Patent Publication Number: US-2012032573-A1

Title: Light emitting diode

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to LEDs (light-emitting diodes), and more particularly to an LED coated with a phosphor layer. 
     2. Description of Related Art 
     An LED (Light-Emitting Diode) as a new type of light source can generate brighter light, and have many advantages, e.g., energy saving, environment friendly and longer life-span, compared to conventional light sources. The LED generally includes an LED chip emitting blue light and a phosphor layer containing phosphor particles deposited on the LED chip. A portion of the blue light emitted by the LED chip and red and green light emitted by the phosphor layer as a result of a partial absorption of the blue light can combine to produce white light. 
     Usually, the white light generated by the LED is not uniform in color temperature. For example, this nonuniformity is a consequence of the variations in the thickness of the phosphor layer. The thickness of the phosphor layer causes nonuniform absorption of blue light and emission of red and green light. Light through thick regions travels longer distance and requires more time than light through thin regions. When light strikes a phosphor particle, the light is either absorbed and re-emitted at a different wavelength or scattered by the phosphor particle. Light that travels a longer distance through the phosphor layer is more likely to be absorbed and re-emitted. Conversely, light that travels a shorter distance through the phosphor layer is more likely to be scattered out of the LED without being absorbed and re-emitted. The thick regions of the phosphor layer absorb more blue light and emit more red and green light than the thin regions of the phosphor layer do. As a result, more blue light is emitted from regions of the LED corresponding to the thin regions of the phosphor layer, and more red and green light is emitted from regions of the LED corresponding to the thick regions of the phosphor layer. The light emitted from the thick regions thus tends to appear yellow or display reddish and greenish blotches, and the light emitted from the thin regions tends to appear bluish. 
     What is needed, therefore, is an LED which can overcome the limitations described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an LED in accordance with a first embodiment of the disclosure. 
         FIG. 2  is a diagram illustrating a luminous intensity distribution of an LED chip of the LED of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of an LED in accordance with a second embodiment of the disclosure. 
         FIG. 4  is a cross-sectional view of an LED in accordance with a third embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an LED  100  in accordance with a first embodiment is shown. The LED  100  is capable of emitting visible light with a first color temperature, and includes a substrate  10 , an LED chip  20  thermally attached to the substrate  10 , an encapsulation  30  encapsulating the LED chip  20 , and a phosphor layer  40  containing phosphor particles and disposed on an outer surface of the encapsulation  30 . 
     The substrate  10  can be made of metallic material, such as copper, copper-alloy, aluminum or aluminum-alloy. The substrate  10  can also be made of a ceramic material with properties of electrically insulating and high thermal conductivity, such as Al 2 O 3 , AlN, SiC or BeO 2 . A groove  12  with a trapeziform cross section is defined in a top face of the substrate  10 . The LED chip  20  is received in the groove  12  and attached to an inner face of the groove  12 . The encapsulation  30  is filled in the groove  12 , and the outer surface of the encapsulation  30  is coplanar with the top face of the substrate  10  adjacent to the groove  12 . 
     The encapsulation  30  can be made of transparent material, such as silicone, epoxy resin, PC (polycarbonate) or PMMA (polymethyl methacrylate). 
     The phosphor layer  40  can be sulfides, aluminates, oxides, silicates, nitrides or oxinitrides. Particularly, the phosphor layer  40  can be of Ca 2 Al 12 O 19 :Mn, (Ca,Sr,Ba)Al 2 O 4 :Eu, Y 3 Al 5 O 12 :Ce 3+ (YAG), Tb 3 Al 5 O 12 :Ce 3+ (TAG), BaMgAl 10 O 17 :Eu 2+ (Mn 2+ ), Ca 2 Si 5 N 8 :Eu 2+ , (Ca,Sr,Ba)S:Eu 2+ , (Mg,Ca,Sr,Ba) 2 SiO 4 :Eu 2+ , (Mg,Ca,Sr,Ba) 3 Si 2 O 7 :Eu 2+ , Ca 8 Mg(SiO 4 ) 4 Cl 2 :Eu 2+ , Y 2 O 2 S:Eu 3+ , (Sr,Ca,Ba)Si x O y N z :Eu 2+ , (Ca,Mg,Y)Si w Al x O y N z :Eu 2+ , CdS, CdTe or CdSe. The phosphor layer  40  has a flat, rectangular cross section. The thickness T of the phosphor layer  40  is preferably 700 μm or lower, and more preferably 500 μm or lower. The phosphor layer  40  can be formed on the outer surface of the encapsulation  30  by a method such as spray coating or screen printing. 
     The LED chip  20  employs a semiconductor material capable of emitting visible light with a second color temperature. Referring to  FIG. 2 , a diagram illustrating a luminous intensity I distribution of the LED chip  20  is shown. The luminous intensity I of light generated by the LED chip  20  and a radiation angle θ are in Lambertian distribution and satisfy following formula: I═I 0 ×cos θ, wherein the radiation angle θ is not lower than 0°, and is not higher than 90°, and I 0  represents a luminous intensity at a central axis O of the LED chip  20 , and the radiation angle θ is an angle between the light generated by the LED chip  20  and the central axis O. 
