite side of the pixel structure section  158 . The contact segments  186 A and  186 B are transverse to the contact segments  186 E,  186 F,  186 G and  186 H. Contact segments  186 E- 186 H correspond to contacts  66 A,  66 B,  66 C and  66 D in FIG.  3 D. The n + GaAs conductors  186 C and  186 D correspond to conductors  66 E and  66 F in FIG.  3 D. 
     Each of the pixel structure sections  154 - 164  has a contact/reflector for electrically connecting the lower contact of the pixel to the corresponding In bump. 
     The region between the bumps is filled with wicked epoxy  182 , such as shown in FIGS. 3C and 3D. 
     In each of the pixel structure sections  154 - 164  immediately below the n + GaAs contact  186  is a diffractive MQW structure  190  which has a segment beneath each of the contact  186  segments. This is essentially the same as shown in FIGS. 3D and 4A. For example, MQW element  190 A is immediately below contact  186 A. 
     A planar contact is formed in a position between the MQW structure  190  and a contact/reflector. The pixel structure sections  154 ,  156 ,  158 ,  160 ,  162  and  164  have respective lower (second) contacts  194 A,  194 B,  194 C,  194 D,  194 E and  194 F. 
     A contact/reflector  196 A is in a lower portion of pixel structure section  154 . Reflector  196 A is physically and electrically in contact with bump  166 . A contact/reflector  196 B is in the lower portion of pixel structure section  158  and is in electrical contact with the bump  168 . A contact/reflector  196 C is in the lower portion of pixel structure section  162  and a contact/reflector  196 D is in the lower portion of pixel structure section  164 . The contacts/reflectors are made preferably of Au and have a preferred thickness of 2,000 Å. Reflectors  196 A- 196 D correspond to reflector  72  in FIG.  3 D. 
     Slots  210 ,  212 ,  214 ,  216  and  218  are etched into the structure of detector array  150  to electrically isolate each pixel structure section. Each of these slots extends upward from the region occupied by the bumps up to the lower surface of the contact  186 . These slots are filled with the epoxy  182 , which is electrically nonconductive. 
     The dimensions and corresponding infrared radiation wavelength bands for each section of the pixel structure  153  is listed as follows: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Band Center 
               
               
                 Section 
                 Spacing (micron) 
                 Width (micron) 
                 Wavelength (micron) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 154 
                 6.4 
                 1.4 
                 8.6 
               
               
                 156 
                 7.1 
                 1.5 
                 9.5 
               
               
                 158 
                 7.7 
                 1.7 
                 10.4 
               
               
                 160 
                 8.4 
                 1.8 
                 11.3 
               
               
                   
               
             
          
         
       
     
     The infrared radiation bandwidths of sections  154 ,  156 ,  158  and  160  increase from approximately 1.0 microns for section  154  to 1.2 microns for section  160 . 
     Each of the pixel structure sections  154 ,  156 ,  158 ,  160 ,  162  and  164  has at the base of one region thereof a rectangular, planar ohmic contact, such as ohmic contact  219  shown for pixel structure section  164 . A region is the portion of a pixel section substantially surrounded by MQW elements. The ohmic contact can extend to have an area greater than that of one region. The ohmic contact  219  is made of Au/Ge and is positioned between the lower n + GaAs contact  194 F and the Au reflector  196 D. The ohmic contact  219  provides an ohmic connection between the n + GaAs contact  194 F and the reflector  196 D. A similar ohmic contact is provided at a similar location in each of the other pixel structure sections. Each of the contacts/reflectors provides an electrical connection to the corresponding bump beneath the detector pixel structure section. Each contact/reflector also functions as an infrared radiation reflector. 
     The detector array  150  is fabricated by use of the materials and steps described in reference to FIGS. 3A-3D. Parameters for the detector array  150 , as shown in FIG. 5, are set out in Table I below. The test results shown in Table II is for a detector in which the pixel size is 56 μm×56 μm and a typical pixel spacing in the array is 60 μm×60 μm. 
     The top contact  186  is connected to through-conductors on the periphery outside the pixel structures. These through-conductors extend down to In bumps, as described above, to terminals of the underlying ROIC. 
     Optimum parameters for section  158  of pixel structure  153  as shown in FIG. 5 are set out in Table I below. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                 OPTIMAL 
               
