Abstract:
An LED chip of the present invention includes a columnar GaP substrate in which a tapered portion whose outer shape is narrowed toward an upper bottom surface side is formed in an outer wall surface thereof, an upper-surface electrode provided in an upper bottom surface of the GaP substrate, a light-emitting layer provided in a lower bottom surface of the GaP substrate, and a lower-surface electrode provided in a surface opposite to the GaP substrate with respect to the light-emitting layer, the lower-surface electrode being arranged in an annular region outside the region opposite to the upper-surface electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-235285, filed Aug. 15, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor light-emitting device, particularly to the semiconductor light-emitting device having a structure in which improvement of light extraction efficiency and an increase in total optical output of the light-emitting device can be achieved.  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 5  is a side view schematically showing a structure of a conventional junction down-mounted LED chip (semiconductor light-emitting device)  100 . The LED chip  100  includes a truncated pyramid-shape GaP substrate  101 , a light-emitting layer  102  provided on a lower surface of the GaP substrate  101 , a lower-surface electrode  103  provided on the lower surface of the light-emitting layer  102 , and an upper-surface electrode  104  provided on an upper surface of the GaP substrate  101 . The GaP substrate  101  has a transparent characteristic to a light emission wavelength. The GaP substrate  101  is tapered such that the emitted light is easily extracted outside the chip.  
         [0006]     In order to evenly emit the light in the surface, an electrode is formed in the entire lower surface of the lower-surface electrode  103 , or the lower-surface electrode  103  is formed by plural thin patterned electrodes  103   b  to  103   d . The upper-surface electrode  104  is formed in a central portion of the upper surface of the GaP substrate  101  in order to perform wire bonding.  
         [0007]     In the LED chip  100 , as shown in  FIG. 5 , the light-emitting layer  102  emits the light by passing current between the lower-surface electrode  103  and the upper-surface electrode  104 .  FIG. 6  shows a light emission intensity distribution of the LED chip  100 . In the light beams emitted from the light-emitting layer  102 , light beams α 1  and α 2  which are located within a total internal reflection angle with respect to each surface of the GaP substrate  101  can be extracted outside the chip, while a light beam α 3  which is located more than the total internal reflection angle is confined in the chip. The light beam α 4  toward the upper-surface electrode  104  is absorbed in the upper-surface electrode  104 . Therefore, there is a problem that light extraction efficiency becomes lower.  
         [0008]     There is disclosed an LED chip, in which a light-emitting layer is selectively formed in a region except for a portion located immediately below an upper electrode and thereby the emitted light is passed through the portion located immediately below the upper electrode to improve the light extraction efficiency (for example, see Japanese Patent No. 2792781).  
         [0009]     There is disclosed an LED chip, in which a reflective film is formed in a surface opposite to a light outgoing surface and thereby the light emitted toward the surface opposite to the light outgoing surface is reflected toward the light outgoing surface to improve the light extraction efficiency (for example, see Japanese Patent No. 3312049).  
         [0010]     In the conventional LED chip, there are the following problems. That is, in the LED chip disclosed in Japanese Patent No. 2792781, it is necessary that an active layer be partially formed by selectively irradiating a growth layer with a laser beam. However, from the technical standpoint, it is difficult to form the partial active layer, which results in the problem that the number of production processes is increased to increase cost.  
         [0011]     The LED chip disclosed in Japanese Patent No. 3312049 is effective when a distance between the light-emitting layer and the surface in which the reflection layer is formed is separated away from each other. However, in the case where the electrode and the reflection layer are formed near the light-emitting layer, the light is emitted in the substantially same shape as the electrode on the light-emitting layer, and the light propagates toward the electrode while not spread. Therefore, there is the problem that the emitted light is absorbed by the electrode while not reflected from the reflection layer.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     In view of the foregoing, an object of the invention is to provide a semiconductor light-emitting device which can realize high-efficiency light emission by decreasing the emitted light confinement in the chip due to the total internal reflection or by decreasing a ratio of which the emitted light is absorbed in the counter electrode.  