     The concentration C of the phosphor particles in the phosphor layer  40  and the radiation angle θ satisfy following formula: C═C 0 ×cos θ, wherein the radiation angle θ is not lower than 0°, and is not higher than 90°, and C 0  is a concentration of the phosphor particles in the phosphor layer  40  at the central axis O of the LED chip  20 , and the radiation angle θ is the angle between light and the central axis O. Namely, the concentration C of the phosphor particles in the phosphor layer  40  and the radiation angle θ are also in Lambertian distribution. 
     Since the concentration C of the phosphor particles in the phosphor layer  40  and the luminous intensity I of light generated by the LED chip  20  relative to the radiation angle θ are in Lambertian distribution, the smaller is the radiation angle θ, the greater are the luminous intensity I and the concentration C. Therefore, a greater proportion of light with the second color temperature emitted from the LED chip  20  would be absorbed and converted into light with a third color temperature by the phosphor layer  40 , and then light with the second color temperature which is scattered by the phosphor layer  40  and the light with the third color temperature combine and finally produce light with the first color temperature. Thus, when the concentration C of the phosphor particles in the phosphor layer  40  and the luminous intensity I of light generated by the LED chip  20  are in Lambertian distribution, the first color temperature of light emitted from the entire LED  100  is likely to be uniformly distributed within the range of the radiation angle θ. 
     In the above, the concentration C of the phosphor particles in the phosphor layer  40  is variable relative to the luminous intensity I of light generated by the LED chip  20  so as to achieve a uniform light distribution of the entire LED  100  with the first color temperature. However, it is understandable that when the particle number N of the phosphor particles in the phosphor layer  40  or the thickness T of the phosphor layer  40  relative to the luminous intensity I of light generated by the LED chip  20  are in Lambertian distribution, a uniform distribution of the first color temperature of light emitted from the entire LED  100  can also be achieved. That is to say, the particle number N of the phosphor particles in the phosphor layer  40  and the radiation angle θ satisfy following formula: N═N 0 ×cos θ, wherein the radiation angle θ is not lower than 0°, and is not higher than 90°, and N 0  is the particle number of the phosphor particles in the phosphor layer  40  at the central axis O of the LED chip  20 , and the radiation angle θ is an angle between light and the central axis O; or the thickness T of the phosphor layer  40  and the radiation angle θ satisfy following formula: T=T 0 ×cos θ, wherein the radiation angle θ is not lower than 0°, and is not higher than 90°, and T o  is the thickness of the phosphor layer  40  at the central axis O of the LED chip  20 , and the radiation angle θ is an angle between light and the central axis O. The smaller is the radiation angle θ, the greater are the luminous intensity I and the particle number N or the thickness T. Accordingly, a greater proportion of light with the second color temperature emitted from the LED chip  20  would be absorbed and converted into light with a third color temperature by the phosphor layer  40 , and then light with the second color temperature which is scattered by the phosphor layer  40  and the light with the third color temperature combine and finally produce light with the first color temperature. Thus, when the particle number N of the phosphor particles in the phosphor layer  40  or the thickness T of the phosphor layer  40  relative to the luminous intensity I of light generated by the LED chip  20  are in Lambertian distribution, the first color temperature of light emitted from the entire LED  100  is likely to be uniformly distributed within the range of the radiation angle θ. 
     When the concentration C of the phosphor particles in the phosphor layer  40  is in Lambertian distribution relative to the radiation angle θ, the thickness T of the phosphor layer  40  can be uniform. When the particle number N of the phosphor particles in the phosphor layer  40  is in Lambertian distribution relative to the radiation angle θ, the thickness T of the phosphor layer  40  can be uniform. When the thickness T of the phosphor layer  40  is in Lambertian distribution relative to the radiation angle θ, the concentration C of the phosphor particles in the phosphor layer  40  can be uniform. When the thickness T of the phosphor layer  40  is in Lambertian distribution relative to the radiation angle θ, the particle number N of the phosphor particles in the phosphor layer  40  can be uniform. 
     Also referring to  FIG. 3 , an LED  200  in accordance with a second embodiment is shown. The LED  200  includes a substrate  10   a , an LED chip  20   a  thermally attached to the substrate  10   a , an encapsulation  30   a  encapsulating the LED chip  20   a , and a phosphor layer  40   a  disposed on an outer surface of the encapsulation  30   a . The differences of the LED  200  relative to the LED  100  in the first embodiment are that: the substrate  10   a  of the LED  200  does not define any groove for receiving the LED chip  20   a , the encapsulation  30   a  is hemispherical, and the phosphor layer  40   a  is curved and covered on the hemispherical outer surface of the encapsulation  30   a.    
     Also referring to  FIG. 4 , an LED  300  in accordance with a third embodiment is shown. The differences of the LED  300  relative to the LED  100  in the first embodiment are that: a transparent protective layer  50  is covered on an outer surface of the phosphor layer  40 , and the protective layer  50  is made of the same material as the encapsulation  30 . 
     It is to be understood, however, that even though numerous characteristics and advantages of certain embodiments have been set forth in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.