Light emitting device and fabricating method thereof

Abstract

A light-emitting device includes a light-emitting element for emitting primary light, and a wavelength conversion unit for absorbing part of the primary light and emitting secondary light having a wavelength longer than that of the primary light, wherein the wavelength conversion unit includes plural kinds of phosphors having light absorption characteristics different from each other, and then at least one kind of phosphor among the plural kinds of phosphors has an absorption characteristic that can absorb the secondary light emitted from at least another kind of phosphor among the plural kinds of phosphors.

Claims

1. A light-emitting device, comprising: a light-emitting element for emitting primary light; and a wavelength conversion unit for absorbing part of the primary light and emitting secondary light having a wavelength longer than that of the primary light, wherein said light-emitting element is formed of a gallium nitride-based semiconductor, said wavelength conversion unit includes plural kinds of phosphors different in light absorption characteristic from each other that are dispersed into a transparent binder resin, said primary light that said light-emitting element emits has a peak wavelength in the range from 400 nm to 500 nm, said secondary light that at least one kind of phosphors among the plural kinds of phosphors emits has a peak wavelength in the range from 500 nm to 550 nm, said secondary light that at least one other kind of phosphors among the plural kinds of phosphors emits has a peak wavelength in the range from 600 nm to 650 nm, said plural kinds of phosphors have different median values of particle sizes from each other, the different median diameters are adjusted so that the plural kinds of phosphors have different settling speeds, and a concentration of the phosphor that emits the secondary light having the peak wavelength in the range from 600 nm to 650 nm adjacent the light-emitting element is higher than a concentration of the phosphor that emits the secondary light having the peak wavelength in the range from 500 nm to 550 nm adjacent the light-emitting element.

2. The light-emitting device of claim 1, wherein said binder resin is silicone-based resin.

3. The light-emitting device of claim 1, wherein said binder resin is epoxy-based resin.

4. A liquid crystal display having said light-emitting device of claim 1 as a backlight light source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic vertical cross sectional view showing a structure of a light-emitting device according to an embodiment of the present invention.

(2) FIG. 2 is a graph showing spectral distribution of excitation and emission of a phosphor for emitting red light.

(3) FIG. 3 is a graph showing spectral distribution of excitation and emission of a phosphor for emitting green light.

(4) FIG. 4 is a graph showing spectral distribution of excitation and emission of a phosphor for emitting blue light.

(5) FIG. 5 is a graph showing emission spectral distribution in a light-emitting device according to an embodiment of the present invention.

(6) FIG. 6 is a graph showing spectral characteristics of color filters used for evaluation of properties of the light-emitting device.

(7) FIG. 7 shows schematic cross sectional views illustrating a fabrication process of a light-emitting device according to another embodiment of the present invention.

(8) FIG. 8 is an xy chromaticity diagram showing the relation between the temperature of blackbody radiation and the chromaticity coordinates.

(9) FIG. 9 is a schematic vertical cross sectional view showing a structure of a light-emitting device according to a further embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Example 1

(10) In FIG. 1, a light-emitting device according to Example 1 of the present invention is illustrated in a schematic vertical cross sectional view. This light-emitting device 10 includes a light-emitting element 11 for emitting primary light, and a wavelength conversion unit 12 that absorbs at least part of the primary light and emits secondary light having a wavelength longer than that of the primary light. Light-emitting element 11 is mounted on a cathode terminal 18 and it is electrically connected to an anode terminal 17 and cathode terminal 18 via gold wires 19.

(11) For light-emitting element 11, it is possible to use a gallium nitride (GaN)-based light-emitting diode having an emission peak wavelength at 410 nm, for example.

(12) Wavelength conversion unit 12 includes a layer 13 that contains a phosphor having a composition of (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3 for emitting red light, a layer 14 that contains a phosphor having a composition of (Ba.sub.0.85Eu.sub.0.15) (Mg.sub.0.80Mn.sub.0.20)Al.sub.10O.sub.17 for emitting green light, and a layer 15 that contains a phosphor having a composition of (Ba.sub.0.80Eu.sub.0.20)MgAl.sub.10O.sub.17 for emitting blue light.

