Abstract:
This invention relates to a semiconductor light-emitting device including a semiconductor light-emitting chip and a transparent carrier. The semiconductor light-emitting chip includes an active layer and transparent substrate. The active layer emits light under a bias. At least a portion of the light emitted from the active layer enters into the transparent carrier through the transparent substrate. The semiconductor light-emitting chip is coupled to the transparent carrier through the transparent substrate. The area of the transparent carrier is larger than that of the active layer.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    The present application claims the right of priority based on Taiwan Application Serial Number 095113326, filed on Apr. 13, 2006, the disclosure of which incorporated herein by reference in its entirety. 
       TECHNICAL FIELD  
       [0002]    This invention relates to a light-emitting semiconductor device and more particularly to a semiconductor light-emitting device with a light extraction region, where the cross-sectional area of the light extraction region is larger than that of an active layer. 
       BACKGROUND  
       [0003]    A light-emitting diode (LED) is a solid-state semiconductor element, which at least comprises one p-n junction that is formed between a p-type semiconductor layer and an n-type semiconductor layer. When the p-n junction receives certain bias, holes in the p-type semiconductor layer and electrons in the n-type semiconductor layer combine to emit light. The region where light is produced is called an active region. The materials of the p-n junction determine the color of the light emitted inform the active region. For example, LEDs of AlGaInP series emit red light to green light, and LEDs of III-V nitride series emits green light to ultraviolet light. 
         [0004]    In general, structures of the active layers comprise single heterostructure (SH), double heterostructure (DH), double-side double heterostructure (DDH), multi-quantum well (MQW), etc. However, such structures still obey the mechanism of the p-n junction. The structure of a commercialized LED production, besides the p-n junction, comprises a growth substrate, a buffer layer, electrodes, a reflection layer, conductive wires, and/or a phosphor, etc. 
         [0005]    The light produced in the active layer goes omnidirectionally. However, users usually need the light with specific directions only, so the reflection layer and mirror are adopted to reflect a portion of light. In addition, the difference of the refraction coefficients between LED materials and environment mediums results in that light illuminating on the boundary of the LED undergoes total reflection over a particular incident angle. Generally, it is difficult to prevent each of the reflected light beams from being re-reflected inside the LED. 
         [0006]    Referring to  FIG. 1A , a prior art LED  100  includes a substrate  110  and epitaxial layers  130 . The epitaxial layers  130  include an active layer  131 . The active layer  131  emits light omnidirectionally when a bias is applied thereon. A reflection layer  150  is formed between the epitaxial layers  130  and the substrate  110  to reflect the light from the active layer  131 . 
         [0007]    A first ray R 1  emits toward the upper side of the LED  100 . When the refraction coefficient of the environment medium is less than that of the LED  100  and the incident angle is larger than the critical angle, the first ray R 1  is reflected totally on the boundary of the epitaxial layers  130  toward the inside of the epitaxial layers  130 . When the first ray R 1  passes through the active layer  131 , a portion of the first ray R 1  is absorbed by the active layer  131 . The other portion of the first ray R 1  proceeds toward the reflection layer  150  and is reflected upward to pass through the active layer  131  again. Therefore, the first ray R 1  oscillates in the epitaxial layers  130  and passes through the active layer  131  repeatedly, then is gradually absorbed. Under the same mechanism, a second ray R 2  emitting to the underside of LED  100  oscillates in the epitaxial layers  130  and passes through the active layer  131  repeatedly, then is absorbed gradually as well. 
         [0008]    Referring to  FIG. 1B , there is no reflection layer between the substrate  110  of the LED  100  and the epitaxial layers  130 , and the substrate  110  is transparent relative to the light emitted from the active layer  131 . There is a mirror layer (not shown) or just air beneath the substrate  110 . If a third ray R 3  reflected form the underside of the substrate  110  illuminates the lateral wall of the substrate  110  with an incident angle Θ I  larger than the critical angle Θ C , it will be reflected to inside of the epitaxial layers  130 . The active layer  131  absorbs a portion of the third ray R 3  back to the epitaxial layers. As described above, the third ray R 3  is possible to be totally reflected from the margin of the epitaxial layers  130  to the inside of the epitaxial layers  130 , and oscillates in the epitaxial layers  130  as well as passes the active layer  131 , then is gradually absorbed. The light absorbed by the active layer  131  definitely reduces the light extraction efficiency of the LED  100  to a certain extent. 
