Abstract:
A luminance control circuit for controlling the luminance levels of different colored light sources that lends itself to easy incorporation into display devices is presented. A light emitting diode (LED) substrate includes a plurality of driving thin film transistors (TFTs) including a semiconductor layer deposited on a substrate. A plurality of LEDs for generating lights of different wavelengths is mounted respectively on the plurality of driving TFTs. A plurality of thin film sensors for sensing the luminous intensities of the plurality of LEDs is formed between the plurality of LEDs and the substrate. A luminance control circuit for controlling the driving TFTs has of a plurality of controlling TFTs including a semiconductor layer deposited on the substrate and is connected to the plurality of thin film sensors.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority from Korean Patent Application No. 2006-0017527 filed in the Korean Patent Office on Feb. 23, 2006, the entire content of which is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a light emitting diode device, and more particularly to a light emitting diode substrate with thin film light sensors integrated on the substrate. 
         [0004]    2. Description of the Related Art 
         [0005]    A liquid crystal display (LCD) device displays an image by using electro-optical properties of liquid crystals. Specifically, the LCD device includes an LCD panel for displaying an image through a pixel matrix and a driving circuit for driving the LCD panel. Since the LCD panel is a non-light-emitting device, the LCD device includes a backlight unit for supplying light to the LCD panel. 
         [0006]    The backlight unit typically uses a lamp as a light source but recently, the trend has been shifting to using a light emitting diode (LED) having high luminance as a point light source. Since the point light source uses red (R), green (G) and blue (B) LEDs of three primary colors, a wide variety of colors can be produced by mixing the primary colors. 
         [0007]    However, color uniformity is less than what is desirable when using the primary color LEDs. The reason for the low color uniformity is that the different-colored point light sources differ in luminous efficiency and aging characteristics. In order to prevent the color uniformity from decreasing, a luminance control circuit for controlling the luminance of the R, G and B LEDs or optical sensors for measuring luminous intensities is included in an LED driver. These additional components complicate the circuit configuration and raises manufacturing cost. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides an LED substrate that reduces manufacturing cost and improves color uniformity, a method of manufacturing the same and an LCD device using the same. A thin film light sensor is integrated on the substrate, together with an LED driving circuit. 
         [0009]    In one aspect, the present invention is provided an LED substrate including a plurality of driving TFTs including a semiconductor layer deposited on a substrate, a plurality of LEDs mounted respectively on the plurality of driving TFTs, for generating lights of different wavelengths, a plurality of thin film sensors formed between the plurality of LEDs and the substrate, for sensing the luminous intensities of the plurality of LEDs, and a luminance control circuit having a plurality of controlling TFTs including a semiconductor layer deposited on the substrate and being connected to the plurality of thin film sensors, thus to control the driving TFTs. 
         [0010]    According to another aspect of the present invention, there is provided a method of manufacturing an LED substrate that entails: forming a plurality of driving TFTs including a semiconductor layer on a substrate, mounting a plurality of LEDs generating lights of different wavelengths on the plurality of driving TFTs, forming a plurality of thin film sensors sensing the luminous intensities of the plurality of LEDs between the plurality of LEDs and the substrate, and forming a luminance control circuit that has a plurality of TFTs including a semiconductor layer deposited on the substrate and is connected to the plurality of thin film sensors and the plurality of driving TFTs. 
         [0011]    According to still another aspect of the present invention, there is provided an LCD device comprising a light source including the above-described LED substrate, and an LCD panel for displaying an image by using light generated from the light source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
           [0013]      FIG. 1  is a cross-sectional view of an LED substrate according to an exemplary embodiment of the present invention; 
           [0014]      FIG. 2  is an equivalent circuit diagram of the LED substrate shown in  FIG. 1 ; 
           [0015]      FIG. 3  is an exploded perspective view of an LCD device to which an LED substrate is applied according to an exemplary embodiment of the present invention; and 
           [0016]      FIG. 4  is an exploded perspective view of an LCD device to which an LED substrate is applied according to another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0017]    The exemplary embodiments of the present invention will now be described with reference to  FIGS. 1 to 4 . 
         [0018]      FIG. 1  is a cross-sectional view illustrating a part of an LED substrate according to an exemplary embodiment of the present invention. 