(13) FIG. 2 shows spectral distribution of excitation and emission of the phosphor for emitting red light used in Example 1; FIG. 3 shows spectral distribution of excitation and emission of the phosphor for emitting green light; and FIG. 4 shows spectral distribution of excitation and emission of the phosphor for emitting blue light. Specifically, in each graph shown in FIGS. 2-4, a horizontal axis represents the wavelength (nm) of light, and a vertical axis represents the relative intensity of light. Further, curves 31, 33 and 35 in the graphs represent spectral distribution of excitation of the phosphors, and curves 32, 34 and 36 represent spectral distribution of emission from the phosphors.

(14) As seen from FIGS. 2-4, each phosphor in Example 1 can very efficiently emits red, green or blue light in response to the primary light having the peak wavelength of 410 nm.

(15) Further, it is recognized that the phosphor for emitting red light can emit the red light by absorbing even the green light (wavelength around 520 nm) and the blue light (wavelength around 450 nm) emitted from the phosphors for emitting green and blue lights, respectively.

(16) However, it is not beneficial to excite the phosphor for emitting red light by the green light (near 520 nm wavelength) and the blue light (near 450 nm wavelength) emitted as the secondary lights from the phosphors excited by the primary light, since it would decrease wavelength conversion efficiency as a whole.

(17) More specifically, to obtain a light-emitting device of high luminance, it is important to stack phosphor layer 13 for red light, phosphor layer 14 for green light and phosphor layer 15 for blue light in this order, as in this Example 1. It is noted that the phosphor for green light and the phosphor for blue light may be mixed to form a single light-emitting layer, since the phosphor for green light cannot be excited by the blue light to emit green light.

(18) In Example 1, each of the phosphor for red light, the phosphor for green light and the phosphor for blue light is kneaded into a (silicone-based or epoxy-based) binder resin, which is then introduced into a cup 16 of the device package shown in FIG. 1 in order of red light-emitting layer 13, green light-emitting layer 14 and blue light-emitting layer 15, which are then set to form a phosphor layered structure 12.

(19) On the inner wall of cup 16, terraces 16a are provided so as to prevent the resins kneaded with the phosphors and introduced into the cup from climbing up by surface tension along the inner wall of the cup, thereby making it possible to obtain a uniform thickness for each light-emitting layer. Further, it is preferable that the inner wall of the cup is made to be inclined more steeply in the upper part than in the lower part with the boundary formed by terrace 16a. This can reduce shade of terrace 16a, which is caused in the upper phosphor layer by light emitted from the lower phosphor layer.

(20) As Comparative Example 1, phosphors of the exactly same kinds as in Example 1 were mixed at a mass ratio of phosphor for blue light: phosphor for green light: phosphor for red light=2.0:1.5:1.0, to form a single light-emitting layer serving as the wavelength conversion unit.

(21) Table 1 shows evaluation results of white light emitted from the light-emitting devices of Example 1 and Comparative Example 1.

(22) TABLE-US-00001 TABLE 1 Brightness (relative value) Tc-duv Example 1 100% 7000K + 0.001 Comparative  70% 7000K + 0.001 Example 1

(23) As seen from Table 1, while the light-emitting devices of Example 1 and Comparative Example 1 emit light of the same color, the light-emitting device of Example 1 is considerably improved in brightness as compared to Comparative Example 1.

(24) Here, “Tc” represents the correlated color temperature of light emitted from the light-emitting device, and “duv” represents the deviation of the chromaticity point of the emitted light from the blackbody radiation locus (i.e., the length of the normal drawn from the chromaticity point of the emitted light down to the blackbody radiation locus on the U*V*W chromaticity diagram (CIE 1964 uniform color space)). It is generally considered that light having duv of less than 0.01 is felt as natural (white) light, similarly to light emitted from a common tungsten filament light bulb and the like. With the blackbody radiation temperature of 7000K, natural white light is obtained, since it is close to the color temperature of the sun.

(25) In each of Example 1 and Comparative Example 1, although the primary light emitted from the light-emitting element has its peak wavelength of 410 nm in the wavelength range effective for excitation of the phosphors, it is low in luminous efficacy and thus does not contribute to brightness of the light-emitting device even when it is leaked outside. Therefore, it is preferable that a sheet 12a coated with an optical film (for example, multi-layered interference film) having the property of reflecting only the primary light is laminated on the outermost surface of phosphor layer structure 12. The primary light reflected by this sheet 12a can again contribute to excitation of the phosphor layers, thereby enabling emission of brighter light from the light-emitting device as a whole.

Example 2

(26) In Example 2 of the present invention, a gallium nitride (GaN)-based light-emitting diode having a peak wavelength at 460 nm was used as the light-emitting element.

