Abstract:
A light emitting diode is disclosed which is small and inexpensive and can emit a variety of intermediate colors of light with a single light emitting element chip and whose current consumption is small. This invention provides a light emitting diode which comprises: a substrate; electrodes formed on the substrate; a light emitting element chip connected to the electrodes and mounted on an upper surface of the substrate; a resin material sealing an upper surface side of the light emitting element chip; and fluorescent particles and coloring particles scatteringly distributed in the resin material. The coloring particles absorb a part of wavelengths of light from the light emitting element chip and a part of wavelengths of light that have been changed by the fluorescent particles.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement in a light emitting diode and more specifically to a light emitting diode which converts a natural color of light from a light emitting element chip into a soft, intermediate pastel color. 
     2. Description of the Prior Art 
     A conventional practice to produce an intermediate color of light involves simultaneously causing two or more light emitting element chips to emit different colors of light and mixing them. An example light emitting diode of this kind is a surface mount type light emitting diode  1  which has two light emitting element chips  3 ,  4  that produce two different colors of light, as shown in FIG.  1 . This light emitting diode  1  has a glass epoxy substrate  2  forming a base, two light emitting element chips  3 ,  4  mounted on the substrate to emit different colors of light, and a resin sealing member  5  covering the light emitting element chips  3 ,  4  from above. On the upper surface of the glass epoxy substrate  2  are provided cathode electrodes  6   a ,  6   b  for securely holding the two light emitting element chips  3 ,  4  and anode electrodes  8   a ,  8   b  connected to the light emitting element chips  3 ,  4  through bonding wires  7 . 
     In the light emitting diode  1 , when the two light emitting element chips  3 ,  4  are made from a blue color chip and a red color chip, respectively, violet light can be produced by causing the two chips to emit light simultaneously and mixing the different colors of emitted light. 
     A light emitting diode  10  is also known which has three light emitting element chips mounted thereon and mixes multiple colors to produce a desired color of light, as shown in FIG.  2 . This light emitting diode  10  comprises three light emitting element chips  12 ,  13 ,  14  mounted on a base  11  that emit light in red, blue and green, respectively, electrode terminals  15 ,  16 ,  17  connected to the light emitting element chips  12 ,  13 ,  14 , respectively, and a bullet-shaped resin sealing member  18  covering the light emitting element chips  12 ,  13 ,  14  from above. The light emitting diode  10  of this construction can produce light of almost any color by combining the three light emitting element chips  12 ,  13 ,  14  that emit light in red, blue and green (see Masakazu Matsumoto, “Electronic Display,” from Ohmsha Co., Ltd., p213, FIGS. 6 &amp; 20, Jul. 7, 1995). 
     In either of the light emitting diodes  1 ,  10  described above, to produce an intermediate color of light requires two or more light emitting element chips that emit different colors of light. This necessarily increases the size of the package of the light emitting diode and requires a control circuit to individually control the mounted light emitting element chips, thus complicating the control method. Depending on the kinds of light emitting element chips combined, an optimum balance in brightness among different colors may not be able to be established. For example, if one of the light emitting element chips to be used in combination has a low brightness, an adjustment must be made to match other light emitting element chips to the low-brightness chip, giving rise to a problem that performances of high-luminance light emitting element chips cannot be fully exploited. 
     The light emitting diodes  1 ,  10  described above also have a problem of a large current consumption resulting from the need to make a plurality of light emitting element chips produce light simultaneously. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a light emitting diode capable of producing a variety of intermediate colors of light with a single light emitting element chip. 
     Another object of the present invention is to provide a small, inexpensive light emitting diode. 
     Still another object of the present invention is to provide a light emitting diode which consumes a small amount of current in the light emitting element chip and which has no need to control the current. 
     According to one aspect, the present invention provides a light emitting diode which comprises: a substrate; a light emitting element chip mounted on the substrate; a wavelength conversion material to receive light from the light emitting element chip and change wavelengths of the received light; and a wavelength absorbing material to receive the light from the light emitting element chip and the wavelength conversion material and absorb a part of the wavelengths of the received light. 
     The substrate is formed with electrodes, which are connected to the light emitting element chip. 
     The wavelength conversion material is made from, for example, fluorescent particles and the wavelength absorbing material is made from, for example, coloring particles. 
     It is preferred that the wavelength conversion material and the wavelength absorbing material be dispersed in a scatteringly distributed condition in a cover member enclosing the light emitting element chip. 
     The cover member may be made from a light transmitting resin material. 
     Because the resin material is mixed with the fluorescent particles for producing light with a wider range of wavelength and with coloring particles for absorbing a desired wavelength range, it is possible to produce a soft, intermediate, pastel color of light based only on the color of light emitted by a single light emitting element chip. 
     According to another aspect of the present invention, the fluorescent particles are made from yttrium aluminum garnet (YAG). 
     Because this invention uses yttrium aluminum garnet (YAG) for the fluorescent particles, a wide range of wavelengths, 400 nm to 700 nm, can be produced stably and efficiently by exciting the fluorescent particles. 