       SUMMARY OF THE INVENTION  
       [0009]    This invention provides a semiconductor light-emitting device and a LED encapsulant structure, for reducing the light absorbed by an active layer. 
         [0010]    The semiconductor light-emitting device comprises a light-emitting structure and a transparent carrier. The light-emitting structure at least comprises the active layer composed of two different semiconductor layers and a transparent substrate. The transparent carrier is coupled to the light-emitting structure on one side of the transparent substrate, and the area of the transparent carrier is larger than that of the active layer. The semiconductor layers emit light when a bias is applied. A portion of the light emits into the transparent carrier through the transparent substrate. The transparent carrier enhances light extraction efficiency of the light-emitting structure. 
         [0011]    The LED encapsulant structure at least comprises the transparent carrier, a base, an anode support, and a cathode support. The transparent carrier is fixed on the base. Alternatively, a reflection structure can be set on the intermediate between the transparent carrier and the base. The transparent carrier is employed to load the LED chip, and the area of the transparent carrier is larger than that of the active layer in the LED chip. The anode and the cathode of the LED chip electrically connect to the anode support and the cathode support respectively. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]      FIGS. 1A and 1B  illustrate light paths in two types of prior art LEDs. 
           [0013]      FIG. 2A  is a cross-sectional view of a light-emitting semiconductor device in accordance with an embodiment of the present invention. 
           [0014]      FIG. 2B  illustrates a light trace path of the light-emitting semiconductor device in  FIG. 2A . 
           [0015]      FIGS. 3A-3C  are cross-sectional views showing a light-emitting semiconductor device in accordance with another embodiment of the present invention. 
           [0016]      FIG. 4  is a cross-sectional view showing a light-emitting semiconductor device in accordance with another embodiment of the present invention. 
           [0017]      FIGS. 5A and 5B  illustrate the area ratio and power ratio of a light-emitting semiconductor device in accordance with the present invention. 
           [0018]      FIG. 6  shows a drawing of an encapsulant structure of the light-emitting semiconductor device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]      FIG. 2A  shows that a semiconductor light-emitting device  200  comprises a light-emitting structure formed on a transparent carrier  210 . The light-emitting structure, as a LED chip, comprises a transparent substrate  220 , a first type semiconductor layer  230 , and a second type semiconductor layer  250 . The first type semiconductor layer  230  and the second type semiconductor  250  are semiconductor materials with different conductivity, such as a p-type and an n-type semiconductors, and form an active layer  240  at the junction thereof. When the light-emitting structure receives a bias, the active layer  240  emits light. In addition, a first electrode  260  and a second electrode  270  electrically connect to the first type semiconductor layer  230  and the second type semiconductor layer  250  respectively for connecting to an outside circuit. 
         [0020]    A transparent carrier  210  couples to the underside of the transparent substrate  220 . The transparent carrier  210  is independent of the light-emitting structure or the LED chip. In other words, the transparent carrier  210  is not formed during the process of the LED chip manufacture. The transparent substrate  220  comprises a first surface  221  opposite to a second surface  222 . The first surface  221  is closer to the active layer  240  than the second surface  222 . The transparent carrier  210  comprises a third surface  213  opposite to a fourth surface  214 . The third surface  213  is closer to the active layer  240  than the fourth surface  214 . 
         [0021]    In the preferred embodiment, the area of the transparent carrier  210  is larger than that of the active layer  240 . If a mirror or a reflection layer is formed on the underside of the transparent carrier  210 , the light from the active layer  240  is reflected. In accordance with the area of the active layer  240  less than that of the transparent carrier  210 , reflected light is more likely to emit out from the region of the transparent carrier  210  uncovered by the active layer  240 , and relatively, less reflected light returns to the active layer  240 . Namely, the light absorbed by the active layer  240  reduces. 
         [0022]      FIG. 2B  shows that a fourth ray R 4  emits from the active layer  240  to the underside of the transparent carrier  210  and is reflected to the lateral side of the transparent carrier  210 . Because the transparent carrier  210  is larger than the active layer  240 , the fourth ray R 4  is less likely to emit directly to the lateral wall of the transparent carrier  210 , comparing with well known technologies, and is more likely to emit to the region of the third surface  213  uncovered by the active layer  240 . Thus, the probability and the amount of the fourth ray R 4  absorbed by the active layer  240  are reduced. 
         [0023]    In the preferred embodiment, the area of the third surface  213  of the transparent carrier  210  is no less than each area of the first surface  221  and the second surface  222 . In addition, the area of the fourth surface  214  is larger than that of the active layer  240 . Preferably, area ratio of the fourth surface  214  to the active layer  240  is no less than  1 . 6 . Namely, when the shapes of the transparent substrate  210  and the active layer  240  are both square, the ratio of their circumferences is about 1.26. Preferably, the area ratio of the transparent carrier  210  to the active layer  240  is between 4˜8. 
         [0024]    The areas of the third surface  213  and the fourth surface  214  can be different. Preferably, both areas are larger than that of the active layer  240 . If the area of the third surface  213  is larger than that of the fourth surface  214 , the transparent carrier  210  is inverted-trapezoid. On the contrary, if the area of the third surface  213  is less than that of the fourth surface  214 , the transparent carrier  210  is trapezoid or frustum, as shown in  FIG. 3A  and  FIG. 3B . 
         [0025]      FIG. 3A  shows that if the transparent carrier  210  is trapezoid or frustum, the lateral surface of that is a ramp. In this embodiment, because of the ramp inclining in an angle Θ A , a fifth ray R 5  is easier to enter the range of a critical angle Θ C  to escape from the transparent carrier  210 . Detailed explanation refers to Taiwan patent application of No. 095103659. 
         [0026]    The areas between the first surface  221  and the fourth surface  214  can extend gradually.  FIG. 3B  shows that outlines of the transparent substrate  220  and the transparent carrier  210  as a whole are trapezoid. As mentioned above, it also aids light to escape from the lateral surfaces of the transparent substrate  220  and the transparent carrier  210 . 
         [0027]      FIG. 3C  shows that the lateral surface of the transparent carrier  210  is not limited to a flat surface, and can be a cambered surface as well. A lateral cambered surface also aids light to escape from the lateral surface of the transparent carrier  210 . 
         [0028]    As mentioned above, the areas of the first surface  221  and the second surface  222  are able to be different. However, both areas must be less than the area of the third surface  213 . If the area of the first surface  221  is larger than that of the second surface  222 , the transparent substrate  220  is inverted-trapezoid. On the contrary, if the area of the first surface  221  is less than that of the second surface  222 , the transparent substrate  220  is trapezoid. Both these two shapes respectively aid light reflected to the active layer  240  in certain angles to escape from the light-emitting structure. The selection of the shapes is in accordance with the thickness of the transparent substrate  220 . 
         [0029]    When light illuminates an interface between the transparent carrier  210  and environment air, it is possible to encounter a total reflection problem. If the interface is rough or uneven, the light is scattered, and therefore the total reflection is reduced and light extraction efficiency is enhanced.  FIG. 4  shows that the region of the third surface  213  uncovered by the active layer  240  is roughened. Besides the third surface  213 , the lateral surface of the transparent carrier  210  can also become a rough surface. In addition, a surface contacting electrodes  260  or  270 , or contacting both of them but uncovered by them, is roughened as well in order to enhance light extraction efficiency. 
         [0030]    If the transparent substrate  220  is a sapphire, the first type semiconductor layer  230  and the second type semiconductor layer  250  can be composed of III-V nitride semiconductor. Because the sapphire is an insulator, the first electrode  260  and the second electrode  270  are formed on the same side of the light-emitting device to conduct the current. 
         [0031]    Materials of the transparent substrate  220  also can be conductors, such as SiC, GaP, GaAsP, or ZnSe. At this time, the corresponding materials of the first type semiconductor layer  230  and the second type semiconductor layer  250  are AlGaInP series. By applying the materials mentioned above, the first electrode  260  and the second electrode  270  are located on different sides of the active layer  240  respectively. For some specific structures, the first electrode  260  and the second electrode  270  can be located on the same side of the transparent substrate  220  as well. 
         [0032]    In addition, by applying AlGaInP series to form the first and second semiconductor layers  230  and  250 , the transparent substrate  220  can be attached to the semiconductor layers by gluing. Then the transparent substrate is fixed onto the transparent carrier  210  after removing the growth substrate. 
         [0033]    The materials of the transparent substrate comprise, but unrestricted to, SiC, GaP, GaAsP, ZnSe, and CVD diamond. The materials for gluing comprise, but unrestricted to, SOG, silicone, BCB, epoxy, polyimide, PFCB, or the combination thereof. 
         [0034]    Light from the active layer  240  has to emit into the transparent carrier  210  through the transparent substrate  220 . Therefore, the materials for attaching the transparent substrate  220  with the transparent carrier  210  must be transparent relative to the light from the active layer  240 . The materials for gluing comprise, but unrestricted to, SOG, silicone, BCB, epoxy, polyimide, PFCB, or the combination thereof. 
         [0035]    Materials of the transparent carrier  210  comprise, but unrestricted, SiC, GaP, sapphire, GaAsP, ZnSe, diamond, or CVD diamond. Taking CVD diamond as an example, its thermal expansion coefficient is about 1.1 ppm/K, and its thermal conductivity coefficient is about 20.0 W/cmK. Preferably, the difference of the thermal expansion coefficients between the transparent carrier  210  and the transparent substrate  220  is less than or the same as 1×1 −5 /° C. Thus, the peeling between the transparent carrier  210  and the transparent substrate  220  because of the heat is avoided. 
         [0036]      FIGS. 5A and 5B  show graphs of area ratio and power ratio of the LED chips of different wavelengths relative to the transparent carrier  210 . 
         [0037]    Naked yellow-light chips of  14 mil with a primary wavelength of about 593 nm are fixed on sapphire carriers of 14 mil, 22 mil, 30 mil, and 40 mil respectively by applying BCB. Round dots of  FIG. 5A  show that when the area ratio of the sapphire carrier to the naked yellow-light chip reaches 4.6 times, comparing to the naked yellow-light chip without a transparent carrier attached, the power of the naked yellow-light chip attached to the sapphire carrier raises 1.8 times. Square dots of  FIG. 5A  show that when the naked yellow-light chip is covered by epoxy, the power of the naked yellow-light chip attached to the sapphire carrier raises 1.4 times. In the meantime, the thickness of the sapphire carrier is about 120 μm 
         [0038]    Naked blue-light chips of  15 mil with a primary wavelength of about 593 nm are fixed on sapphire carriers of 14 mil, 22 mil, 30 mil, and 40 mil respectively by applying BCB. Round dots of  FIG. 5B  show that, comparing to the naked blue-light chip without a transparent carrier attached, the power of the naked blue-light chip attached to the sapphire carrier can be raised. In present experiments, the power ratio exceeds 1.3 times. Square dots of  FIG. 5B  show that when the naked blue-light chip is covered by epoxy, the power of the naked blue-light chip attached to the sapphire carrier raises 1.25 times. In the meantime, the thickness of the sapphire carrier is about 120 μm. 
         [0039]    The structure mentioned above is suitable to an LED encapsulant.  FIG. 6  shows that the completed light-emitting structure and the transparent carrier  210  are fixed on a base  300  by silver glue or scotch glue (not shown here). The reflection layer (not shown here) is formed under the transparent carrier  210  to reflect light. An anode and a cathode are connected to an anode support  302  and a cathode support  301  by a first conductive wire  303  and a second conductive wire  304 . The structure mentioned above can also be covered by the encapsulant materials. A phosphor is doped into the encapsulant materials or covers the LED chip to convert the original color light of the LED chip. The encapsulant materials comprise, but unrestricted to, epoxy, acrylic, silicon, or the combination thereof. 
         [0040]    In the encapsulant structure mentioned above, the transparent carrier  210  is set on the base  300  and attaches to the light-emitting structure then. Thus, the commercial LED chips available in the market can be applied to the light-emitting device as shown in abovementioned embodiments. 
         [0041]    In accordance with the economic consideration, a backlight module is looking forward lighting, thinning, low power consumption, great luminance and low cost. Therefore, the light-emitting device and the encapsulant structure mentioned above are applicable to the backlight module. 
         [0042]    It should be noted that the proposed various embodiments are not for the purpose to limit the scope of the invention. Any possible modifications without departing from the spirit of the invention may be