(27) For the wavelength conversion unit, there were used a phosphor for green light represented by a composition of (Sr.sub.0.75Ba.sub.0.24Eu.sub.0.01).sub.2SiO.sub.4 and a phosphor for red light represented by a composition of (Ca.sub.0.985Eu.sub.0.015)AlSiN.sub.3. It is recognized from FIG. 2 that the phosphor for red light can absorb green light (wavelength near 550 nm) emitted from the phosphor for green light and then can emit red light.

(28) The light-emitting device of Example 2 was fabricated using the light-emitting diode and the phosphors, in a similar manner as in Example 1. As Comparative Example 2, there was fabricated a light-emitting device having a wavelength conversion unit formed of a single light-emitting layer made by mixing exactly the same kinds of phosphors as in Example 2.

(29) Properties of the light-emitting devices of Example 2 and Comparative Example 2 were evaluated, of which results are shown in Table 2.

(30) TABLE-US-00002 TABLE 2 Brightness (relative value) Tc-duv Example 2 100% 6000K − 0.001 Comparative  75% 6000K − 0.001 Example 2

(31) As seen from Table 2, while the light-emitting device of Example 2 emits light having the same color as that emitted from the light-emitting device of Comparative Example 2, its brightness is considerably improved.

Examples 3-8

(32) Light-emitting devices of Examples 3-8 and Comparative Examples 3-8 were fabricated in a similar manner as in Example 1 and Comparative Example 1, except that the emission peak wavelengths of the light-emitting elements and the kinds of phosphors used were changed. Evaluation results of their properties are shown in Table 3.

(33) TABLE-US-00003 TABLE 3 Peak wavelength Brightness of primary light Phosphors (relative value) Tc-duv Ex. 3 410 nm red: (Ca.sub.0.94Sr.sub.0.05Eu.sub.0.01)AlSiN.sub.3 100% 6500K − 0.003 green: (Sr.sub.0.70Ba.sub.0.28Eu.sub.0.02).sub.2SiO.sub.4 blue: (Sr.sub.0.74Ba.sub.0.20Ca.sub.0.05Eu.sub.0.01).sub.10(PO.sub.4).sub.6•Cl.sub.2 Com. ″ red: (Ca.sub.0.94Sr.sub.0.05Eu.sub.0.01)AlSiN.sub.3  73% ″ Ex. 3 green: (Sr.sub.0.70Ba.sub.0.28Eu.sub.0.02).sub.2SiO.sub.4 blue: (Sr.sub.0.74Ba.sub.0.20Ca.sub.0.05Eu.sub.0.01).sub.10(PO.sub.4).sub.6•Cl.sub.2 Ex. 4 410 nm red: (Ca.sub.0.99Eu.sub.0.01)(Al.sub.0.90Ga.sub.0.10)SiN.sub.3 100% 7500K + 0.001 green: (Ba.sub.0.86Eu.sub.0.14)(Mg.sub.0.75Mn.sub.0.25)Al.sub.10O.sub.17 blue: (Ba.sub.0.86Eu.sub.0.14)(Mg.sub.0.99Mn.sub.0.01)Al.sub.10O.sub.17 Com. ″ red: (Ca.sub.0.99Eu.sub.0.01)(Al.sub.0.90Ga.sub.0.10)SiN.sub.3  70% ″ Ex. 4 green: (Ba.sub.0.86Eu.sub.0.14)(Mg.sub.0.75Mn.sub.0.25)Al.sub.10O.sub.17 blue: (Ba.sub.0.86Eu.sub.0.14)(Mg.sub.0.99Mn.sub.0.01)Al.sub.10O.sub.17 Ex. 5 450 nm red: (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3 100% 5800K − 0.002 green: (Sr.sub.0.72Ba.sub.0.25Ca.sub.0.01Eu.sub.0.02)2SiO.sub.4 Com. ″ red: (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3  75% ″ Ex. 5 green: (Sr.sub.0.72Ba.sub.0.25Ca.sub.0.01Eu.sub.0.02)2SiO.sub.4 Ex. 6 450 nm red: (Ca.sub.0.97Ba.sub.0.01Eu.sub.0.02)(Al.sub.0.99In.sub.0.01)SiN.sub.3 100% 7500K − 0.003 green: Mg.sub.3(Al.sub.0.85Ce.sub.0.15).sub.2(SiO.sub.4).sub.3 Com. ″ red: (Ca.sub.0.97Ba.sub.0.01Eu.sub.0.02)(Al.sub.0.99In.sub.0.01)SiN.sub.3  70% ″ Ex. 6 green: Mg.sub.3(Al.sub.0.85Ce.sub.0.15).sub.2(SiO.sub.4).sub.3 Ex. 7 465 nm red: (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3 100% 4200K + 0.002 green: Ca.sub.3(Y.sub.0.80Al.sub.0.10Ce.sub.0.10).sub.2(SiO.sub.4).sub.3 Com. ″ red: (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3  72% ″ Ex. 7 green: Ca.sub.3(Y.sub.0.80Al.sub.0.10Ce.sub.0.10).sub.2(SiO.sub.4).sub.3 Ex. 8 460 nm red: (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3 100% 3500K − 0.002 green: (Sr.sub.0.76Ba.sub.0.22Eu.sub.0.02).sub.2SiO.sub.4 Com. ″ red: (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3  75% ″ Ex. 8 green: (Sr.sub.0.76Ba.sub.0.22Eu.sub.0.02).sub.2SiO.sub.4

