Abstract:
The present invention relates to a white light emitting diode comprising a blue light emitting diode chip; a green light emitting diode chip; and a molding member that encapsulates the blue light emitting diode chip and a green light emitting diode chip, the molding member containing a red fluorescent substance.

Description:
[0001]     This application claims priority under 35 USC 119 by virtue of Korean Patent Application No. 10-2005-0116520 filed on Dec. 1, 2005, the disclosure of which is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a white light emitting diode, and more particularly, to a white light emitting diode, wherein a molding member with a red fluorescent substance mixed therein is formed on a blue light emitting diode chip and a green light emitting diode chip.  
         [0004]     2. Description of the Related Art  
         [0005]     As red, orange, green, blue and white light emitting diodes with high luminance have recently appeared with the development of compound semiconductor technology, light emitting diodes, which was mainly used for simple indicators, dial plates and the like in the prior art, have been applicable to a variety of fields in which full colors, high reliability, low power consumption and miniaturization are required. In particular, as the illumination efficiency of a white light emitting diode is superior to that of an incandescent bulb, the appearance of the white light emitting diode shows possibility of use as an illumination light source, and the white light emitting diode has been also used as a backlight source of a liquid crystal display (LCD) There have been developed a variety of methods for implementing such white light emitting diodes, which include a method of obtaining white light by coupling a fluorescent substance on a blue or ultraviolet light emitting diode chip as a single chip type, a method of obtaining white light by combining two or three light emitting diodes as a multi-chip type, and the like.  
         [0006]     Among conventional techniques, in a case where a blue light emitting diode is used, the blue light emitting diode is used as an excitation light source and excited light is passed through a fluorescent substance of YAG (Yttrium Aluminum Garnet) emitting yellow light so as to implement a white light emitting diode. However, this method has disadvantages in that a flicker effect frequently occurs due to color separation resulting from a wide wavelength interval between blue and yellow light, and it is difficult to control color temperature and a color rendering evaluation index. Further, there is a method of implementing a white light emitting diode by applying red, green and blue fluorescent substances on an ultraviolet light emitting diode chip. However, there is a problem in that the ultraviolet light emitting diode chip has a short life span and low efficiency.  
         [0007]     Meanwhile, although red, green and blue light emitting diode chips are combined to implement a white light emitting diode with multi chips, there is a problem in that the operation voltage of each of the chips is not uniform, and the output of each of the chips varies depending on an ambient temperature.  
         [0008]      FIGS. 1A and 1B  are perspective and sectional views of a white light emitting diode according to a prior art. Referring to  FIGS. 1A and 1B , there is shown a case where a blue light emitting diode is used as an excitation light source and excited light is passed through a fluorescent substance of YAG (Yttrium Aluminum Garnet) emitting yellow light so as to implement a white light emitting diode. The white light emitting diode shown in  FIGS. 1A and 1B  is a side view type light emitting diode used as a backlight source of an LCD. The white light emitting diode comprises a substrate  110 , a first lead terminal  120 , a second lead terminal  130 , a blue light emitting diode chip  140 , a reflection portion  150 , a wire  160 , a fluorescent substance  170  and a molding member  180 .  
         [0009]     In the side view type white light emitting diode, the first lead terminal  120  and the second lead terminal  130  are arranged on the substrate  110 , and the blue light emitting diode chip  140  is mounted on the second lead terminal  130 . The reflection portion  150  for laterally reflecting light emitted from the blue light emitting diode chip  140  is formed on the substrate  110 , the molding member  180  encapsulates the blue light emitting diode chip  140 , and the fluorescent substance  170 , e.g., YAG (Yttrium Aluminum Garnet) is mixed in the molding member  180 . As a result, blue light emitted from the blue light emitting diode chip  140  excites the fluorescent substance  170  to generate yellow light, and white light is emitted due to constructive interference of the yellow light and the blue light. Thus, since only the blue light and the yellow light exist, there is limitation on expression of red and green, which causes a problem of deterioration of color reproducibility. Further, as described above, since white light emitting diodes implemented according to the prior art have problems, there is a current need for white light emitting diodes that solve these problems.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention solves the aforementioned problems by providing a white light emitting diode wherein red, green and blue lights are emitted to improve color reproducibility. According to an aspect of the present invention, the white light emitting diode comprises a blue light emitting diode chip; a green light emitting diode chip; a molding member such as a resin that encapsulates the blue light emitting diode chip and the green light emitting diode chip and which contains a red fluorescent substance. The red fluorescent substance may advantageously comprise (Sr 1 -xEux)S or (Ca 1 -xEux)S, where x is 0.001 to 0.02. The white light emitting diode may further include a reflection portion arranged on the substrate to reflect light emitted from the light emitting diode chips in a predetermined direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0012]      FIGS. 1A and 1B  are perspective and sectional views of a white light emitting diode according to the prior art;  
         [0013]      FIGS. 2A and 2B  are perspective and sectional views of a white light emitting diode according to a first embodiment of the present invention;  
         [0014]      FIGS. 3A and 3B  are plan and sectional views of a white light emitting diode according to a second embodiment of the present invention;  
         [0015]      FIG. 4  is a sectional view of a white light emitting diode according to a third embodiment of the present invention; and  
         [0016]      FIGS. 5A and 5B  are views showing emission spectra of the white light emitting diodes according to the prior art and the present invention, respectively, and  FIGS. 5C and 5D  are views showing color coordinates according to the prior art and the present invention, respectively. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Light emitting diodes have been fabricated in various forms and fabricated as surface mount devices (SMD) adapted to be mounted directly on printed circuit boards. A SMD type light emitting diode is fabricated as a side view type or a top view type according to the desired application. SMD type white light emitting diodes of side view and top view will be described below with reference to  FIGS. 2 and 3 .  