     According to still another aspect of the present invention, the coloring particles are made from a dye which absorbs a part of wavelengths of light from the light emitting element chip and a part of wavelengths of light that were changed by the fluorescent particles. 
     Because this invention uses a dye for the coloring particles, it is possible to produce a soft, intermediate, pastel color of light with low cost. 
     According to a further aspect of the present invention, the light emitting element chip is a blue light emitting diode element formed of a gallium nitride compound semiconductor. 
     These and other features and advantages of the present invention will be described in more detail by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing an example of a conventional surface mount type multicolor light emitting diode. 
     FIG. 2 is a perspective view showing an example of a conventional lead frame type multicolor light emitting diode. 
     FIG. 3 is a perspective view showing a light emitting diode as a first embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of the light emitting diode of the first embodiment taken along the line A—A of FIG. 3 when the light emitting diode is mounted on a mother board. 
     FIG. 5 is an explanatory diagram showing how light emitted from the light emitting element chip in the light emitting diode is changed in wavelength. 
     FIG. 6 is a CIE chromaticity diagram showing a result of chromaticity measurements using the light emitting diode of the present invention. 
     FIG. 7 is an explanatory diagram showing how light emitted from a light emitting element chip in a light emitting diode of a second embodiment is changed in wavelength. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, embodiments of a light emitting diode according to the present invention will be described in detail by referring to the accompanying drawings. FIG. 3 to FIG. 5 represent the first embodiment of the invention which is implemented as a surface mount type light emitting diode. A surface mount type light emitting diode  31  of this embodiment has a rectangular substrate  32 , a pair of electrodes (a cathode electrode  33  and an anode electrode  34 ) patterned on the substrate  32 , and a light emitting element chip  41  arranged almost at a center on the upper surface of the substrate  32 . The light emitting diode  31  is surface-mounted on a mother board  37  by having its electrodes  33 ,  34  soldered at  40  on the underside to printed wires  38 ,  39  (see FIG.  4 ). 
     The light emitting element chip  41  is secured to the substrate  32  by a bonding agent  51  applied to the underside thereof. The light emitting element chip  41  is a blue light emitting element made of a gallium nitride compound semiconductor which is grown by diffusing an n-type semiconductor  44  and a p-type semiconductor  45  over the upper surface of an element substrate  43  of sapphire glass, as shown in FIG.  5 . The n-type semiconductor  44  and the p-type semiconductor  45  have an n-type electrode  46  and a p-type electrode  47 , respectively, which are connected through bonding wires  48 ,  49  to the cathode electrode  33  and the anode electrode  34 , respectively, provided on the substrate  32 . Supplying a predetermined amount of current causes the light emitting element chip  41  to emit light in blue. 
     This invention is characterized by a wavelength conversion material which, upon receiving light from the light emitting element chip  41 , changes the wavelength of the received light and also by a wavelength absorbing material which, upon receiving light from the light emitting element chip and the wavelength conversion material, absorbs a part of the wavelengths of the received light. To describe in more detail, in this embodiment the wavelength conversion material is made from fluorescent particles  52  and the wavelength absorbing material is made from coloring particles  53 . 
     The fluorescent particles  52  and the coloring particles  53  are preferably scattered in a cover member, for example, a light transmitting resin material  55 , which encloses the light emitting element chip  41  and seals and protects the substrate  32 . More specifically, the resin material  55  is a transparent base material of silicone resin which is mixed with appropriate amounts of yttrium aluminum garnet (abbreviated YAG), the material for the fluorescent particles  52 , and of dye, the material for the coloring particles  53 . The added materials are uniformly scattered. The resin material  55  containing the fluorescent particles  52  and the coloring particles  53  is formed in a rectangular parallelepiped shape over the upper surface of the substrate  32  except for through-hole portions  35  in the cathode electrode  33  and anode electrode  34 . 
     The dye used as the coloring particles  53  is made as follows. Appropriate amounts of, for example, four compounds—phthalocyanine compound, anthraquinone compound, azo compound and quinophthalone compound—are mixed to generate six colors in advance, i.e., blue, green, yellow, orange, red and violet. In this case, a blue is produced from the phthalocyanine compound, and a green is produced by mixing the phthalocyanine compound and the anthraquinone compound. A yellow is made from the quinophthalone compound, and an orange and a red are produced by mixing the anthraquinone compound and the azo compound. A violet is made either by using the anthraquinone compound or by mixing the phthalocyanine compound, anthraquinone compound and azo compound. In this embodiment, these six colors produced in this manner in advance are further mixed to generate a desired chromaticity. The coloring particles  53  are not limited to the above-described dye but a pigment may also be used. 