(34) As seen from Table 3, the light-emitting devices of Examples 3-8 of the present invention are considerably improved in brightness as compared to Comparative Examples 3-8, respectively.

Example 9

(35) FIG. 1 may be referred to also for a white light-emitting device according to Example 9 of the present invention. A white light-emitting device 10 of Example 9 includes a light-emitting element 11 for emitting primary light, and a wavelength conversion unit 12 that absorbs at least part of the primary light and emits secondary light having a wavelength longer than that of the primary light. For light-emitting element 11, it is possible to use a gallium nitride (GaN)-based light-emitting diode having a peak wavelength at 410 nm.

(36) For wavelength conversion unit 12, a resin layer 13 that contains a phosphor for red light represented by a composition of (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3, a resin layer 14 that contains a phosphor for green light represented by a composition of (Ba.sub.0.85Eu.sub.0.15) (Mg.sub.0.80Mn.sub.0.20)Al.sub.10O.sub.17, and a resin layer 15 that contains a phosphor for blue light represented by a composition of (Ba.sub.0.80Eu.sub.0.20)MgAl.sub.10O.sub.17 were stacked successively in this order, with the three kinds of phosphors being contained at a mass ratio of 1:1:1.

(37) FIG. 5 is a graph showing emission spectral distribution of the white light-emitting device of Example 9. Specifically, in the graph, a horizontal axis represents emission wavelength (nm), and a vertical axis represents radiant intensity (a.u.: arbitrary unit) of light. It is found in this graph that the light from the phosphor for emitting green light (peak wavelength around 520 nm) as the secondary light has a narrow spectral width corresponding to the light emitted from Mn.

(38) In the white light-emitting device of Example 9, brightness and color reproducibility (NTSC ratio) were evaluated. FIG. 6 shows the spectral characteristics of the color filters of blue, green and red that were used for measurement of the color reproducibility. In this graph, a horizontal axis represents the wavelength (nm) of light, and a vertical axis represents the amount (a.u.) of transmitted light. Further, curves 37, 38 and 39 in the graph represent the characteristics of the blue filter, the green filter and the red filter, respectively.

(39) For the purpose of comparison with Example 9, a light-emitting device of Comparative Example 9 was fabricated. The light-emitting device of Comparative Example 9 differs from that of Example 9 only in that a single resin layer was used for the wavelength conversion unit, which contains a phosphor for blue light represented by a composition of (Ba.sub.0.80Eu.sub.0.20)MgAl.sub.10O.sub.17 and a phosphor for yellow light represented by a composition of (Sr.sub.0.92Ba.sub.0.05Ca.sub.0.01Eu.sub.0.02).sub.2SiO.sub.4 mixed at a mass ratio of 1.5:1.0.

(40) Table 4 shows evaluation results of the white light emitted by the light-emitting devices of Example 9 and Comparative Example 9.

(41) TABLE-US-00004 TABLE 4 Brightness Color (relative reproducibility value) Tc-duv (NTSC ratio) Ex. 9 100.0% 6600K + 0.007 85.0% Com. Ex. 9  97.3% 6600K + 0.007 48.1%

(42) As seen from Table 4, the light-emitting device of Example 9 is considerably improved not only in brightness but also in color reproducibility (NTSC ratio) as compared to Comparative Example 9.