               
                 PARAMETER 
                 UNITS 
                 VALUE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 PEAK SPECTRAL RESPONSE 
                 μm 
                 9.0 
               
               
                 SPECTRAL FULL WIDTH AT HALF MAX 
                 μm 
                 1.2 
               
               
                 GaAs WELL PARAMETERS 
               
               
                 THICKNESS 
                 A 
                 45 
               
               
                 DOPING (n-TYPE) 
                 cm −3   
                 1 × 10 18   
               
               
                 NUMBER OF WELLS 
                   
                 18 
               
               
                 Al x Ga 1−x As BARRIER PARAMETERS 
               
               
                 x-VALUE 
                   
                 ·0.27 
               
               
                 THICKNESS 
                 A 
                 500 
               
               
                 NUMBER OF BARRIERS 
                   
                 19 
               
               
                 STRUCTURAL PARAMETERS 
               
               
                 TOP (PATTERNED) CONTACT THICKNESS 
                 μm 
                 0.4 
               
               
                 (n-doping N D  = 2 × 10 18  cm −3 ) 
               
               
                 MQW THICKNESS 
                 μm 
                 1.03 
               
               
                 BOTTOM (UNPATTERNED) CONTACT 
                 μm 
                 0.4 
               
               
                 THICKNESS 
               
               
                 (n-doping N D  = 2 × 10 18  cm −3 ) 
               
               
                 2-D MQW GRATING PERIOD 
                 μm 
                 8 
               
               
                 MQW GRATING STRIP WIDTH 
                 μm 
                 1.2 
               
               
                   
               
             
          
         
       
     
     Test results for a section of a pixel structure fabricated with a strip periodicity of 8.00 microns and a strip width of 1.2 microns is shown in Table II below. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
                   
                   
                 ARRAY 
                 STANDARD 
               
               
                   
                 TEMP 
                 BIAS 
                 AVERAGE 
                 DEVIATION 
               
               
                 PARAMETER 
                 (K) 
                 (V) 
                 (58 SAMPLES) 
                 (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 PEAK 
                 77.3 
                 0.5 
                 8.4 
                   
               
               
                 SPECTRAL 
               
               
                 RESPONSE 
               
               
                 (μm) 
               
               
                 SPECTRAL 
                   
                   
                 1.0 
               
               
                 FWHM) 
               
               
                 (μm) 
               
               
                 D*(500K) 
                 77.3 
                 0.5 
                 3.95 × 10 9   
                 4.7 
               
               
                 (cmHz 1/2 /W) 
                   
                 1.5 
                 4.67 × 10 9   
                 4.1 
               
               
                 FOV = 23° 
                 83 
                 0.5 
                 2.07 × 10 9   
                 3.1 
               
               
                   
                   
                 1.5 
                 2.32 × 10 9   
                 4.5 
               
               
                 DARK 
                 77.3 
                 0.5 
                 5.5 × 10 −5   
                 3.4 
               
               
                 CURRENT 
                   
                 1.5 
                 1.6 × 10 −4   
                 3.8 
               
               
                 DENSITY 
                 83 
                 0.5 
                 1.6 × 10 −4   
                 4.4 
               
               
                 (A/cm 2 ) 
                   
                 1.5 
                 5.9 × 10 −4   
                 3.6 
               
               
                   
               
             
          
         
       
     