         [0013]     In order to achieve the object, a semiconductor light-emitting device of the invention is configured as follows.  
         [0014]     A semiconductor light-emitting device according to one aspect of the invention comprises: a columnar substrate in which a tapered portion is formed in an outer wall surface, an outer shape of the tapered portion is narrowed toward an upper bottom surface side; an upper-surface electrode provided in an upper bottom surface of the substrate; a light-emitting layer provided in a lower bottom surface of the substrate; and a lower-surface electrode provided in a surface opposite to the substrate with respect to the light-emitting layer, the lower-surface electrode being arranged in an annular region outside the region opposite to the upper-surface electrode.  
         [0015]     A semiconductor light-emitting device according to another aspect of the invention comprises: a columnar substrate in which a tapered portion is formed in an outer wall surface, an outer shape of the tapered portion is narrowed toward an upper bottom surface side; an upper-surface electrode provided in an upper bottom surface of the substrate; a light-emitting layer provided in a lower bottom surface of the substrate; and a lower-surface electrode provided in a surface opposite to the substrate with respect to the light-emitting layer, wherein the light-emitting layer is arranged in an annular region outside the region opposite to the upper-surface electrode.  
         [0016]     According to the invention, the high-efficiency light emission can be realized by decreasing the emitted light confinement in the chip due to the total internal reflection or by decreasing a ratio of which the emitted light is absorbed in the counter electrode.  
         [0017]     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0018]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
         [0019]      FIG. 1  is a side view schematically showing an LED chip according to a first embodiment of the invention;  
         [0020]      FIG. 2  is a graph showing a light emission intensity distribution of the LED chip;  
         [0021]      FIG. 3  is a side view schematically showing an LED chip according to a second embodiment of the invention;  
         [0022]      FIG. 4  is a side view schematically showing an LED chip according to a third embodiment of the invention;  
         [0023]      FIG. 5  is a side view schematically showing a conventional semiconductor light-emitting device; and  
         [0024]      FIG. 6  is a graph showing a light emission intensity distribution of the conventional semiconductor light-emitting device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 1  is a side view schematically showing an LED chip (semiconductor light-emitting device)  10  according to a first embodiment of the invention, and  FIG. 2  is a graph showing a light emission intensity distribution of the LED chip  10 . The LED chip  10  includes a truncated pyramid-shape GaP substrate  11 , a light-emitting layer  12  provided on the lower surface of the GaP substrate  11 , a lower-surface electrode  13  provided on the lower surface of the light-emitting layer  12 , a lower-surface electrode  13  provided on the lower surface of the light-emitting layer  12 , an upper-surface electrode  14  provided on the upper surface of the GaP substrate  11 , and a reflective film  15  provided on the lower surface of the lower-surface electrode  13 . The GaP substrate  11  has a transparent characteristic to a light emission wavelength. For example, the light-emitting layer  12  is made of InAlGaP.  
         [0026]     In the GaP substrate  11 , a tapered portion  11   a  having an angle of θ is provided such that the emitted light is easily extracted outside the chip. It is assumed that a height of the GaP substrate  11  is H.  
         [0027]     The upper-surface electrode  14  is formed in the central portion to perform the bonding of wire (not shown).  
         [0028]     On the other hand, the lower-surface electrode  13  is arranged so as to satisfy the following three conditions. First, the lower-surface electrode  13  is arranged at a position where the lower-surface electrode  13  does not face the upper-surface electrode  14 . This is because the light emission in the light-emitting layer  12  is prevented from being absorbed by the upper-surface electrode  14 .  
         [0029]     Second, the lower-surface electrode  13  is arranged at a position where the lower-surface electrode  13  is located away from an outer periphery of the GaP substrate  11  by L defined in the formula (1). That is, assuming that W is a distance between the outer periphery of a lower bottom surface of the GaP substrate  11  and the outer periphery of the upper-surface electrode  14 , n 1  is a refractive index of the GaP substrate  11 , and n 2  is a refractive index of the outside of the GaP substrate  11 , the following formula (1) is obtained: 
 