(43) In Table 4, the blackbody radiation temperature of 6600K, which is close to the temperature of the sun, means that natural white light can be obtained.

(44) For the purpose of reference, an xy chromaticity diagram of FIG. 8 shows the relation between the blackbody radiation temperature and the xy chromaticity coordinates. In this chromaticity diagram, a plurality of circles represent the spectrum locus, and a plurality of triangles represent the blackbody radiation locus.

(45) The light-emitting device of Example 9, for which FIG. 1 may be referred to, is fabricated more specifically as follows. GaN-based light-emitting element 11 is mounted on one of a pair of lead frames (thin metal plates) 17, 18, and is electrically connected to lead frames 17, 18 via a pair of wires 19.

(46) A bowl-shaped cup 16 is formed surrounding light-emitting element 11 by using a resin of white color having high reflectivity for visible light. On the inner wall of the bowl-shaped cup, terraces 16a are provided to stabilize the levels of the liquid resins containing the phosphors. As a result, each of the resin layers 13, 14 and 15 containing the phosphors can have approximately a predetermined uniform thickness.

(47) Light-emitting element 11 is molded with a resin layer 13 added with a phosphor for secondary light of red, a resin layer 14 added with a phosphor for secondary light of green, and a resin layer 15 added with a phosphor for secondary light of blue, successively in this order. After molding of a resin layer added with one kind of phosphor, it may be set provisionally before introducing the next resin layer. This can suppress reduction of production efficiency while preventing the adjacent resin layers from being mixed with each other.

(48) Although the inner wall of cup 16 may be left with the white color resin material, it is more preferable to coat the inner wall with a metal having high reflectivity for visible light, such as silver, aluminum or the like, in order to further improve the luminous efficiency of the light-emitting device.

(49) In FIG. 7, an alternative fabrication process of a light-emitting device related to Example 9 is illustrated in schematic cross sections.

(50) In FIG. 7(A), GaN-based light-emitting element 11 is mounted on a hard wiring-board 21, and electrically connected thereto via wire 22. As shown in FIG. 7(B), light-emitting element 11 is molded successively with a resin layer added with a phosphor for secondary light of red, a resin layer added with a phosphor for secondary light of green, and a resin layer added with a phosphor for secondary light of blue, and thus is covered with a resin dome 23 containing these resin layers.

(51) Resin dome 23 can be formed without use of a die, a metallic mold or the like, by using a resin having high thixotropy. As shown in FIG. 7(C), however, a die assembly may be used to accurately regulate the thicknesses of the resin layers contained in resin dome 23.

(52) In FIG. 7(C), light-emitting element 11 is covered with resin layer 23a for red light that contains the phosphor for emitting red light. Resin layer 23a is provisionally set before being pressed by a press mold 25. This allows the thickness t of resin layer 23a above the top face of light-emitting element 11 to be a predetermined value, as shown in FIG. 7(D). Similarly, resin layer 23b for green that contains the phosphor for emitting green light and resin layer 23c for blue that contains the phosphor for emitting blue light are formed using press mold 25, whereby making it possible to obtain a white light-emitting device including light-emitting element 11 covered with three resin layers 23a, 23b and 23c having the controlled thicknesses, as shown in FIG. 7(E). In this case as well, it is possible to suppress reduction of production efficiency while preventing the adjacent resin layers from being mixed with each other, because light-emitting element 11 is covered with one kind of resin layer which is then provisionally set before being covered with the next resin layer.

Example 10

(53) In Example 10 of the present invention, a gallium nitride (GaN)-based light-emitting diode having a peak wavelength of 390 nm was used as the light-emitting element. For the wavelength conversion unit, there were used a resin layer that contains a phosphor for red light represented by a composition of (Ca.sub.0.985Eu.sub.0.015)AlSiN.sub.3, a resin layer that contains a phosphor for green light represented by a composition of (Ba.sub.0.70Sr.sub.0.10Eu.sub.0.20)(Mg.sub.0.75Mn.sub.0.25)Al.sub.10O.sub.17, and a resin layer that contains a phosphor for blue light represented by a composition of (Ba.sub.0.80Eu.sub.0.20)MgAl.sub.10O.sub.17.