     The detector array  150  shown in FIG. 5 is non-polarized because the MQW diffractive grating elements are transverse to each other within each pixel structure. Therefore, all polarizations of incident infrared radiation are received. 
     Referring to FIG. 6, there is shown a polarized detector array  240  which is similar to the detector array  150  shown in FIG. 5, but has elongate diffractive MQW elements extending in only one direction. The detector array  240  includes detector pixel structure  241  which includes pixel structure sections  242 ,  244 ,  246  and  248 . Pixel structure sections  250  and  252  are in adjacent pixel structures. Each pixel structure section has a corresponding In bump for connection to an ROIC  260 . Bump  262  connects pixel structure section  242  to a terminal (not shown) of ROIC  260 . In a similar fashion, there is provided a bump  264  for pixel structure section  246 , a bump  266  for pixel structure section  250  and a bump  268  for pixel structure section  252 . There is a corresponding bump for each of the pixel sections within the detector array  240 . Each pixel structure section therefore generates a different signal which is provided to the ROIC  260 . In this embodiment of the invention, there are four pixel structure sections for each pixel structure. 
     The space between the bumps  262 - 268  is filled with wicked epoxy  270  for bonding the surrounding elements together. 
     The detector array  240  has a layer comprising an n + GaAs array contact  274  on the upper surface thereof which comprises a plurality of elongate contact strips, all of which are electrically and physically interconnected to each other. The pixel structure section  246  has elongate contacts  274 A and  274 B. 
     Below the contact  274 , there is a diffractive MQW structure  276  similar to the structure  190  shown in FIG.  5  and the structure  68  shown in FIG.  3 D. The pixel structure section  246  includes MQW elements  276 A and  276 B, which have the n + GaAs strip contacts  274 A and  274 B on the upper surfaces thereof. The contact strips  274 A and  274 B extend between wider n + GaAs conductors  274 C and  274 D. 
     A lower (second) planar n + GaAs contact for each pixel structure section extends across the detector array  240  but is divided into separate contacts for each of the pixel structure sections. This consists of contact  280 A for section  242 , contact  280 B for section  244 , contact  280 C for section  246 , contact  280 D for section  248 , contact  280 E for section  250  and contact  280 F for section  252 . Contact  280  corresponds to the contact  70  shown in FIGS. 3A-3D. The contacts  280 A-F are separated by slots  282 ,  284 ,  286 ,  288  and  290 . 
     An Au contact/reflector is also divided by the slots  282 - 290  in individual contact/reflectors corresponding to each pixel structure section. Pixel structure section  242  has reflector  298 A, section  246  has reflector  298 B, section  250  has reflector  298 C and pixel section  252  has reflector  298 D. As described for detector array  150  in FIG. 5, each of the reflectors provides an electrical connection to the corresponding bump beneath the pixel section. Each reflector also functions as an infrared radiation reflector. 
     Each of the pixel structure sections  242 ,  244 ,  246 ,  248 ,  250  and  252  has five longitudinal sections and one longitudinal section thereof is provided with an ohmic contact, such as ohmic contact  292  shown for pixel structure section  250  and contact  294  shown for pixel structure section  252 . Each ohmic contact is a rectangular structure comprising Au/Ge having a thickness of 900 Å. Each ohmic contact is formed between the corresponding lower contact, such as contact  280 E for pixel structure section  250  and the underlying reflector  298 C. The ohmic contact provides a good ohmic connection between the overlying second n + GaAs contact and the underlying Au reflector. 
     The physical characteristics and infrared radiation bands of response of sections  242 ,  244 ,  246  and  248  correspond respectively to sections  154 ,  156 ,  158  and  160  shown in FIG.  5 . 
     The detector array  240  shown in FIG. 6 has elongate MQW elements running in only one direction. It is therefore sensitive to only one polarity of infrared radiation. 
     The pixel structures shown in FIGS. 5 and 6 each have four different wavelength sections set in a quadrature arrangement. However, the present invention can be implemented in a wide variety of configurations. 
     FIGS. 7,  8  and  9  illustrate other possible pixel structures which show applications of the present invention with different physical dimensions, different numbers of sections and different shapes. 
     FIG. 7 is a further configuration of a pixel structure  302  in accordance with the present invention. The pixel structure  302  has four sections  302 A,  302 B,  302 C and  302 D. Sections  302 A and  302 B have six-by-six elements and Sections  302 C and  302 D have five-by-five elements. The spacing of sections  302 A and  302 B is 7.0 microns and the spacing of the elements in sections  302 C and  302 D is 9.0 microns. The thickness of the elements in section  302 A is 1-2 microns. The thickness of the elements in sections  302 B and  302 C is 1.4 microns, and the thickness of the elements in section  302 D is 1.8 microns. The pixel structure  302  is fabricated and implemented in the same manner as the pixel structure  153  described in reference to FIG.  5 . 