( H/ 2)tan θ−( H/ 2)tan(θ+α−90°)&lt; L   (1) 
 
         [0030]     where α=sin −1 (n 2 /n 1 )  
         [0031]     The formula (1) is one in which a condition that the light emitted from a thin-wire electrode  13   e  on the outer-most peripheral side is incident within the total internal reflection angle with respect to the tapered surface of the GaP substrate  11  is geometrically determined.  
         [0032]     Third, the lower-surface electrode  13  is arranged at a position where the lower-surface electrode  13  is located away from the outer periphery of the GaP substrate  11  by L defined in the following formula (2): 
 
 L &lt;( H/ 2)tan θ+( H/ 2)tan(−θ+α+90°), and L&lt;W  (2) 
 
 The formula (2) is one in which a condition that the light emitted from a thin-wire electrode  13   a  on the inner-most peripheral side is incident within the total internal reflection angle with respect to the tapered surface of the GaP substrate  11  or the lower-surface electrode  13  does not face the upper-surface electrode  14  is geometrically determined. 
 
         [0033]     In the LED chip  10  having the above configuration, the light is emitted near the lower-surface electrode  13  in the light-emitting layer  12  by passing the current between the upper-surface electrode  14  and the lower-surface electrode  13 . At this point, because the lower-surface electrode  13  is formed by the thin-wire electrodes  13   a  to  13   e , the light emitted not only from the portions corresponding to the thin-wire electrodes  13   a  to  13   e  but also from the portions located between the electrodes. As a result, the light is substantially evenly emitted from the region where the thin-wire electrodes  13   a  to  13   e  are provided as a whole. In  FIG. 2 , a solid line indicates the light emission intensity distribution at this time, and a broken line indicates the light emission intensity distribution of the conventional LED chip shown in  FIG. 5  as a comparative example.  
         [0034]     The thin-wire electrodes  13   a  to  13   e  are arranged at the positions satisfying the above three conditions, the emitted light absorbed by the upper-surface electrode  14  can be suppressed at a minimum while being incident to each surface of the GaP substrate  11  within the total internal reflection angle.  
         [0035]     On the other hand, in the light emitted from the light-emitting layer  12 , the light leaking onto the lower side of  FIG. 1  is reflected from the reflective film  15  to return to the GaP substrate  11 , and the light is extracted outside the chip. At this point, because the lower-surface electrode  13  is formed in the thin wire, the light is effectively reflected from the reflective film  15 .  
         [0036]     Thus, according to the LED chip  10  of the first embodiment, in the light emitted from the light-emitting layer  12 , the light confined in the chip or the light absorbed by the upper-surface electrode  11  can be suppressed at a minimum, the light extraction efficiency can be enhanced, and the total optical output can be increased in the light-emitting device. Furthermore, when the electrode structure is determined as described above, it is not necessary to selectively form the active layer with increase in cost, which allows production cost to be reduced.  
         [0037]     Other materials may be used as the GaP substrate  11  and light-emitting layer  12  depending on a color and purpose to be emitted. For example, when sapphire is used instead of the GaP substrate  11 , GaN may be used as the light-emitting layer  12 .  
         [0038]      FIG. 3  is a side view schematically showing an LED chip  20  according to a second embodiment of the invention. In  FIG. 3 , the same functional component as those of  FIG. 1  is designated by the same numeral, and the detailed description will be omitted.  
         [0039]     In the LED chip  20 , light-emitting layer  21   a  to  21   e  provided in the lower surface of the GaP substrate  11  is formed in agreement with the positions of the thin-wire electrodes  13   a  to  13   e . In the LED chip  20 , the same effect as the first embodiment can also be obtained.  
         [0040]      FIG. 4  is a side view schematically showing an LED chip  30  according to a third embodiment of the invention. In  FIG. 4 , the same functional component as those of  FIG. 1  is designated by the same numeral, and the detailed description will be omitted.  
         [0041]     The light-emitting layers  21  provided at the positions of the thin-wire electrodes  13   a  to  13   e  and an insulating member  32  are arranged In the LED chip  30 , and a plate-shape lower-surface electrode  33  is also arranged.  
         [0042]     In the LED chip  30  having the configuration of  FIG. 4 , electric power supplied to the lower-surface electrode  33  is supplied only to the portion where the light-emitting layer  21  is provided, and only the position is emitted. Accordingly, the same effect as the first embodiment can also be obtained.  
         [0043]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.