(54) The light-emitting device of Comparative Example 10 prepared for comparison with Example 10 differs from that of Example 10 only in that there was used a single resin layer which contains a mixture of a phosphor for blue light represented by a composition of (Ba.sub.0.80Eu.sub.0.20)MgAl.sub.10O.sub.17 and a phosphor for yellow light represented by a composition of (Y.sub.0.52Gd.sub.0.35Ce.sub.0.13).sub.3Al.sub.5O.sub.12.

(55) In Example 10 and Comparative Example 10 also, similarly as in the case of Example 9, the light-emitting devices as shown in FIG. 1 were fabricated and their properties were evaluated. Table 5 shows the evaluation results.

(56) TABLE-US-00005 TABLE 5 Brightness Color (relative reproducibility value) Tc-duv (NTSC ratio) Ex. 10 100.0% 7700K − 0.001 83.8% Com. Ex. 10  96.8% 7700K − 0.001 47.9%

(57) As seen from Table 5, the light-emitting device of Example 10 is also considerably improved not only in brightness but also in color reproducibility (NTSC ratio) as compared to Comparative Example 10.

Examples 11-15

(58) Light-emitting devices of Examples 11-15 of the present invention and Comparative Examples 11-15 for comparison therewith were fabricated similarly as in Example 9 and Comparative Example 9. Evaluation results of their properties are shown in Table 6. In Examples 11-15 and Comparative Examples 11-15, the light-emitting devices as shown in FIG. 1 were fabricated, varying the emission peak wavelengths of the light-emitting elements and the compositions of the phosphors used.

(59) TABLE-US-00006 TABLE 6 Color reproducibility Peak wavelength Brightness (NTSC of primary light Phosphors (relative value) Tc-duv ratio) Ex. 11 425 nm red: (Ca.sub.0.97Ba.sub.0.01Eu.sub.0.02)(Al.sub.0.99In.sub.0.01)SiN.sub.3 100.0% 9000K − 0.002 85.2% green: (Ba.sub.0.50Sr.sub.0.35Eu.sub.0.15)(Mg.sub.0.80Mn.sub.0.20Al.sub.10O.sub.17 blue: (Sr.sub.0.64Ba.sub.0.30Ca.sub.0.05Eu.sub.0.01).sub.10(PO.sub.4).sub.6•Cl.sub.2 Com. ″ blue: (Sr.sub.0.64Ba.sub.0.30Ca.sub.0.05Eu.sub.0.01).sub.10(PO.sub.4).sub.6•Cl.sub.2  97.2% 9000K − 0.002 48.3% Ex. 11 yellow: (Sr.sub.0.85Ba.sub.0.13Ca.sub.0.01Eu.sub.0.02SiO.sub.4 Ex. 12 400 nm red: (Ca.sub.0.94Sr.sub.0.05Eu.sub.0.01)AlSiN.sub.3 100.0% 8300K + 0.002 85.3% green: (Ba.sub.0.80Sr.sub.0.05Eu.sub.0.15)(Mg.sub.0.80Mn.sub.0.20Al.sub.10O.sub.17 blue: (Ba.sub.0.86Eu.sub.0.14)(Mg.sub.0.99Mn.sub.0.01)Al.sub.10O.sub.17 Com. ″ blue: (Ba.sub.0.86Eu.sub.0.14)(Mg.sub.0.99Mn.sub.0.01)Al.sub.10O.sub.17  96.7% 8300K + 0.002 47.8% Ex. 12 yellow: (Y.sub.0.62Gd.sub.0.25Ce.sub.0.13).sub.3(Al.sub.0.90Ga.sub.0.10).sub.5O.sub.12 Ex. 13 405 nm red: (Ca.sub.0.99Eu.sub.0.01)(Al.sub.0.90Ga.sub.0.10)SiN.sub.3 100.0% 6500K − 0.002 84.8% green: (Ba.sub.0.84Sr.sub.0.01Eu.sub.0.15)(Mg.sub.0.75Mn.sub.0.25)Al.sub.10O.sub.17 blue: (Ba.sub.0.82Sr.sub.0.03Eu.sub.0.15)MgAl.sub.10O.sub.17 Com. ″ blue: (Ba.sub.0.82Sr.sub.0.03Eu.sub.0.15)MgAl.sub.10O.sub.17  97.1% 6500K − 0.002 48.2% Ex. 13 yellow: (Sr.sub.0.77Ba.sub.0.20Ca.sub.0.01Eu.sub.0.02).sub.2SiO.sub.4 Ex. 14 430 nm red: (Ca.sub.0.99Eu.sub.0.01)AlSiN.sub.3 100.0% 7500K + 0.001 84.9% green: Ma.sub.0.45Sr.sub.0.35Eu.sub.0.20(Mg.sub.0.80Mn.sub.0.20Al.sub.10O.sub.17 blue: (Sr.sub.0.62Ba.sub.0.35Ca.sub.0.01Eu.sub.0.02).sub.10(PO.sub.4).sub.6•Cl.sub.2 Com. ″ blue: (Sr.sub.0.62Ba.sub.0.35Ca.sub.0.01Eu.sub.0.02).sub.10(PO.sub.4).sub.6•Cl.sub.2  96.6% 7500K + 0.001 48.1% Ex. 14 yellow: (Sr.sub.0.61Ba.sub.0.35Ca.sub.0.03Eu.sub.0.02SiO.sub.4 Ex. 15 395 nm red: (Ca.sub.0.98Eu.sub.0.02)AlSiN.sub.3 100.0% 8100K − 0.001 85.1% green: (Ba.sub.0.75Sr.sub.0.15Eu.sub.0.10)(Mg.sub.0.75Mn.sub.0.25)Al.sub.10O.sub.17 blue: (Ba.sub.0.75Sr.sub.0.15Eu.sub.0.10)MgAl.sub.10O.sub.17 Com. ″ blue: (Ba.sub.0.75Sr.sub.0.15Eu.sub.0.10)MgAl.sub.10O.sub.17  97.0% 8100K − 0.001 47.7% Ex. 15 yellow: (Y.sub.0.62Gd.sub.0.25Ce.sub.0.13).sub.3(Al.sub.0.80Ga.sub.0.20).sub.5O.sub.12

(60) As seen from Table 6, the light-emitting devices of Examples 11-15 containing the phosphors according to the present invention are considerably improved not only in brightness but also in color reproducibility (NTSC ratio), as compared to Comparative Examples 11-15 each including a combination of a blue light-emitting element with a divalent europium-activated (Sr,Ba,Ca).sub.2SiO.sub.4 phosphor or a trivalent cerium-activated (Y,Gd).sub.3(Al,Ga).sub.5O.sub.12 phosphor that is excited by the blue light and emits yellow light.

(61) The present invention requires that the peak wavelength of the primary light emitted from the light-emitting element falls within the range from 380 nm to 450 nm The light-emitting element having the peak wavelength within the range from 390 nm to 420 nm is more suitable for the light-emitting device of the present invention.

(62) In order to significantly improve the color reproducibility (NTSC ratio), it is desirable to narrow the half bandwidth of emission spectrum of the phosphor for emitting green light (or components for emitting green light contained therein). To this end, light emission due to divalent manganese (Mn) as in the present invention is suitable, and then aluminate is suitable for the matrix of the phosphor for emitting green light.

(63) One of features of the above-described Examples is that the resin layers are arranged from the side closer to the light-emitting element in decreasing order of the wavelength of the secondary light emitted by the phosphors contained therein. In the case that a single resin layer containing a mixture of such phosphors for three colors as in the present invention is used for the wavelength conversion unit, however, the color reproducibility (NTSC ratio) is not adversely affected, though the brightness is reduced considerably. Thus, such a single resin layer containing the mixed phosphors of three colors may be used when improvement in color reproducibility (NTSC ratio) alone is pursued.

(64) In the above-described examples, the resin layer containing the phosphor for blue light is stacked on the resin layer containing the phosphor for green light. In the phosphor for green light in the present invention, however, intensity of the second light emitted by excitation with the primary blue light having a wavelength around 450 nm is weak. This means that even if the phosphor for green light and the phosphor for blue light are mixed together and contained in a single resin layer, the white light is hardly decreased in brightness, and thus the functions and effects of the present invention can be maintained.

Example 16

(65) In FIG. 9, a main part of a light-emitting device according to Example 16 of the present invention is illustrated in a schematic vertical cross sectional view. The light-emitting device of FIG. 9 differs from that of FIG. 1 in that a light-emitting element 11 is mounted at the bottom of a bowl-shaped cup 46 having a reflective surface (inner peripheral surface) made of a white color resin and having no terraces, and in that light-emitting element 11 is sealed with a transparent resin 42 containing phosphor particles 43, 44 and 45 distributed in a prescribed manner.