         [0019]    The LED substrate illustrated in  FIG. 1  includes first to third thin film transistors (TFTs) T 1 , T 2  and T 3  for respectively driving R, G and B LEDs  20 ,  30  and  40 , first to third thin film sensors  22 ,  24  and  26  for respectively detecting the luminous intensities of the R, G and B LEDs  20 ,  30  and  40 , and a fourth TFT T 4  contained in a luminance control circuit for controlling the first to third TFTs T 1 , T 2  and T 3 . The first to fourth TFTs T 1  to T 4  and the first to third thin film sensors  22 ,  24  and  26  may use any one of polysilicon, polysilicon-germanium, amorphous silicon, and amorphous silicon-germanium thin films. However, for simplicity of explanation, the description will focus on the polysilicon thin film by way of example. 
         [0020]    The first to third TFTs T 1 , T 2  and T 3  for respectively driving the R, G and B LEDs  20 ,  30  and  40 , and the fourth TFT T 4  of the luminance control circuit include semiconductor layers  12 ,  14 ,  16  and  18  using the polysilicon thin film. Specifically, the first to fourth TFTs T 1  to T 4  include the semiconductor layers  12 ,  14 ,  16  and  18  formed on a substrate  2  with a buffer layer  4  interposed therebetween, gate electrodes  52 ,  54 ,  56  and  58  overlapping the semiconductor layers  12 ,  14 ,  16  and  18  with a first insulating layer  6  interposed therebetween, source electrodes  34 ,  38 ,  44  and  48  connected to source regions of the semiconductor layers  12 ,  14 ,  16  and  18 , and drain electrodes  32 ,  36 ,  42  and  46  connected to drain regions of the semiconductor layers  12 ,  14 ,  16  and  18 . The source and drain regions of the semiconductor layers  12 ,  14 ,  16  and  18  are doped with n-type or p-type impurities and are thus electrically conductive. The source electrodes  34 ,  38 ,  44  and  48  are connected to the source regions of the semiconductor layers  12 ,  14 ,  16  and  18  by penetrating at least one insulating layer  6 , and similarly, the drain electrodes  32 ,  36 ,  42  and  46  are connected to the drain regions of the semiconductor layers  12 ,  14 ,  16  and  18  by penetrating at least one insulating layer  6 . The source electrodes  34 ,  38  and  44  of the first to third TFTs T 1  to T 3  penetrate a second insulating layer  8  and are extended to be connected to anodes  60 ,  64  and  68  of the R, G and B LEDs  20 ,  30  and  40 , respectively. 
         [0021]    The gate electrodes  52 ,  54  and  56  of the first to third TFTs T 1 , T 2  and T 3  are connected to the luminance control circuit including the fourth TFT T 4  and receive a luminance control signal from the luminance control circuit. The drain electrodes  32 ,  36  and  42  of the first to third TFTs T 1 , T 2  and T 3  are connected to a driving voltage (VDD) supply line (not shown) from a power source, and the source electrodes  34 ,  38  and  44  thereof are connected to the anodes  60 ,  64  and  68  of the R, G and B LEDs  20 ,  30  and  40  to supply an LED driving signal. Such first to third TFTs T 1 , T 2  and T 3  control the amount of current supplied to the R, G and B LEDs  20 ,  30  and  40  from the VDD supply line according to the control signal of the luminance control circuit, thereby driving the R, G and B LEDs  20 ,  30  and  40 . The luminance control circuit consists of a plurality of switching TFTs (as will be described below in reference to  FIG. 2 ) such as the fourth TFT T 4  and controls the first to third TFTs T 1 , T 2  and T 3 . 
         [0022]    The R, G and B LEDs  20 ,  30  and  40  are formed in a chip type configuration using semiconductor crystal, and have anodes  60 ,  64  and  68  formed at their lower surfaces and cathodes  63 ,  65  and  67  formed at their upper surfaces. The R, G and B LEDs  20 ,  30  and  40  are mounted on the second insulating layer  8  so that their anodes  60 ,  64  and  68  are respectively connected to the source electrodes  34 ,  38  and  44  of the first to third TFTs T 1 , T 2  and T 3 . The anodes  60 ,  64  and  68  of the R, G and B LEDs  20 ,  30  and  40  are respectively connected to the source electrodes  34 ,  38  and  44  of the first to third TFTs T 1 , T 2  and T 3  through a conductive adhesive, for example, an anisotropic conductive film (ACF). The cathodes  63 ,  65  and  67  formed at the upper surfaces of the R, G and B LEDs  20 ,  30  and  40  are respectively connected to bonding wires  72 ,  74  and  76  and to a ground electrode  62  formed on the second insulating layer  8 . The R, G and B LEDs  20 ,  30  and  40  emit lights in response to the amount of current supplied through the first to third TFTs T 1 , T 2  and T 3 , thus to generate R, G and B colored lights each having luminance proportional to the received current. An organic insulating layer  10  covering the R, G and B LEDs  20 ,  30  and  40  is coated on the second insulating layer  8  on which the R, G and B LEDs  20 ,  30  and  40  are mounted. The organic insulating layer  10  is formed in a convex (or concave) lens shape and therefore can improve the luminous efficiencies of the R, G and B LEDs  20 ,  30  and  40 . 