         [0018]      FIGS. 2A and 2B  are perspective and sectional views of a white light emitting diode according to a first embodiment of the present invention. The white light emitting diode shown in  FIGS. 2A and 2B  is a side view type white light emitting diode, which may be used as a backlight source of an LCD. The white light emitting diode comprises a substrate  210 , a first lead terminal  220 , a second lead terminals  230 , a blue light emitting diode  240   a , a green light emitting diode  240   b , a reflection portion  250 , wires  260   a  and  260   b , a red fluorescent substance  270  and a molding member  280 . The first lead terminal  220  and the second lead terminal  230  are formed on the substrate  210 . The blue light emitting diode chip  240   a  and the green light emitting diode  240   b  are mounted on the first lead terminal  220 , and the blue light emitting diode chip  240   a  and the green light emitting diode  240   b  are connected to the second lead terminal  230  through the wires  260   a  and  260   b . Although the blue light emitting diode chip  240   a  and the green light emitting diode  240   b  are connected in series in this embodiment, they are not limited thereto but may be connected in parallel.  
         [0019]     The reflection portion  250  is formed on the substrate such that one side of the reflection portion is open to allow light emitted from the light emitting chips to be reflected in a lateral direction. The molding member  280  is contained in the reflection portion  250  so as to function to encapsulate the blue light emitting diode chip  240   a  and the green light emitting diode chip  240   b  and to fix the first lead terminal  220  and the second terminal  230 . At this time, a transparent resin such as epoxy resin or silicone resin is used for the molding member.  
         [0020]     The red fluorescent substance  270  is included in the molding member  280 , and light emitted from the blue light emitting diode chip  240   a  or the green light emitting diode chip  240   b  excites the red fluorescent substance  270  to generate red light. Thus, blue, green and red light are mixed with one another so that white light can be emitted. Here, (Sr 1-x Eu x )S or (Ca 1-x Eu x )S where x is 0.001 to 0.02 is used as the red fluorescent substance  270 . Besides, a variety of red substances may be used.  
         [0021]     The blue light emitting chip  240   a  may be fabricated using IV group semiconductors such as silicone carbide (SiC), II-IV group semiconductors such as ZnSe and ZnS, GaN-based III-V group semiconductors, or the like, preferably using GaN-based semiconductors. In case of GaN, red light to near ultraviolet rays can be emitted depending on a composition ratio of In. Thus, it is possible to fabricate not only a blue light emitting diode chip but also a green light emitting diode chip.  
         [0022]      FIGS. 3A and 3B  are plan and sectional views of a white light emitting diode according to a second embodiment of the present invention. The white light emitting diode shown in  FIGS. 3A and 3B  is a top view type white light emitting diode. This embodiment is different from the first embodiment in that light is emitted upwardly contrary to the first embodiment in which light is emitted in a lateral direction. Remaining components are similar to each other.  
         [0023]     The white light emitting diode comprises a substrate  310 , first lead terminals  320   a  and  320   b , second lead terminals  330   a  and  330   b , a blue light emitting diode chip  340   a , a green light emitting diode chip  340   b , a reflection portion  350 , wires  360   a  and  360   b , a red fluorescent substance  370 , and a molding member  380 .  
         [0024]     The two first lead terminal  320   a  and  320   b , and the two second lead terminal  330   a  and  330   b  are formed on the substrate  310 . The blue light emitting diode chip  340   a  is mounted on the first lead terminal  320   a , and the green light emitting diode chip  340   b  is mounted on the first lead terminal  320   b . The blue light emitting diode chip  340   a  and the green light emitting diode chip  340   b  are connected to the second lead terminals  330   a  and  330   b  through the wires  360   a  and  360   b , respectively. The reflection portion  350  is formed to surround the blue and green light emitting diodes  340   a  and  340   b  on the substrate  310 . At this time, to improve luminance and a light focusing ability, an inner wall of the reflection portion  350  surrounding the blue and green light emitting diode chips  340   a  and  340   b  may be formed to have a predetermined slope. As described above, the reflection portion  350  is formed so that reflection of light emitted from the blue and green light emitting diode chips  340   a  and  340   b  can be maximized.  
         [0025]     The molding member  380  is applied in the reflection portion  350  so as to function to encapsulate the blue light emitting diode chip  340   a  and the green light emitting diode chip  340   b  and to fix the first lead terminals  320   a  and  320   b  and the second lead terminals  330   a  and  330   b . At this time, a transparent resin such as epoxy resin or silicone resin is used for the molding member  380 .  
         [0026]     The red fluorescent substance  370  is included in the molding member  380 , and light emitted from the blue light emitting diode chip  340   a  or the green light emitting diode chip  340   b  excites the red fluorescent substance  370  to generate red light. Thus, blue, green and red light are mixed with one another so that white light can be emitted. As described above, (Sr 1-x Eu x )S or (Ca 1-x Eu x )S wherein x is 0.001 to 0.02 is used as the red fluorescent substance  370 .  
         [0027]      FIG. 4  is a sectional view of a white light emitting diode according to a third embodiment of the present invention. The white light emitting diode according to the third embodiment is a chip type light emitting diode. This embodiment is different from the second embodiment in that there is no reflection portion, and remaining components are similar to each other. Further, although the molding member  380  is shown in the form of a hemisphere in this embodiment, it is not limited thereto but may be fabricated in various forms. Although white light emitting diodes in various forms are shown in  FIGS. 2A  to  4 , the present invention is not limited thereto but may be applied to white light emitting diodes in other forms except these illustrative forms.  
         [0028]      FIGS. 5A and 5B  are views showing emission spectra of white light emitting diodes according to the prior art and the present invention, respectively, and  FIGS. 5C and 5D  are views showing color coordinates according to the prior art and the present invention.  FIG. 5A  shows the emission spectrum of the white light emitting diode according to the prior art, i.e., shown in  FIG. 1 , and  FIG. 5B  shows the emission spectrum of the white light emitting diode according to the present invention. As shown in  FIG. 5A , emission peaks exist only in the vicinity of blue and yellow, and there is no emission peak in the vicinity of green and red. Thus, there is limitation on expression of red and green, resulting in deterioration of color reproducibility.  
         [0029]     Referring to  FIG. 5B , contrary to the emission spectrum shown in  FIG. 5A , it can be seen that emission peaks exist in the vicinity of not only blue but also green and red. Thus, red and green can be readily expressed and color reproducibility is improved.  
         [0030]     Referring to  FIGS. 5C and 5D , it can be seen that there is little difference between the RGB color coordinates of white light emitted from the white light emitting diode according to the prior art and those of white light emitted from the white light emitting diode according to the present invention. In this simulation, CIE 1931(International Commission for Illumination) color coordinate is used, and a color reproducibility defined in the NTSC(National Television Standard Committee) is used. The color reproducibility is defined as follows;
 
Color reproducibility=(the triangular area of RGB color coordinate/the triangular area of NTSC standard color coordinate)*100%
 
         [0031]     The values of color coordinate shown in  FIGS. 5C and 5D  are shown in the following Table 1.  
                                                                                             TABLE 1                                       Prior art       Present invention                    Coordinates                    Color   x   y   x   y                       White   0.324   0.344   0.326   0.363           Red   0.620   0.365   0.673   0.316           Green   0.351   0.588   0.284   0.648           Blue   0.146   0.096   0.145   0.131                Color   56.2%   78.1%           Reproducibility                      
 
         [0032]     The color reproducibility of the white light emitting diode according to the prior art is about 56.2%, but that of the white light emitting diode according to the present invention is about 78.1%. Thus, according to the present invention, the color reproducibility of a white light emitting diode is improved considerably.  
         [0033]     The foregoing is merely exemplary embodiments of the white light emitting diode according to the present invention. The present invention is not limited to the aforementioned embodiments, and it will be understood by those skilled in the art that various modifications and changes can be made thereto without departing from the technical spirit and scope of the present invention as defined by the appended claims.