     In the light emitting diode  31  of this embodiment, when a current flows through the light emitting element chip  41 , blue light is produced at a boundary plane between the n-type semiconductor  44  and the p-type semiconductor  45 . The blue light is emitted as blue rays of light  56  in various directions, upward, sideways and downward. The blue rays  56  emitted upward in particular excite the fluorescent particles  52  scattered in the resin material  55  to produce wavelength-changed yellow rays  57  with a wider range of wavelength which in turn propagate through the resin material  55  in all directions. At the same time, when the yellow rays  57  and the blue rays  56  pass through the coloring particles  53  distributed in the resin material  55 , the coloring particles  53  absorb a part of the wavelengths of the yellow rays  57  and blue rays  56 , thus producing a variety of intermediate colors of rays  58 . Any intermediate color of rays  58  can be formed by appropriately selecting the kind and amount of a dye, the material for the coloring particles  53  scatteringly distributed in the resin material  55 . The brightness of an intermediate color can be adjusted by controlling the current flowing through the light emitting element chip  41 . 
     FIG. 6 is a CIE chromaticity diagram showing a result of chromaticity measurement when various intermediate colors of rays  58  are produced by the light emitting diode  31 . The diagram has found that although the color of light produced varies depending on the kind of the coloring particles  53  used, many rays are observed in an intermediate color range and few rays light up in those areas on or around the lines connecting the red LED, blue LED and green LED which are close to primary colors. 
     FIG. 7 shows a light emitting diode according to the second embodiment of the invention. In this embodiment, the bonding agent  51  that bonds the light emitting element chip  41  to the substrate  32  is also mixed with the fluorescent particles  52  to form a fluorescent bonding layer  60  so that the rays of light propagating downward from the light emitting element chip  41  can also be changed in wavelength, thereby producing a brighter intermediate color of light. Since other constitutional elements are similar to those of the first embodiment shown in FIGS. 3 to  5 , their detailed explanations are omitted. In this embodiment, because the fluorescent particles  52  are mixed in the bonding agent  51 , the blue rays  56  propagating downward from the light emitting element chip  41  excite the fluorescent particles  52  distributed in the fluorescent bonding layer  60  and at the same time the blue rays  56  propagating upward from the light emitting element chip  41  excite the fluorescent particles  52  mixed in the resin material  55 , thus producing brighter yellow rays  57 . Further, because the amount of rays of an intermediate color  58  produced by the emitted light passing through the coloring particles  53  increases, the brightness of the intermediate color also increases. 
     Next, the features of light emission in the light emitting diode  61  of the second embodiment will be explained by referring to FIG.  7 . When a current flows through the light emitting element chip  41 , blue light is produced at the boundary plane between the n-type semiconductor  44  and the p-type semiconductor  45 . The blue light is emitted as blue rays of light  56  in various directions, upward, sideways and downward. The blue rays  56  emitted downward in particular excite the fluorescent particles  52  scatteringly distributed in the fluorescent bonding layer  60  to produce wavelength-changed yellow rays  57  which in turn propagate in all directions. Then, the yellow rays  57  and the blue rays  56  are absorbed by the coloring particles  53  distributed in the resin material  55 , thus producing a desired intermediate color of light  58 . As with the first embodiment, a desired intermediate color of light  58  can be obtained by selecting an appropriate kind and amount of a dye, the material for the coloring particles  53  contained in the resin material  55 . Controlling the current applied to the light emitting element chip  41  can adjust the brightness of the light produced. 
     In either of the embodiments above, we have explained about a chip type light emitting diode directly surface-mounted on the printed wires  38 ,  39  provided on the upper surface of the mother board  37  as shown in FIG.  4 . It should be noted that the light emitting diode according to this invention can also be applied to a lead frame type light emitting diode. That is, a desired intermediate color of light can be produced by rigidly mounting a blue light emitting element chip of gallium nitride compound semiconductor on a base and scatteringly distributing appropriate amounts of the fluorescent particles  52  and the coloring particles  53  in a bullet-shaped resin sealing member. 
     While, in the embodiments above, we have described the case where the light emitting element chip and the electrodes are connected by wire bonding, the present invention is not limited to this method of connection but also includes other connection methods such as flip-chip mounting using solder bumps. 
     As described above, by scatteringly distributing the fluorescent particles and coloring particles in the resin material that seals the light emitting element chip, the present invention can provide a light emitting diode that can emit an intermediate color of light with a single light emitting element chip. Further, by adjusting a volume ratio between the fluorescent particles and the coloring particles scatteringly distributed in the resin material, the range of wavelength to be absorbed and the amount of wavelength absorption can be changed, thus producing a variety of intermediate colors of light. 
     Further, because there is no need to incorporate two or more light emitting element chips with different colors of emitted light, as required by the conventional technology, the size of the light emitting diode can be reduced. 
     Further, because the color of light emitted by the light emitting diode can be selected by the fluorescent particles and coloring particles mixed in the resin material, there is no need to control the current applied to the light emitting element chip, as is required by the conventional light emitting diode. 
     The light emitting diode according to the present invention is advantageously used as a chip type light emitting diode for emitting an intermediate color of light and also has a structure suited for mass production. 
     It should be appreciated that the present invention is not limited to the embodiments described above and that various changes and modifications may be made without departing from the scope of this invention.