         [0023]    The first to third thin film sensors  22 ,  24  and  26  for sensing the luminous intensities of the R, G and B LEDs  20 ,  30  and  40  are formed of a polysilicon thin film under the R, G and B LEDs  20 ,  30  and  40 , together with the semiconductor layers  12 ,  14 ,  16  and  18  of the first to fourth TFTs T 1 , T 2 , T 3  and T 4 . Each of the first to third thin film sensors  22 ,  24  and  26  is formed as a photodiode type of a p-i-n structure in which p-type and n-type impurities are injected into both terminals of an intrinsic semiconductor using polysilicon. Regions of the first to third thin film sensors  22 ,  24  and  26  into which the p-type impurities are injected are connected to the ground electrode  62  formed on the second insulating layer  8  through a contact electrode (not shown) penetrating the first and second insulating layers  6  and  8 , and regions into which the n-type impurities are injected has a floated structure. In addition, the regions of the first to third thin film sensors  22 ,  24  and  26  into which the p-type impurities are injected are also connected to the luminance control circuit including the fourth TFT T 4  through another contact electrode (not shown). These first to third thin film sensors  22 ,  24  and  26  sense the luminous intensities of the R, G and B LEDs  20 ,  30  and  40  through through-holes  51 ,  53  and  55  penetrating the anodes  60 ,  64  and  68  of the R, G and B LEDs  20 ,  30 ,  40 , generate sensing signals proportional to the luminous intensities, and supply the sensing signals to the luminance control circuit. 
         [0024]    The luminance control circuit including the fourth TFT T 4  controls the first to third TFTs T 1 , T 2  and T 3  by using a luminance control signal received from an external component and luminous intensity sensing signals from the first to third thin film sensors  22 ,  24  and  26 . Therefore, a luminous intensity ratio becomes constant regardless of an aging characteristic of each of the R, G and B LEDs  20 ,  30 ,  40  and color uniformity is improved. 
         [0025]    Each of the R, G and B LEDs  20 ,  30  and  40  is formed of a chip in a polygonal shape and the anodes  60 ,  64  and  68  of their lower surfaces are formed in a polygonal band or circular band shape having the through-holes  51 ,  53  and  55  at the center aligned with each of the first to third thin film sensors  22 ,  24  and  26 . The semiconductor layers  12 ,  14  and  16  of the first to third TFTs T 1 , T 2  and T 3 , the gate electrodes  52 ,  54  and  56 , the source electrodes  34 ,  38  and  44 , and the drain electrodes  32 ,  36  and  42  may also be formed in a polygonal band or circular band shape encompassing the peripheral parts of the first to third thin film sensors  22 ,  24  and  26  based on each of the first to third thin film sensors  22 ,  24  and  26 . 
         [0026]    A method of manufacturing the LED substrate having the above-described configuration will now be described. 
         [0027]    The buffer layer  4  is formed on the substrate  2 , and the semiconductor layers  12 ,  14 ,  16  and  18  of the first to fourth TFTs T 1  to T 4 , and the first to third thin film sensors  22 ,  24  and  26  are formed on the buffer layer  4  by using a polysilicon thin film. The substrate  2  may be made of an insulating substrate, such as quartz, glass, ceramic, an organic film, or a metal substrate, such as stainless steel and tungsten. The buffer layer  4  is formed by depositing an inorganic insulating material, such as oxide silicon, on the substrate  2  by a deposition method such as plasma enhanced chemical vapor deposition (PECVD). The semiconductor layers  12 ,  14 ,  16  and  18  and the first to third thin film sensors  22 ,  24  and  26  are formed by forming an amorphous silicon thin film on the buffer layer  4  by PECVD, crystallizing the amorphous silicon thin film by laser annealing to form a polysilicon thin film, and patterning the polysilicon thin film by a mask process. Thereafter, n-type or p-type impurities are injected into both terminals of each of the semiconductor layers  12 ,  14 ,  16  and  18  of the first to fourth TFTs T 1  to T 4  to form the source regions and drain regions by another mask process, and n-type and p-type impurities are injected into both terminals of each of the first to third thin film sensors  22 ,  24  and  26  to form anodes and cathodes. In this case, the n-type and p-type impurities are injected by different mask processes. 
         [0028]    Next, at least two insulating layers  6  and  8 , the gate electrodes  52 ,  54 ,  56  and  58 , the source electrodes  34 ,  38 ,  44  and  48 , the drain electrodes  32 ,  36 ,  42  and  46  of the first to fourth TFTs T 1  to T 4 , and the ground electrode  62  are formed by a plurality of mask processes on the buffer layer  4  on which the semiconductor layers  12 ,  14 ,  16  and  18 , and the first to third thin film sensors  22 ,  24  and  26  are formed. The drain electrodes  32 ,  36 ,  42  and  46  of the first to fourth TFTs T 1  to T 4  at least partially penetrate the first insulating layer  6  to be connected to the drain regions of the semiconductor layers  12 ,  14 ,  16  and  18 , and the source electrodes  34 ,  38  and  44  of the first to third TFTs T 1  to T 3  penetrate the first and second insulating layers  6  and  8  to be connected to the source regions of the semiconductor layers  12 ,  14  and  16 . The source electrode  48  of the fourth TFT T 4  penetrates the first insulating layer  6  to be connected to the source region of the semiconductor layer  18 . For example, the first insulating layer  6  including a contact hole is formed by one mask process and the gate electrodes  52 ,  54 ,  56  and  58 , the drain electrodes  32 ,  36 ,  42  and  46 , and the source electrode  48  of the fourth TFT T 4  are formed on the first insulating layer  6  by another mask process. Thereafter, the second insulating layer  8  including a contact hole is formed by a further mask process, and the source electrodes  34 ,  38  and  44  and the ground electrode  62  are formed on the second insulating layer  8  by still another mask process. 
         [0029]    Next, the R, G and B LEDs  20 ,  30  and  40  are respectively mounted on the first to third TFTs T 1  to T 3 . Then the anodes  60 ,  64  and  68  of the R, G and B LEDs  20 ,  30  and  40  are connected to the source electrodes  34 ,  38  and  44  of the first to third TFTs T 1  to T 3  through a conductive adhesive. The bonding wires  72 ,  74  and  76  connect the cathodes  63 ,  65  and  67  of the R, G and B LEDs  20 ,  30  and  40  to the ground electrode  62  by a bonding process, respectively. Thereafter, the organic insulating layer  10  covering the R, G and B LEDs  20 ,  30  and  40  is coated on the second insulating layer  8  on which the R, G and B LEDs  20 ,  30  and  40  are formed. The organic insulating layer  10  is formed in a convex (or concave) lens shape in the unit of the R, G and B LEDs  20 ,  30  and  40  by a mask process, thereby improving the luminous efficiencies of the R, G and B LEDs  20 ,  30  and  40 . 
         [0030]    As described above, the LED substrate according to the present invention integrates, on a single substrate  2 , the first to third TFTs T 1 , T 2  and T 3  for driving the R, G and B LEDs  20 ,  30  and  40 , the luminance control circuit consisting of a plurality of fourth TFTs T 4  for controlling the first to third TFTs T 1 , T 2  and T 3 , and the first to third thin film sensors  22 ,  24  and  26  for sensing the luminous intensities of the R, G and B LEDs  20 ,  30  and  40  by using a polysilicon thin film, and a plurality of conductive layers and insulating layers, thus saving manufacturing cost. 
         [0031]      FIG. 2  is an equivalent circuit diagram of the LED substrate shown in  FIG. 1 . 
         [0032]    Referring to  FIG. 2 , the LED substrate includes the first to third TFTs T 1 , T 2 , and T 3  for forming a current path between the VDD supply line and the R, G and B LEDs  20 ,  30  and  40 , the first to third thin film sensors  22 ,  24  and  26  having cathodes floated and having anodes connected to the ground via current limiting resistors R, for detecting the luminous intensities of the R, G and B LEDs  20 ,  30  and  40 , and the luminance control circuit having a plurality of amplifiers A 1  to A 4  and comparators C 1  and C 2  connected between the first to third thin film sensors  22 ,  24  and  26  and the first to third TFTs T 1 , T 2 , and T 3 . 
         [0033]    The luminance control circuit uniformly controls the luminous intensity ratios of G LED  30  and the R LED  20  based on the B LED  40  having the smallest luminous intensity ratio in order to improve color uniformity. The third TFT T 3  for driving the B LED  40  is controlled by the first amplifier A 1  receiving an overall luminance control signal. The second TFT T 2  for driving the G LED  30  controls a G luminous intensity to be substantially the same as a B luminous intensity, by having the first comparator C 1  compare a B luminous intensity sensing signal from the third thin film sensor  26  with a G luminous intensity sensing signal from the second thin film sensor  24 . The first TFT T 1  for driving the R LED  20  controls an R luminous intensity to be substantially the same as the B luminous intensity, by having the second comparator C 2  compare the B luminous intensity sensing signal from the third thin film sensor  26  to an R luminous intensity sensing signal from the first thin film sensor  22 . The luminance control circuit is comprised of a plurality of TFTs such as the fourth TFT T 4  shown in  FIG. 1  and is integrated on the substrate  2 , together with the first to third TFTs T 1  to T 3  and the first to third thin film sensors  22 ,  24  and  26 . 
         [0034]    Specifically, the third TFT T 3  is controlled by the first amplifier A 1  receiving the overall luminance control signal from an external component and drives the B LED  40  by controlling the amount of current supplied to the B LED  40  from the VDD supply line. The third thin film sensor  26  generates a luminous intensity sensing signal in proportion to the light quantity of the B LED  40  and supplies the sensing signal to the first comparator C 1  through the second amplifier A 2 . The second TFT T 2  is controlled by the first comparator C 1  and drives the G LED  30  by controlling the amount of current supplied to the G LED  30  from the VDD supply line. The first comparator C 1  compares the B luminous intensity sensing signal supplied from the third thin film sensor  26  through the second amplifier A 2  with the G luminous intensity sensing signal supplied from the second thin film sensor  24  through the third amplifier A 3  and adjusts the control signal of the second TFT T 2  to make the luminous intensity of the G LED  30  to be about equal to that of the B LED  40 . The third amplifier A 3  may be controlled by a relative luminance control signal from an external component. The first TFT T 1  is controlled by the second comparator C 2  and drives the R LED  20  by controlling the amount of current supplied to the R LED  20  from the VDD supply line. The second comparator C 2  compares the B luminous intensity sensing signal supplied from the third thin film sensor  26  through the second amplifier A 2  with the R luminous intensity sensing signal supplied from the first thin film sensor  22  through the fourth amplifier A 4  and adjusts the control signal of the first TFT T 1  to make the luminous intensity of the R LED  20  about the same as that of the B LED  40 . The fourth amplifier A 4  may be controlled by a relative luminance control signal from an outside component. 
         [0035]    As described above, the thin film sensors  22 ,  24  and  26  for sensing the luminous intensities of the R, G and B LEDs  20 ,  30  and  40  and the luminance control circuit are mounted on the LED substrate according to the present invention. With this arrangement, the color uniformity can be improved by keeping the luminous intensity ratio constant without regard to the different luminous efficiencies and aging characteristics of the R, G and B LEDs  20 ,  30  and  40 . The LED substrate is applicable to various appliances using an LED, for example, to an LED display device or a backlight unit of an LCD device. Hereinafter, an example of the backlight unit of the LCD device to which the LED substrate is applied will be explained. 
         [0036]      FIG. 3  is an exploded perspective view of an LCD device to which the LED substrate is applied according to an exemplary embodiment of the present invention. 
         [0037]    Referring to  FIG. 3 , the LCD device includes an LCD panel  120  for displaying an image, an edge-type backlight unit  180  for supplying light from the back of the LCD panel  120 , and top and bottom chassis  110  and  170  for fixing the LCD panel  120  and the backlight unit  180 . 
         [0038]    The LCD panel  120  has a structure in which an upper substrate  121  on which color filters are formed is bonded to a lower substrate  122  on which TFTs are formed, with liquid crystal disposed therebetween. The LCD panel  120  includes subpixels that are independently driven by the TFTs and arranged in a matrix format to display an image. Since the LCD panel  120  is a non-light-emitting device, it uses light from the backlight unit  180 . A driver  125  is connected to the lower substrate  122  of the LCD panel  120 . The driver  125  includes circuit films  126  on which driving chips  127  are mounted, for driving data lines and gate lines formed on the lower substrate  122  of the LCD panel  120 . One side of the circuit film  126  is connected to the lower substrate  122 , and another side is connected to a printed circuit board (PCB)  128 . In  FIG. 3 , the circuit films  126  supporting the driving chips  127  are illustrated to have a chip-on-film (COF) or tape carrier package (TCP) structure. However, in other embodiments, the driving chips  127  may be directly mounted on the lower substrate  122  by a chip-on-glass (COG) method or may be formed on the lower substrate  122  during a TFT forming process. 
         [0039]    The backlight unit  180  includes an edge-type light source  150  arranged separately from a light guide plate  156 , and a plurality of optical sheets  130  and a reflective plate  160  arranged at the upper and lower parts of the light guide plate  156  to raise light efficiency. 
         [0040]    The edge-type light source  150  generates lights by using R, G and B LEDs  154 . The edge-type light source  150  has a structure in which the R, G and B LEDs  154  are mounted on a substrate  152  where a driving circuit for driving the R, G and B LEDs  154 , thin film sensors for sensing the luminous intensities of the R, G and B LEDs  154 , and a luminance control circuit are integrated. The luminance control circuit controls the driving circuit by using a luminance control signal received from an outside component and luminous intensity sensing signals from the thin film sensors so that the luminous intensities of the R, G and B LEDs  154  can be constant. The edge-type light source  150  generates R, G and B lights having substantially equal luminous intensity ratio by compensating for the different luminous efficiencies and aging characteristics of the R, G and B LED  154 . Accordingly, there is no change in color and luminous characteristic is stabilized, thus improving color uniformity. 
         [0041]    The light guide plate  156  converts light having an optical distribution of a point light source form generated from the edge-type light source  150  into light having an optical distribution of a surface light source form to move light toward the LCD panel  120 . The plurality of optical sheets  130  includes a diffuser sheet  131 , a prism sheet  132  and a protector sheet  133 . The optical sheets  130  diffuse and collect light traveling toward the LCD panel  120  from the light guide plate  156  to have a uniform light distribution, thus improving the efficiency. The reflective plate  160  reflects light traveling toward the back (corresponding to the bottom in  FIG. 3 ) of the light guide plate  156  toward the LCD panel  120  to improve the light efficiency. 
         [0042]    The LCD panel  120  and the backlight unit  180  are fixed in an internal space provided by engaging the bottom chassis  170  and the top chassis  110 . A mold frame (not shown) holding the peripheral parts of the LCD panel  120  and the backlight unit  180  may be additionally provided. 
         [0043]      FIG. 4  is an exploded perspective view of a direct-type LCD device using an LED substrate according to another exemplary embodiment of the present invention. 
         [0044]    The direct-type LCD device shown in  FIG. 4  has substantially the same configuration as the edge-type LCD device shown in  FIG. 3  except for a backlight unit  280 , and therefore, any repetitive description will be omitted. 
         [0045]    The direct-type backlight unit  280  shown in  FIG. 4  includes a direct-type light source  140 , and the plurality of optical sheets  130  arranged between the direct-type light source  140  and the LCD panel  120 . A reflective plate (not shown) is arranged at the back of the direct-type light source  140  or the inner surface of the bottom chassis  170  is coated with a reflective material. The direct-type light source  140  generates lights by using R, G and B LEDs  144  arranged in columns and rows. The direct-type light source  140  has a structure in which the R, G and B LEDs  144  are mounted on a substrate  142  where a driving circuit for driving the R, G and B LEDs  144 , thin film sensors for sensing the luminous intensities of the R, G and B LEDs  144  and, a luminance control circuit are integrated. The luminance control circuit controls the driving circuit by using a luminance control signal received from an external component and luminous intensity sensing signals from the thin film sensors so that the luminous intensities of the R, G and B LEDs  144  can be constant. The direct-type light source  140  generates R, G and B lights having a constant intensity ratio by compensating for the different luminous efficiencies and aging characteristics of the R, G and B LED  144 . Accordingly, any change in color and luminous characteristic is stabilized, improving color uniformity. 
         [0046]    As is apparent from the foregoing description, since the driving circuit for driving the R, G and B LEDs, the sensors for sensing the luminous intensities, and the luminance control circuit are integrated on one substrate, manufacturing cost is reduced and a uniform luminous intensity ratio is obtained irrespective of the different luminous efficiencies and aging characteristics of the R, G and B LEDs. Therefore, color uniformity can be improved. 
         [0047]    The LED substrate according to the present invention is applicable to various appliances using an LED, and may be used in the backlight unit of the LCD device to enhance picture quality and improve color uniformity. 
         [0048]    While the invention has been shown and described with reference to certain preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.