     FIG. 8 is a further configuration of a pixel structure  304  in accordance with the present invention. The structure  304  has four sections  304 A,  304 B,  304 C and  304 D which correspond respectively to the sections  302 A,  302 B,  302 C and  302 D described in reference to FIG.  7 . The pixel structure  304  has an elongate configuration as opposed to the square configuration shown for pixel structure  302  in FIG.  7 . The pixel structure  304  can be fabricated and implemented in the same manner as the pixel structure  153  described in reference to FIG.  5 . 
     A still further pixel structure  306  in accordance with the present invention is illustrated in FIG.  9 . The pixel structure  306  has sections  306 A,  306 B,  306 C and  306 D. However, the sections  306 A and  306 D are identical in physical configuration and the sections  306 B and  306 C are identical in physical configuration. Therefore, the pixel structure  306  is responsive to only two bands of infrared radiation. This is in contrast to the four bands of infrared radiation received by the pixel structures shown in FIGS. 5-8. The sections  306 A and  306 D correspond to the section  304 A of pixel structure  304  shown in FIG.  8 . The sections  306 B and  306 C correspond to the pixel structure section  304 B shown in FIG.  8 . The pixel structure  306  can be fabricated and implemented in the same manner as the pixel structure  153  described in reference to FIG.  5 . 
     Referring to FIG. 10, there is shown a focal plane array  310  which comprises a plurality of pixel structures, as previously described in accordance with the present invention. This array preferably has 640 pixel structures horizontally and 480 pixel structures vertically. One pixel structure  312 , as an example, corresponds to pixel structure  153  described in reference to FIG.  5 . Each of the pixel structure sections within the array  310  has a separate electrical output signal so that a complete image having 640×480 elements can be produced. Each section of the pixel structure provides information related to a different wavelength (color) of infrared radiation. 
     The electrical connection of pixel structure sections to an ROIC for the present invention is illustrated in FIG.  11 . Reference is made to the detector array  150  described in FIG. 5. A top n + GaAs contact, such as contact  186 , is connected to an electrically common point which is shown by a ground symbol. A particular diffractive MQW element  320  which corresponds, for example, to the diffractive MQW element  190 A, is connected between the top contact  186  and the lower contact, which corresponds to n + GaAs contact  194 C. The lower contact is ohmically connected to the Au reflector  196 B and connected through the bump  168  to the ROIC  180 . For a preferred embodiment this is a “direct injection” ROIC. 
     The signal transmitted through the bump  168  is provided to a MOSFET PREAMP transistor  322  which has a gate terminal connected to a V bias  supply. Transistor  322  functions as a preamplifier. The amplified signal produced by transistor  322  is integrated by a capacitor  324 . The integrated signal is provided to a multiplexer (not shown) which selectively samples the signal produced by each of the pixel structure sections throughout the array. The MQW element  320  is provided with the noted bias voltage through the transistor  322  and intervening conductors to the MQW element  320 . 
     When infrared radiation is absorbed by the diffractive MQW element  320 , carriers are produced which change the conductivity of the MQW element. This changes the output signal from the element. This change in signal corresponds to the received infrared radiation. The collection of all of the signals from the pixel elements produces an image representative of the received infrared radiation. 
     Although each of the MQW elements described above has a linear configuration, such elements may also have a curved and elongate configuration. 
     In summary, the present invention comprises an improved structure that provides enhanced performance for receiving multiple bands of infrared radiation. The co-planar configuration of adjacent pixel sections for one pixel uses common MQW material but have different physical configurations for reception of different bands of infrared radiation. 
     The present invention has been described using a Gallium Arsenide/Aluminum Gallium Arsenide MQW operating in the long wave infrared spectral band of 8-12 μm. However, through the proper selection of parameters and of MQW material systems, the invention is applicable to others spectral bands. Alternate MQW material systems include but are not limited to Indium Gallium Arsenide/Indium Aluminum Arsenide, Indium Gallium Arsenide/Indium Phosphide and strained Indium Gallium Arsenide/Gallium Arsenide. 
     Although several embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention.