Patent Publication Number: US-2004041160-A1

Title: High power, high luminous flux light emitting diode and method of making same

Description:
FIELD OF INVENTION  
       [0001] The present invention relates generally to light emitting devices using compound semiconductor materials. More particularly, the present invention relates to high power, high luminous flux light emitting diodes.  
       BACKGROUND OF INVENTION  
       [0002] Light emitting diode (LED) technology has revolutionized lighting equipment in recent years. Due to the advantages offered by light emitting diodes (LEDs), many applications now incorporate LEDs instead of conventional incandescent lighting sources. These applications include, but are not limited to, traffic signaling, electronic signs, medical applications, instrumentation, and general illumination. LEDs generally consume much less power as equally luminous incandescent lamps, and LEDs are also much more durable than conventional incandescent lighting sources. This leads to less frequent replacements and lower maintenance costs. Also, less electrical power consumption by the LEDs translates into less strain on a power source, such as an alternator or battery. LEDs are also insensitive to vibration and have lower switch-on time in comparison to most incandescent lighting sources.  
       [0003] For LEDs to replace incandescent lighting sources in applications as described above, the LEDs will have to provide high luminous output while maintaining reliability, low power consumption and low manufacturing cost. In many of the above-described applications, the LEDs are in the form of LED chips having an edge length of around 300 μm. An individual LED chip of this type usually has low power output and can only be subjected to low injection current. As a result, these LED chips need to be assembled into clusters or arrays to achieve the required luminous flux level.  
       [0004] Multiple clusters or arrays of LED chips are generally mounted onto a board and then integrated with a lamp housing, electronics, and various lenses. Due to the small size of these LED chips and the limited amount of luminous flux that each can generate, the number of LED chips necessary to achieved the required flux levels is generally quite large. This increases the complexity in packaging and installing LED chips for a particular application, in terms of both time and manufacturing cost. For example, much time and manufacturing cost are needed for mounting, optical collecting, and focusing the emissions from the LED chips. Extra time and cost are also required to install and aggregate the LED chips in a specific arrangement as required by a specific application.  
       [0005] Attempts have been made to manufacture LED chips that are capable of creating higher luminous flux than the ˜300 μm edge length LED chips. One approach is to increase the edge length and make each LED chip larger. The larger size allows more current to flow over and through the LED chip, and higher luminous flux is generated per LED chip as a result. Although the larger size simplifies packaging and installation of the LED chips because a fewer devices are required to be packaged and installed, reliability and power consumption become problematic. Specifically, larger size LED chips currently available are limited in their power and luminous flux output. For example, several commercial devices currently available are limited to a current dissipation of approximately 350 mA.  
       [0006] The primary limiting factor in larger LED chips is the inability for current to spread evenly over and through the entire structure of an LED chip. Rather, the current accumulates at specific spots on the LED chip, preventing the efficient use of the available light-emitting semi-conductive material. This phenomenon is commonly referred to as “current crowding.” Current crowding tends to occur at points on electrical contacts of an LED chip because of the tendency of charge carriers to travel a path of least resistance. Current crowding may also occur in certain regions of the electrical contacts depending on the capacity for each of the regions to accept and spread current. Current crowding leads to unstable luminous flux output with bright spots and dim spots on the LED chip. Current crowding also necessitates more current to be injected into the LED chip, which leads to high power consumption and can cause breakdown in the LED chip. As a result, light is not emitted efficiently, and power consumption is not minimized. Moreover, the larger size LED chips currently available include additional limiting factors that further contribute to its limited power and limited luminous flux output. These limiting factors include ineffective heat dissipation, deficient light enhancing structure, and limited number of light emitting regions that results in high light re-absorption within the device structure. Therefore, high power, high luminous flux LED chips cannot be achieved using conventional means.  
       SUMMARY OF INVENTION  
       [0007] Aspects of the present invention relate to high power, high luminous flux light emitting diodes and the methods of making them. In one embodiment, the light-emitting diode comprises a substrate, a light-emitting structure disposed above the substrate along a vertical axis, a P electrode having a number of legs extending in one direction along a substantially horizontal axis perpendicular to the vertical axis, and an N electrode having a number of legs extending substantially horizontally in the direction opposite to the direction of the legs of the P electrode. The light-emitting structure includes a P cladding layer, an active layer and an N cladding layer. The P electrode is in contact with the P cladding layer of the light-emitting structure, while the N electrode is in contact with the N cladding layer of the light-emitting structure. The N electrode is disposed at a lower surface than the P electrode, where the lower surface is defined by a mesa etch process, forming a mesa edge separating the N electrode from the P electrode. A thin metal layer is under the P electrode, which is overlapped and in contact with the P electrode and separated from the N electrode by the mesa edge. The P and N electrodes are designed in such a manner that portions of the legs of the P electrode are interspersed with and spaced apart from portions of the legs of the N electrode.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008]FIG. 1 illustrates a top level view of an LED  100  constructed according to an embodiment of the present invention;  
     [0009]FIG. 2 illustrates a top level view of an LED  200  constructed according to an embodiment of the present invention;  
     [0010]FIG. 3 illustrates a top level view of an LED  300  constructed according to an embodiment of the present invention;  
     [0011]FIG. 4 illustrates a top level view of an LED  400  constructed according to an embodiment of the present invention;  
     [0012]FIG. 5 shows a cross-sectional side view of an LED  500  constructed according to an embodiment of the present invention;  
     [0013]FIG. 6 shows a cross-sectional side view of an LED  600 , showing channels, constructed according to an embodiment of the present invention;  
     [0014]FIGS. 7 a  and  7   b  illustrate a plurality of LEDs arranged in exemplary relationships according to embodiments of the present invention;  
     [0015]FIG. 8 illustrates a method of making the LED shown in FIG. 1 according to an embodiment of the present invention;  
     [0016]FIG. 9 illustrates a method of making the LED shown in FIG. 2 according to an embodiment of the present invention; and  
     [0017]FIG. 10 illustrates a top level view of an LED  1000  constructed according to an embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION  
     [0018]FIG. 1 illustrates a top level view of an LED  100  constructed according to an embodiment of the present invention. The top view of the LED  100  shows an N electrode  110 , a P electrode  120 , and a region  150  capable of passing light defined by the P electrode  120  and the N electrode  110 . A thin, substantially translucent metal layer  130  is disposed above the region  150  and between the N electrode  110  and the P electrode  120 , which is overlapped with the P electrode  120 , and separate from the N electrode  110  by the mesa edge  160 . Although the LED  100  is shown to retain a square shape in the embodiment of FIG. 1, it is noted that any shape may be employed depending on the specific application. In one embodiment, the LED  100  is a square and has an edge length of around 1.20 mm ˜1.26 mm.  
     [0019] Although not shown in FIG. 1, disposed below the thin metal layer  130  and the region  150 , along a vertical axis, is a light-emitting structure with an N cladding layer and a P cladding layer. The N electrode  110  is in contact with the N cladding layer at outside of the mesa edge  160 , while the P electrode  120  is in contact with the P cladding layer and overlap with the thin metal current spreading layer  130 . In operation, a voltage difference is applied between the P electrode  110  and N electrode  120  to activate the light-emitting structure of the LED  100 , and current flows from the P electrode  110  to the N electrode  120  and the current spreaded from the P electrode  110  to the thin metal layer  130  diffuses through the layers of the LED  100 . The spreading of the current from the P electrode  110  to the N electrode  120  is enhanced by the layout design of and/or specific features on the P and N electrodes  110 ,  120  as well as the thin metal layer  130 . With the current spread and flowing through the active region of the LED, recombination of charge carriers occurs resulting in the release of light energy through the region  150  and out into the environment.  
     [0020] In the embodiment, the N electrode  110  has a contact portion  117  and three substantially straight tapered legs  112 ,  114 ,  116  extending to the left along a horizontal axis, and the P electrode has a contact portion  127  and two straight tapered legs  122 ,  124  extending to the right along the horizontal axis. The two legs  122 ,  124  of the P electrode  120  are interspersed with and spaced apart from the three legs  112 ,  114 ,  116  of the N electrode  110 . As viewed from above, the legs  112 ,  114 ,  116 ,  112 ,  124  appear to be parallel to each other. In this configuration, the leg  122  of the P electrode  120  is disposed between the legs  112 ,  114  of the N electrode  110 , while the leg  124  of the P electrode  120  is disposed between the legs  114 ,  116  of the N electrode  110 . On the other hand, the leg  114  of the N electrode  110  is disposed between the legs  122 ,  124  of the P electrode. Although the P electrode  120  is shown to have two legs and the N electrode  110  is shown to have three legs, the placement of the P electrode  120  and the N electrode  110  may be interchanged according to embodiment of the present invention. That is, a P electrode would be the right electrode with three legs and a larger total surface area, while an N electrode would be the left electrode with two legs and a smaller total surface area.  
     [0021] One feature of the embodiment in FIG. 1 is the legs of the N and P electrodes  110 ,  120  being tapered, with wide ends being closer to the electrode contact portions  117 ,  127  of the N and P electrodes  110 ,  120 , respectively, and narrow ends being further away from the electrode contact portions  117 ,  127  of the N and P electrodes  110 ,  120 , respectively. In FIG. 1, the legs  122 ,  124  of the P electrode  120  are tapered to the right, while the legs  112 ,  114 ,  116  of the N electrode  110  are tapered to the left. Because the tapering in the legs  112 ,  114 ,  116  of the N electrode  110  runs in the opposite direction to the tapering in the legs  122 ,  124  of the P electrode  120 , the legs  122 ,  124  of the P electrode  120  taper off to the right and decrease in width while the legs  112 ,  114 ,  116  of the N electrode  110  expand to the right and increase in width.  
     [0022] In one embodiment, the decrease in width in the P electrode legs  122 ,  124  along the length of said legs in one direction is proportional to the increase in width in the N electrode legs  112 ,  114 ,  116  along the length of said legs in the same direction. Thus, each of the P electrode legs  122 ,  124  is spaced apart from its neighboring N electrode leg in substantially equal distance along each of the P electrode legs  122 ,  124  and its neighboring N electrode leg. For example, in looking at the leg  122  of the P electrode  120  and the leg  114  of the N electrode  110 , the P electrode leg  122  tapers in direction opposite to that of the N electrode leg  114 . This tapering arrangement allows the narrowing of the P electrode leg  122  in one direction to be compensated by the widening of the N electrode leg  114  in the same direction. This makes the distance between the P electrode leg  122  and the N electrode leg  114  substantially equal along the length of the two legs  114 ,  122 , and variations in this distance are minimized. Thus, when current flows from the P electrode leg  122  through the thin film  130  to the N electrode leg  114 , the current traverses substantially the same distance along the length of the two legs and, hence a substantially equally resistive path. This promotes a uniform current spreading along the length of the two legs  122 ,  114  in the rectangular shaped region define by the two legs  122 ,  114 .  
     [0023] The layout design of the P electrode  120  and the N electrode  110  defines the region  150 , which substantially retains a M shape according to the embodiment shown in FIG. 1. In this configuration, the M shape is rotated 90° clockwise. The region  150  is capable of passing light produced from the LED  100 . The thin metal layer  130  is formed above the region  150  and disposed between the P electrode  120  and the N electrode  110 . In one embodiment, the thin metal layer  130  overlaps with the P electrode  120  and separate from the N electrode  110  by the mesa edge  160 . The thin metal layer  130  comprises Nickel and Gold (Ni/Au). Alternatively, other material that has current spreading characteristics and does not significantly obstruct light produced from the LED  100  may also be used.  
     [0024] The thin metal layer  130  promotes current spreading therethrough as well as current diffusion down the layers therebelow. Through the thin metal layer  130 , current spreads initially from the wide end of the P electrode leg  122  to portions of the region  150  next to the wide end. The wide end provides more area for the initial high current to start spreading, avoiding current crowding near the electrode contact portion  127  and the thin metal layer  130 . The current spreads outward to the portion of the region  150  next to the electrode leg  122  as the current propagates toward the narrow end of the P electrode leg  122 . Because less and less current is present as the current spreads to the region  150  along the P electrode leg  122  and propagates toward the narrow end, the P electrode leg  122  is made narrower. As the taper progresses along an electrode leg, resistance in the conductor increases, and less current passes. Consequently, current escapes from the electrode into the conductive layer substantially evenly along the edge of the electrode rather than from one point. This again has the advantage of promoting even current spreading along the length of the legs of the P and N electrodes. Similarly, the P electrode leg  122  and the N electrode leg  112  function in likewise fashion as described above for the P electrode leg  122  and the N electrode leg  114 . An added benefit of making the legs tapered is to enlarge the region  150 , creating extra area for light to emit from the LED  100 . This further improves luminous efficiency.  
     [0025] In one embodiment, the leg  114  of the N electrode  110  includes an enlarged portion  115  at its end, while the outer leg  112  of the N electrode  110  includes an enlarged portion  113  toward the end of the outer leg  112 . Similarly, the leg  122  of the P electrode also includes an enlarged portion  125  and an extension  126  toward the end of the leg  122 . In one embodiment, the enlarged portions  113 ,  115 ,  125  encourage current distribution along the length of their respective legs and toward the legs&#39; respective narrow ends. This again promotes current spreading and avoids current crowding in the LED  100 . In another embodiment, the enlarged portions  113 ,  115 ,  125  and/or the extension  126  provide better anchoring of their respective legs by increasing the contact area between the legs and the layer below. This promotes to decrease the contact resistance and increase reliability of the device. Although the enlarged portions  113 ,  115 ,  125  are shown to have either a semicircular or circular shape, it is noted that the enlarged portions  113 ,  115 ,  125  may have another shape, such as a square, rectangular, triangular and elliptical shape. In other embodiments, different sizes and different shapes of the enlarged portions may also be employed in a single LED or among different LEDS in multiple arrays of LEDs.  
     [0026]FIG. 2 illustrates a top level view of an LED  200  constructed according to another embodiment of the present invention. The LED  200  has substantially the same structure as that of the LED  100 . The top view of the LED  200  shows an N electrode  210 , a P electrode  220 , a region  250  capable of passing light defined by the P electrode  220  and the N electrode  210 , and a plurality of channels  264  disposed in the region  250 . The N electrode  210  has three straight tapered legs  212 ,  214 ,  216  extending to the left, and the P electrode  220  has two straight tapered legs  222 ,  224  extending to the right. For illustration purpose only, the region  250  is shown in black, while the P and N electrodes  220 ,  210  and the channels  264  are shown in white. The two legs  222 ,  224  of the P electrode  220  are interspersed with and spaced apart from the three legs  212 ,  214 ,  216  of the N electrode  210 .  
     [0027] The region  250  substantially retains an M shape, rotated  90 ° clockwise, according to the embodiment shown in FIG. 2. The region  250  is capable of passing light produced from a light-emitting structure disposed below the surface of the LED  200 . Disposed within the region  250  are a number of channels  264  that further divide the region  250  into sub-regions. For examples, with respect to the top portion of the region  250  defined by the P electrode legs  212 ,  214  and the N electrode leg  222 , the channels  264  divide this portion into six substantially rectangular shaped sub-regions  251 - 256 . In other embodiments, different shapes may be employed for the sub-regions. The channels  264  are openings or trenches within the region  250 , and they provide additional surface area to the region  250  for light to escape. The channels  264  do not have absorption materials above them to limit light output from the light-emitting structure. Examples of the absorption materials include the thin metal layer  230  and the light emitting structure and P and N electrodes. Thus, light emits from the channels  264  in a more efficient manner. This improves luminous efficiency of the LED  200 . The channels  264  further minimize contacts between the sub-regions themselves, allowing current spreading to be focused within a sub-region, between a respective portion of a leg of the P electrode  220  and a respective portion of a leg of the N electrode  210  of the sub-region.  
     [0028] In the exemplary configuration shown in FIG. 2, current spreads from the P electrode leg  222  out toward the sub-regions  251 - 256  to either the N electrode leg  212  or the N electrode leg  214 . The tapering of the P electrode legs  222 ,  224  along the length of those legs run opposite to the tapering of the N electrode legs  212 ,  214 ,  216  along the length of the N electrode legs  212 ,  214 ,  216 . The sub-regions  251 ,  254  are near the wide end of the P electrode leg  222 , while the sub-region  253  is near the wide end of the N electrode leg  214 , and the sub-region  256  is near the wide end of the N electrode leg  212 . As the current comes in from the wide end of the P electrode leg  222 , the current starts spreading into the region closest to the wide end, i.e., subregions  251 ,  254 , and moving toward to the narrow ends of the N electrode legs  212 ,  214 . The current propagates along the length of the P electrode leg  222 , and then current spreading occurs in sub-regions  252 ,  255 . In the same manner, current spreading occurs in sub-regions  253 ,  256  when current propagates to the narrow end of the P electrode leg  222 .  
     [0029] Although not readily shown from the top view of the LED  200 , the channels may have vertical walls or angled walls according to different embodiments of the present invention. Although the channels  264  are shown to be straight and either horizontal or vertical when viewed from above in FIG. 2, it is noted that channels may retain a different line shape or may be slanted or curved when viewed from above in other embodiments. The number of channels may also vary, dividing the region  250  into more or fewer than the twelve sub-regions shown in FIG. 2. Channels with different lengths and widths may also be employed in a single LED or among different LEDS in multiple arrays of LEDs according to other embodiments.  
     [0030]FIG. 3 illustrates a top level view of an LED  300  constructed according to an embodiment of the present invention. The LED  300  has an electrode design of the N and P electrodes that is different from those illustrated in FIGS. 1 and 2. In the embodiment, some of the legs of or portions of the legs of the LED  300  are curved, creating a region  350  with rounded portions shown in FIG. 3. The top view of the LED  300  shows an N electrode  310 , a P electrode  320 , and the region  350  capable of passing light defined by the P electrode  320  and the N electrode  310 . The N electrode  310  has a straight leg  314  and two curved legs  312 ,  316  extending to the northeast corner, and the P electrode  320  has two curved segments  322 ,  324  extending to the southwest corner. In particular, the P electrode  320  includes a straight arm  325  that branches into the curved segments  322 ,  324 . For illustration purpose only, the region  350  is shown in white, while the P and N electrodes  320 ,  310  are shown in black. The two segments  322 ,  324  of the P electrode  320  are interspersed with and spaced apart from the three legs  312 ,  314 ,  316  of the N electrode  310 .  
     [0031] In the embodiment, the leg  314  of the N electrode  310  includes an enlarged portion  315  at its end, which has similar characteristics as the enlarged portion  115  shown in FIG. 1. Although the enlarged portion  315  is shown to have a circular shape, it is noted that another shape may be employed in other embodiments. Although the legs/segments of the P and N electrodes  320 ,  310  are not tapered and channels are not provided in the LED  300 , legs/segments of an LED with a similar electrode design as that of the LED  300  may be tapered and/or channels may be provided according to other embodiments of the present invention.  
     [0032]FIG. 4 illustrates a top level view of an LED  400  constructed according to an embodiment of the present invention. The LED  400  presents yet another electrode design of the N and P electrodes. In the embodiment, some of the legs or portions of the legs of the LED  400  are angled, creating a region  450  with triangular portions shown in FIG. 4. The top view of the LED  400  shows an N electrode  410 , a P electrode  420 , and the region  450  capable of passing light defined by the P electrode  420  and the N electrode  410 . The N electrode  410  has a straight leg  414  and two angled legs  412 ,  416  extending to the southwest corner, and the P electrode  420  has two angled segments  422 ,  424  extending to the northeast corner. In particular, the P electrode  420  includes a straight arm  425  that branches into the angled segments  422 ,  424 . For illustration purpose only, the region  450  is shown in white, while the P and N electrodes  420 ,  410  are shown in black. The two segments  422 ,  424  of the P electrode  420  interspersed with and spaced apart from the three legs  412 ,  414 ,  416  of the N electrode  410 .  
     [0033] In the embodiment, the leg  414  of the N electrode  410  includes an enlarged portion  415  at its end, which has similar characteristics as the enlarged portion  115  shown in FIG. 1. Although the enlarged portion  415  is shown to have a square shape, it is noted that another shape may be employed in other embodiments. Although the legs/segments of the P and N electrodes  420 ,  410  are not tapered and channels are not provided in the LED  400 , legs/segments of an LED with a similar electrode design as that of the LED  400  may be tapered and/or channels may be provided according to other embodiments of the present invention.  
     [0034]FIG. 5 shows a cross-sectional side view of an LED  500  constructed according to an embodiment of the present invention. If the LED  500  were to represent the LED  200  shown in FIG. 2 or an embodiment similar to the LED  200  when looking from above, this cross-sectional side view would represent a view obtained by cutting across Line AA shown in FIG. 2. The cross-sectional side view of the LED  500  shows a substrate  20 , a reflective layer  10 , a light-emitting structure  60 , a well  80 , a thin metal layer  230 ′, a P electrode  220 ′ and an N electrode  210 ′. In one embodiment, the LED  500  is Gallium Nitride (GaN) based, and the substrate  20  is made of sapphire, silicon carbide, or another suitable crystalline material. The reflective layer  10  is disposed below the substrate  20  along a vertical axis. The reflective layer  10  reflects light back toward the top surface, or the emitting surface, of the LED  500 . In one embodiment, the reflective layer  10  acts as a mirror and is made of aluminum. In other embodiments, other types of metal or material that provides the similar reflective effect may be utilized. According to an embodiment of the present invention, the reflective layer  10  is made of material that further provides thermal benefit to the LED  500  by improving the heat dissipation capability of the LED  500 . In the embodiment, the reflective layer  10  tends to draw heat produced in the LED  500  during operation and radiate it into the surrounding environment in an efficient manner.  
     [0035] The light-emitting structure  60  is disposed above the substrate  20 . In one embodiment, the light-emitting structure  60  comprises an active layer  50  sandwiched in between an N cladding layer  30  and a P cladding layer  40 . In operation, the forward biasing of the LED  500  causes light  5  to be emitted from the active layer  50 . Light emits in various directions as shown by the arrows in FIG. 5. Light that travels toward the substrate  20  will be reflected back by the reflective substrate  10 . Within the light-emitting structure  60 , the N cladding layer  30  is disposed above the substrate  20  along the vertical axis, and the P cladding layer  40  is disposed above the N cladding layer  30  along the vertical axis. In one embodiment, the P cladding layer  40  comprises Aluminum Gallium Nitrite (AlGaN), and the N cladding layer  30  comprises silicon doped Gallium Nitrite (Si:GaN). The P cladding layer  40  and the N cladding layer  30  form parts of the light-emitting structure of the LED  500 . The thin metal layer  230 ′ is disposed above the P cladding layer  40  of the light-emitting structure along the vertical axis and in contact with the P cladding layer  40 . Although the P cladding layer  40  is shown to be on top of the N cladding layer  30  in LED  500 , their positions may be reversed in other embodiments.  
     [0036] In the embodiment shown in FIG. 5, the P electrode  220 ′ is disposed above the P cladding layer  40  of the light-emitting structure along the vertical axis. Being in contact with the P cladding layer  40  at one end, the P electrode  220 ′ extends through the thin metal layer  230 ′ along the vertical axis at the other end. On the other hand, the N electrode  210 ′ is disposed in the well  80  that has an exposed surface  35  of the N cladding layer  30 . The N electrode  210 ′ is in contact with the surface  35  of the N cladding layer  30  in the well  80 . Because the N electrode  210 ′ is disposed in the well  80 , which is at a lower elevation than the top of the LED  500 , the N electrode  210 ′ is at a lower elevation than the P electrode  220 ′. In another embodiment, the location of the P cladding layer  40  and the P electrode  220 ′ may be switched with that of the N cladding layer  30  and the N electrode  210 ′, respectively, making the N electrode  210 ′ be at a higher elevation than the P electrode  220 ′. In yet another embodiment, the well  80  is not present, and there is no elevation offset between the P electrode  220 ′ and the N electrode  210 ′.  
     [0037] In one embodiment, the LED  500  may further include other layers disposed above and/or below the light-emitting structure  60 . These layers, along with the layers shown presently in FIG. 5, may be grown in a Metal Organic Chemical Vapor Deposition (MOCVD) reactor. A buffer layer(s) may, for example, be inserted somewhere between the substrate  20  and the light-emitting structure  60  to compensate the crystal lattice mismatch between layers and/or to allow formation of high quality materials at the beginning of crystal growth of the LED  500 . In one embodiment, a window structure formed of layers of GaN doped with different concentration of Magnesium may be formed between the light-emitting structure  60  and the P electrode  220 ′. In this case, even though the P electrode  220 ′ is not in direct contact with the P cladding layer  40 , they are still electrically connected with each other. The precise structure, composition and doping of the additional layers, as well as the layers presently shown in FIG. 5, are dependent on the required wavelength of the light-emission to be generated and need to be appropriately adapted in each individual case.  
     [0038]FIG. 6 shows a cross-sectional side view of an LED  600  constructed according to an embodiment of the present invention. In particular, channels  264 ″ are illustrated in this cross-sectional side view. If the LED  600  were to represent the LED  200  shown in FIG. 2 or an embodiment similar to the LED  200  when looking from above, this cross-sectional side view would represent a view obtained by cutting across Line B-B shown in FIG. 2. The cross-sectional side view of the LED  600  shows a substrate  20 ″, a reflective layer  10 ″, an N cladding layer  30 ″, a P cladding layer  40 ″, a mesa  80 ″, channels  264 ″, a thin metal layer  230 ″, a P electrode  220 ″ and an N electrode  210 ″. In the embodiment, the reflective layer  10 ″ is disposed below the substrate  20 ″ along a vertical axis. The reflective layer  10 ″ reflects light back toward the top surface, or the side emitting surface, of the LED  600 . The N cladding layer  30 ″ is disposed above the substrate  20 ″, and the P cladding layer  40 ″ is disposed above the N cladding layer  30 ″. In operation, the forward biasing of the LED  600  causes light  5 ″ to be emitted therefrom. In one embodiment, the P cladding layer  40 ″ comprises AlGaN, and the N cladding layer  30 ″ comprises InGaN. The thin metal layer  230 ″ is disposed above the P cladding layer  40 ″ along the vertical axis and in contact with the P cladding layer  40 ″. Although the P cladding layer  40 ″ is shown to be on top of the N cladding layer  30 ″ in LED  600 , their positions may be reversed in other embodiments.  
     [0039] In the embodiment shown in FIG. 6, the P electrode  220 ″ is disposed above the P cladding layer  40 ″ of the light-emitting structure along the vertical axis. Being in contact with the P cladding layer  40 ″ at one end, the P electrode  220 ″ extends through the thin metal layer  230 ″ along the vertical axis at the other end. On the other hand, the N electrode  210 ″ is disposed in the outside of mesa  80 a″ that has an exposed surface  35 ″ of the N cladding layer  30 ″. The N electrode  210 ″ is in contact with the surface  35 ″ of the N cladding layer  30 ″ in the outside of mesa  80 a″, which is at a lower elevation than the top of the LED  600 , the N electrode  210 ″ is at a lower elevation than the P electrode  220 ″. In the embodiment, a well  80 ″ b  is also provided next to the P electrode  220 ″, providing extra opening to the side of the LED  600 .  
     [0040] In one embodiment, the channels  264 ″ cut through the thin metal layer  230 ″ and the P cladding layer  40 ″ to the N cladding layer  30 ″, wherein a small portion of the N cladding layer  40 ″ is also removed. The channels  264 ″ may, for example, have the same depth as that of the wells  80   a ″,  80   b ″. This allows the channels  264 ″ and the wells  80   a ″,  80   b ″ to be formed together simultaneously in the same processing steps. The channels  264 ″, which shape similar to trenches, are openings that provide additional surface area for light to emit from the LED  600 . As compare to light that exits from the top surface of the LED  600 , which must past through the P cladding layer  40 ″ and the thin metal layer  230 ″, light that exits from the channels  264 ″ does not have to pass through such absorption material. The wells  80   a ″,  80   b ″ also provide non-absorbing area for light to exit. The wells  80   a ″,  80   b ″ allow light to exit from the side, without having to pass through the P cladding layer  40 ″ or the thin metal layer  230 ″ and the active layer. Together, the channels  264 ″ and the wells  80   a ″,  80   b ″ further improve luminous efficiency of the LED  600 .  
     [0041]FIGS. 7 a  and  7   b  illustrate a number of LED chips arranged in exemplary relationships according to embodiments of the present invention. In these embodiments, a number of LED chips are assembled into multiple clusters or arrays, which are then mounted onto a board and then integrated with a lamp housing, electronics, and/or various lenses to form a product. The LED chips may be placed in various arrangements, and FIGS. 7 a  and  7   b  show two examples of such arrangements. In FIG. 7 a,  the LED chips  710 - 740  are placed edge to edge, essentially forming a bigger square/rectangle. The wiring  745  provides the required electrical connection for the LED chips  710 - 745 . In FIG. 7 b,  the LED chips  750 - 790  are placed substantially in a cross arrangement. The wiring  795  provides the required electrical connection for the LED chips  750 - 795 . The arrangement of the LED chips is dependent on, for example, the required light-emission to be generated or the shape of the housing, and it is appropriately adapted according to individual cases.  
     [0042]FIG. 8 illustrates a method of making the LED  100  shown in FIG. 1 according to an embodiment of the present invention. In step P 800 , a substrate is provided. In one embodiment, the substrate comprises sapphire. In block P 810 , a light-emitting structure is formed above the substrate. This includes the formation of a first cladding layer and a second cladding layer, preferably an N cladding layer and a P cladding layer, respectively. In one embodiment, the P cladding layer is formed above the N cladding layer. In block P 820 , a thin metal layer is formed above the light-emitting structure and coupled to the light-emitting structure.  
     [0043] In block P 830 , an opening is created in the thin metal layer, exposing a portion of the first cladding layer of the light-emitting structure. In one embodiment, viewed from above, the opening resembles the U shape of the P electrode shown in FIG. 1, with two straight tapered opening portions extending to the right and having enlarged regions toward the ends of the portions. In the embodiment, the opening is created by conventional masking and etching techniques. In block P 840 , another opening, in the form of a well when viewed from the side of the LED  100 , is created. The well exposes a portion of the second cladding layer of the light-emitting structure. The surface of the well is at a lower elevation than the surface of the opening formed in block P 830 . In one embodiment, viewed from above, the well resembles the M shape of the N electrode shown in FIG. 1, with three straight tapered opening portions extending to the left and having enlarged regions toward the ends of the portions. In the embodiment, the opening/well is created by conventional masking and etching techniques. In block P 850 , a P electrode is coupled to the first cladding layer via the opening etched in block P 830  and overlap with the thin metal layer at connection area. In block P 860 , an N electrode is coupled to the second cladding layer via the opening, or the well, etched in block P 840 .  
     [0044] In block P 870 , a reflective layer is disposed below the substrate. The reflective layer reflects light back toward the top surface, or the emitting surface, of the LED  100 . In one embodiment, the reflective layer is also made of material that further provides thermal benefit to the LED  100  by improving the heat dissipation capability of the LED  100 .  
     [0045]FIG. 9 illustrates a method of making the LED  200  shown in FIG. 2 according to an embodiment of the present invention. In step P 900 , a substrate is provided. In block P 910 , a light-emitting structure is formed above the substrate, including the formation of a P cladding layer, an active layer, and an N cladding layer. In block P 920 , a thin metal layer is formed above the light-emitting structure and coupled to the light-emitting structure. In block P 930 , a first opening is created in the thin metal layer, exposing a portion of the P cladding layer. In one embodiment, viewed from above, the opening resembles the U shape of the P electrode shown in FIG. 2, with two straight tapered opening portions extending to the right and having enlarged regions toward the ends of the portions.  
     [0046] In block P 940 , a second opening, in the form of a well when viewed from the side of the LED  200 , is created. The second opening exposes a portion of the N cladding layer of the light-emitting structure. The surface of the well is at a lower elevation than the surface of the opening formed in block P 930 . In one embodiment, viewed from above, the well resembles the M shape of the N electrode shown in FIG. 1, with three straight tapered opening portions extending to the left and having enlarged regions toward the ends of the portions.  
     [0047] In block P 950 , a number of straight-line openings, each in the form of a well when viewed from the side of the LED  200 , are created. In one embodiment, the straight-line openings expose a portion of the N cladding layer of the light-emitting structure. The straight-line openings, which may be vertical or horizontal when viewed from above, serve as the channels of LED  200 , dividing the region defined by the P electrode and the N electrode into sub-regions. The top surface of the straight-line openings is at a lower elevation than the surface of the opening formed in block P 930 .  
     [0048] In block P 960 , an edge opening is formed along the edge of the LED  200 . The fourth opening also represents a well when viewed from the side of the LED  200 . Viewed from above, the edge opening resembles a hollow square. The top surface of the edge opening is at a lower elevation than the surface of the opening formed in block P 930 . In one embodiment, the openings formed in blocks P 950  and P 960  have the same depth as the one formed in block P 940 , allowing the three openings formed in blocks P 940 -P 960  to be formed simultaneously during the same etching processes.  
     [0049] In block P 970 , a P electrode is coupled to the first cladding layer via the first opening etched in P 930 . In block P 980 , an N electrode is coupled to the second cladding layer via the second opening, or the well, etched in P 940 . The third opening is left unchanged. In block P 990 , a reflective layer is disposed below the substrate to reflect light travels toward it back toward the top surface, or the emitting surface, of the LED  200 .  
     [0050]FIG. 10 illustrates a top level view of an LED  1000  constructed according to an embodiment of the present invention. The top view of the LED  1000  shows an N electrode  1100 , a P electrode  1200 , and a region  1500  capable of passing light defined by the P electrode  1200  and the N electrode  1100 . A thin, substantially translucent metal layer  1300  is disposed above the region  1500  and between the N electrode  1100  and the P electrode  1200 , which is overlapped with the P electrode  1200 , and separate from the N electrode  1100  by a mesa edge  1600 . Although the LED  1000  is shown to retain a square shape in the embodiment of FIG. 10, it is noted that any shape may be employed depending on the specific application.  
     [0051] Although not shown in FIG. 10, disposed below the thin metal layer  1300  and the region  1500 , along a vertical axis, is a light-emitting structure with an N cladding layer and a P cladding layer. The N electrode  1100  is in contact with the N cladding layer, while the P electrode  1200  is in contact with the P cladding layer and overlaps with the thin metal current spreading layer  1300 . The operation of the LED  1000  has been disclosed hereinabove with respect to similar embodiments and as such shall not be discussed further herein.  
     [0052] The spreading of the current from the P electrode  1100  to the N electrode  1200  is enhanced by the layout design and relative positioning of the P and N electrodes  1100 ,  1200  as well as the thin metal layer  1300 .  
     [0053] In the embodiment depicted in FIG. 10, the N electrode  1100  has a contact portion  1170  and a plurality of legs  1120 ,  1140 ,  1160  extending from the contact portion  1170  along a horizontal axis. The P electrode  1200  has a contact portion  1270  and at least two legs  1220 ,  1240  extending from the contact portion  1270  along the horizontal axis in a direction opposite the plurality of legs  1120 ,  1140 ,  1160 .  
     [0054] The at least two legs  1220 ,  1240  of the P electrode  1200  are interdigitated with and spaced apart from the three legs  1120 ,  1140 ,  1160  of the N electrode  1100 . As viewed from above, the legs  1120 ,  1140 ,  1160 ,  1120 ,  1240  appear to be parallel to each other. The P electrode  1200  and N electrode  1100  may be interchanged and the current flow reversed and the LED  1000  will still function.  
     [0055] Each leg  1120 ,  1140 ,  1160 ,  1220 ,  1240  has an outer edge as defined by the periphery thereof. As depicted in FIG. 10, the minimum distance from the outer edge of any one leg of the N electrode  1100  to the outer edge of at least one leg of the P electrode  1200  is substantially the same for all points along the outer edge of each leg  1120 ,  1140 ,  1160 ,  1220 ,  1240 . External edges  1180  of the N electrode legs  1120 ,  1140 ,  1160  that are at the periphery of the LED  1000  are not considered in determining the minimum travel distances.  
     [0056] By maintaining the same minimum distance between the outer edges of the N and P electrode legs respectively, current crowding due to differences in resistive distance is minimized and potentially eliminated.  
     [0057] Additionally, the spread of current flow through the active region may be maximized by ensuring that there exists a one to one correspondence between a point on the outer edge of each leg  1120 ,  1140 ,  1160  of the N electrode  1100 , and the outer edge of each leg  1220 ,  1240  of the P electrode  1200 , such that current will flow through the entire region  1500 .  
     [0058] With the electrode designs of the present invention and specific characteristics, the optical output efficiency or the luminous efficiency is improved. The LEDs are also able to operate reliably at its current level while minimizing current crowding. The specific structures of the elements on the LEDs also allow emission of light from a number of additional places within the LEDs. With the reflective layer, the LEDs are also able to have increased illumination and improved heat dissipation capability. Embodiments of the present invention are suitable for implementation in, for example, a large area GaN LED with dimensions of 0.5 mm×0.5 mm to 5 mm×5 mm. Embodiments of the present invention are also suitable for implementation in applications such as those related to traffic lights, electronic signs, high power displays, medicine and dentistry.  
     [0059] It should be emphasized that the above-described embodiments of the invention are merely possible examples of implementations set forth for a clear understanding of the principles of the invention. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Variations and modifications may be made to the above-described embodiments of the invention without departing from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the invention and protected by the following claims.