(66) For transparent resin 42, it is possible to use epoxy resin, silicone resin or the like. At least two kinds of particles selected from phosphor particles 43 for red light, phosphor particles 44 for green light and phosphor particles 45 for blue light are included in transparent resin 42, and these phosphor particles are distributed as being separated approximately into layers depending on their kinds. Here, the phosphor particles for emitting secondary light of a shorter wavelength are distributed farther from light-emitting element 11.

(67) More specifically, transparent resin 42 includes large size phosphor particles 43, medium size phosphor particles 44, and small size phosphor particles 45, which are separated into layers. In this case, light emitted from the larger size phosphor particles can be scattered by the smaller size phosphor particles, thereby becoming uniform radiation.

(68) The light-emitting device as shown in FIG. 9 can be fabricated in the following manner. As described above, the phosphors contained in transparent resin 42 are those of at least two kinds selected from phosphor particles 43 for red light, phosphor particles 44 for green light and phosphor particles 45 for blue light. The phosphor particles of the different kinds have their particle sizes adjusted such that they differ in settling speed in the transparent resin of a liquid phase before being set.

(69) The settling speed of the phosphor particles in the liquid resin is determined according to the gravity acting on the phosphor particles and the magnitude of friction force due to the liquid resin in contact with the surfaces of the particles. The gravity is proportional to cubic of the particle size, and the friction force is proportional to square of the particle size. Thus, the particle size considerably affects the settling speed of the particles. The friction force due to the liquid resin does not much depends on the kind of resin, but mainly depends on the surface state of the phosphor particle and thus varies depending on the material and the surface treatment of the phosphor.

(70) In the case of the phosphor particles for which no special surface treatment has been performed, the surface area per unit mass is generally greater in the finer particles, of which settling speed is slower than that of the coarser particles (see WO 02/059982 A1). Incidentally, in the case that primary phosphor particles coagulate to form secondary particles, the settling speed is determined not depending on the size of the primary particle but depending on the diameter of the secondary particle. It is considered that the above-described idea generally holds, although the actual phosphor particles are not of an ideal sphere shape and simple comparison cannot be done.

(71) To cause the particles distributed in the liquid resin to be separated depending on their sizes by utilizing the difference in settling speed as described above, it is practically desirable to use phosphor particles of inorganic material having sizes in the order of several μm. For example, if the particle size is decreased to the order of Bohr radius as disclosed in Japanese Patent Laying-Open No. 2004-071357, the separation will take a long time, which is not practical.

(72) In Example 16, it is possible to use the phosphor particles for red, green and blue lights having their respective median sizes of, e.g., 13 μm, 9.5 μm and 6.5 μm. Here, the median size refers to the central value in distribution of the particle size. The averaged particle diameter d.sub.50 is often used as a parameter indicating the particle size as well. Whether the former or the latter is used, not much influence is caused on the effects of the present invention. Incidentally, it is needless to say that narrower distribution of the particle size in each kind of the phosphors is more preferable from the standpoint of clearer separation between the different kinds of phosphors.

(73) More specifically, referring again to FIG. 9, different kinds of phosphor particles 43, 44 and 45 having different settling speeds are kneaded together into transparent resin 42 of a liquid phase, which is introduced into cup 46 provided with light-emitting element 11. Thereafter, transparent resin 42 containing phosphor particles 43, 44 and 45 is left at rest for a prescribed time period. Then, phosphor particles 43 having a faster settling speed are distributed such that the concentration thereof is higher near the bottom of cup 46 and decreases as the distance from the bottom increases. In contrast, the phosphor particles 45 having a slower settling speed are distributed such that the concentration thereof is lower near the bottom of cup 46 and increases as the distance from the bottom increases. As such, the concentration distribution of the phosphor particles can be formed depending on the settling speeds thereof.

(74) It is noted that the chemical formulae representing the phosphors and composition ratios thereof shown in the above Examples are merely representative, and that the effects of the present invention can be achieved as long as the phosphors satisfy the compositions and composition ratios shown in the “Summary of the Invention” section above.

(75) As described above, the present invention can provide a light-emitting device that includes a wavelength conversion unit containing plural kinds of phosphors for efficiently emitting light in response to the ultraviolet light or blue light emitted from a light-emitting element, in which it is easy to set the color of emission light and it is possible to provides high luminance. Further, it is possible to provide a light-emitting device that can radiate white light excellent in color reproducibility (NTSC ratio).

(76) Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.