Abstract:
An arrangement of light emitting diodes including a first micro-die, a second micro-die, a first bridge, a second bridge and a substrate supporting the first micro-die and the second micro die. The first micro-die includes a first edge having a first end and a second end, a second edge opposite and not parallel to the first edge, a first connecting portion near the first end, and a second connecting portion near the second end.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of co-pending application Ser. No. 12/149,852, filed on May 9, 2008, which is a Divisional of application Ser. No. 10/994,361, filed on Nov. 23, 2004, now U.S. Pat. No. 7,531,843, for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 093126201 filed in Taiwan, R.O.C. on Aug. 31, 2004 under 35 U.S.C. §119, the entire contents of all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a structure of light-emitting diodes (LED), and particularly to a structure of LED dies with an alternating current (AC) loop (a structure of AC LED dies). 
     2. Related Art 
     A light-emitting diode (LED) emits light based on its semi-conductive characteristics, in contrast to the heating light-emitting principle of a fluorescent lamp, and is thus called a cold light. The LED provides a number of advantages such as high endurance, long lifetime, compactness, low power consumption and so forth. Furthermore, no pernicious material such as mercury is contained in the LED. Therefore, there are high expectations with respect to the LEDs for being a light source in daily life in the current lighting market. 
     However, prior LEDs are generally limited in their acceptable power levels. Most LEDs may be fed with only low DC voltages and may be damaged if high voltages or AC voltages are applied thereon. Because of this, a DC voltage conversion circuit is generally used to transform the external power supply used by such LEDs. In operating an LED by use of a low DC voltage, the LED has its characteristic curve of the current-voltage relation, as shown in  FIG. 1A . As shown, when the voltage is forwardly applied, the LED is conducted and light is emitted there from. On the other hand, if a reverse voltage is applied, the LED breaks down and no light is emitted. Further, in practical usage the LED is often connected in series or parallel with several such LEDs, such as those used in traffic light apparatuses such as stop light apparatuses. As shown in  FIG. 2 , the externally supplied AC voltage  11  is first reduced in its level by means of a conversion circuit  12  and then converted into a DC voltage corresponding thereto. Then the converted DC voltage is fed into a plurality of LEDs connected with one another in series or in parallel as mentioned above, in which LEDs cannot be used when reverse power is supplied. 
     However, once a single LED arranged among the plurality of LEDs is damaged, the set of LEDs in which the damaged LED resides is also likely to become damaged and the whole of the loop formed with the damaged LED included is badly affected. To reduce this occurrence, the number of LEDs connected in series is generally reduced as much as possible. Unfortunately, the total amount of wires used for these LEDs in a specific application is unavoidably increased and the power consumption increases correspondingly. Furthermore, the voltage at an end of one of the wires is insufficient and thus causes uneven luminance of the LEDs. 
     There is another serious problem with a low DC voltage operated ALInGaN LED. When such a LED is assembled and processed, electrical static discharge (ESD) is apt to occur. When this occurs, an instantaneous high reverse voltage is burst forth and the LED is damaged. 
     To resolve the above-mentioned shortcomings, circuit assembly and die manufacturing are two generally adopted solutions. 
     The circuit assembly scheme may be seen in U.S. Pat. No. 6,547,249. This patent discloses an additional diode arranged in a reverse orientation and connected in parallel to protect an LED-based circuit to prevent sudden ESD or an exceptional current or voltage attack. In another U.S. Pat. No. 5,936,599, LEDs in an LED based circuit are arranged in a reverse orientation and connected in parallel, and inductors and capacitors are introduced in the circuit. In this case, an AC voltage and a high voltage may be used by the LEDs. However, although the problem of high power consumption may be overcome by such circuit assembly schemes, the corresponding large volume of the LED based circuit considerably limits its actual applicable range. 
     An example of the die manufacturing scheme may be seen in U.S. Pat. No. 6,547,249, in which LED dies are manufactured as a matrix form and connections of the LED dies are arranged in the same orientation in series and in parallel. Although such LEDs may be operated with a high voltage, they may still not be applied with an AC voltage. In this patent, an arrangement for protection of breakdown of the LEDs is also provided by connecting a diode with the LEDs in a variety of combinations where the LEDs may also be arranged in mutually reverse orientations and connected with each other but should be disposed over a submount and then connected with the LED matrix in parallel. According to this patent, the LED die  91  has a structure shown in  FIG. 1B , and has an equivalent circuit shown in  FIG. 1C , in which the LED  91  is connected in parallel with two mutually oriented Zener diodes  92  and  93 , or a connection may be provided to form a loop as shown in  FIG. 1D . The current-voltage relation curves corresponding to the equivalent circuits in  FIG. 1C  and  FIG. 1D  are shown in  FIG. 1E  and  FIG. 1F  respectively. 
     Also referring to U.S. Pat. No. 6,635,902, the LED dies are also manufactured as a matrix form while the LEDs are oriented the same and connected in series. Although the LEDs may be operated with a high voltage, they also have the problem of not being capable of operation with AC voltage. 
     SUMMARY OF THE INVENTION 
     An object of the invention is therefore to provide a structure of light-emitting diode (LED) dies having an alternating current (AC) loop abbreviated as a structure of AC LED dies, on which an AC power supply may be applied directly to considerably broaden applicable range. 
     In one embodiment, an arrangement of light emitting diodes includes a substrate, and four adjoining light-emitting units forming a light-emitting array on the substrate, wherein each unit includes at least one micro-die. Each micro-die includes a lower semiconductor layer of first conductivity on the substrate, an upper semiconductor layer of second conductivity opposite to the first conductivity on the lower semiconductor layer, a first connecting portion disposed on the lower semiconductor layer, and a second connecting portion disposed on the upper semiconductor layer. The first connecting portion and the second connecting portion are respectively located near different sides of the micro-die. The four adjoining light-emitting units include a first group of the connecting portions having four connecting portions disposed substantially at a center of the light-emitting array, and a second group of the connecting portions including another four connecting portions disposed substantially at the periphery of the light-emitting array. 
     In another embodiment, an arrangement of light emitting diodes includes a first micro-die, a second micro-die, a first bridge, a second bridge, and a substrate supporting the first micro-die and the second micro die. The first micro-die includes a first edge having a first end and a second end, a second edge opposite and not parallel to the first edge, a first connecting portion near the first end, and a second connecting portion near the second end. Besides, the first bridge covers the first connecting portion, and electrically connects the first micro-die with the second micro-die. The second bridge covers the second connecting portion and electrically connects the first micro-die with the second micro-die. 
     In practical usage, the structure of the AC LED dies may be provided in a flipped form or a faced-up form. Also, each of the LED dies in the structure of the AC LED dies may correspond to the same wavelength or different wavelengths with those of the other LEDs in the unit of AC LED dies. Thus the structure of AC LED dies may be used in a wider applicable range. 
     The objects, constructions, features and functions of the invention may be better understood through the following detailed description with respect to the preferred embodiments thereof in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a characteristic curve diagram of a prior light-emitting diode (LED) die; 
         FIGS. 1B-1D  are schematic illustrations of a prior LED die produced by Lumileds; 
         FIGS. 1E-1F  are characteristic curve diagrams of the LED die shown in  FIGS. 1B-1D ; 
         FIG. 2  is an illustration of the prior LED in use; 
         FIG. 3  is a schematic diagram of a structure of LED dies having an alternating current (AC) loop (a structure of AC LED die) according to the invention; 
         FIG. 4A  is an equivalent circuit diagram of the structure of AC LED dies shown in 
         FIG. 3 ; 
         FIG. 4B  is a characteristic curve diagram of the structure of AC LED dies shown in 
         FIG. 3 . 
         FIG. 5  is a schematic diagram describing a manufacturing of the structure of AC LED dies; 
         FIG. 6  is a schematic diagram illustrating a package of the structure of AC LED dies shown in  FIG. 3 ; 
         FIG. 7  is a schematic diagram illustrating a flip-chip structure of the AC LED dies shown in  FIG. 3 ; 
         FIG. 8  is a variant of the equivalent circuit shown in  FIG. 4A ; 
         FIG. 9A  is the structure of AC LED dies according to another embodiment of the invention; 
         FIG. 9B  is a variant of the structure of AC LED dies shown in  FIG. 9A ; 
         FIGS. 10A and 10B  are illustrations of a plurality of structures of AC LED dies connected in a matrix form according to the invention; 
         FIG. 11  is an equivalent circuit diagram of the matrix-formed plurality of structures of AC LED dies shown in  FIGS. 10A and 10B ; and 
         FIGS. 12A-12F  are illustrations of a process flow of the manufacturing of the structure of AC LED dies according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A structure of light-emitting diode (LED) dies having an alternating current (AC) loop, which may be fed with a direct AC power supply, is disclosed in the invention (abbreviated as a structure of AC LED dies). The structure of AC LED dies comprises at least a unit  50  of AC LED micro-dies, which will be described in the following. Referring to  FIG. 3 , the unit of AC LED micro-dies comprises a first LED micro-die and a second LED micro-die  21  and  22  arranged in mutually reverse orientations and connected in parallel. The unit of AC LED micro-dies has an equivalent circuit as shown in  FIG. 4A . Since the first and second LED micro-dies  21  and  22  are oriented reversely and connected in parallel, the first LED micro-die  21  emits light when a positive-half wave voltage in the AC power supply is applied, while the second LED micro-die  22  emits light when a negative-half wave voltage in the AC power supply is applied. Therefore, the unit of LED micro-dies may emit light continuously whenever a proper AC power supply is provided. For this reason, the above-mentioned terms “AC loop”, “AC LED dies” and “AC LED micro-dies” are used. 
     As is shown in  FIG. 3 , the unit  50  of micro-dies is fabricated on a substrate  100 . Two layers  102  and  104  of a first conductivity type are supported on the substrate and separated from each other. The layer  102  has a wide end  102 W and a narrow end  102 N. Similarly, the layer  104  has a wide end  104 W and a narrow end  102 N. Two layers  106  and  108  of a second conductivity type are supported respectively on the two layers  102  and  104  with the first conductivity type. Like the layers  102  and  104 , the layers  106  and  108  have wide ends ( 106 W and  108 W) and narrow ends ( 106 N and  108 N). A rear pad  110  is formed on the layer  104  of the first conductivity type, adjacent its narrow end  104 N. A rear pad  112  is formed on the layer  106  of the second conductivity type, adjacent its wide end  106 W. A rear conductive bridge  114  connects the rear pads  110  and  112 . A front pad  116  is formed on the layer  102  of the first conductivity type, adjacent its narrow end  102 N. A front pad  118  is formed on the layer  108  of the second conductivity type, adjacent its end  108 W. A front bridge  120  connects the front pads  116  and  118 . 
     As is also shown in  FIG. 3 , the substrate  100  is rectangular and has two pairs of parallel sides, including front and rear parallel sides  100 F and  100 R. The micro-die  21  has an edge  122 , and an anode (at pad  118 ) and a cathode (at pad  110 ) disposed adjacent opposite ends of the edge  122 . As  FIG. 3  illustrates, the micro-dies  21  and  22  complement each other in shape so as to occupy a substantially rectangular region. The micro-die  22  has an edge  124  and an anode and cathode disposed adjacent opposite ends of the edge  124 . The edge  122  is disposed at an acute angle α (illustrated with the aid of a dotted line) to the front and rear edges  100 F and  100 R. Similarly, the edge  124  is disposed at an acute angle (corresponding to the angle α and illustrated with the aid of a dotted line, but not marked with a reference character in order to avoid cluttering the drawing) with respect to the front and rear edges  100 F and  100 R. 
     Furthermore, the characteristic curve associated with the current-voltage relation of the unit of AC LED micro-dies is provided in  FIG. 4   b . Since each LED micro-die in the unit is operated forwardly, the structure of AC LED dies also provides protection from electric static charge (ESD) without the need of an additional circuit, as in the prior art, or a diode fixed on a sub-mount and connected with the LEDs, as in U.S. Pat. No. 6,547,249. Therefore, the purpose of cost saving may be achieved. 
       FIG. 5  illustrates the manufacturing of the structure of AC LED dies in a related embodiment. First, two unconnected n-type light-emitting layers  62   a  and  62   b , such as a n-InGaN layer, are first formed on a substrate  61  made of Al 2 O 3 , GaAs, GaP or SiC, etc. Next, two p-type light-emitting layers  63   a  and  63   b , such as an p-InGaN layer, are formed on portions of the n-type light-emitting layers  62   a  and  62   b  respectively. Next, n-type pads  67   a  and  67   b  are formed on other portions of the n-type light-emitting layers  62   a  and  62   b  respectively. Then, p-type pads  66   a  and  66   b  are formed on the p-type light-emitting layers  63   a  and  63   b  respectively. Then a conductive bridge  65  is formed to connect the n-type pad  67   a  and the p-type pad  66   b , and an insulating layer  64  is formed to avoid short-circuiting between the n-type pad  67   a , the p-type pad  66   b  and the conductive bridge  65 . Finally, the p-type pad  67   b  is connected to the n-type pad  66   a.    
     Specifically, the manufacturing of the structure of AC LED dies is illustrated as follows with reference to  FIGS. 12A-12F . First, a substrate  61  is provided. On the substrate  61 , n-type light-emitting layers  62   a  and  62   b  and p-type light-emitting layers  63   a  and  63   b  are provided (from bottom to top), as shown in  FIG. 12A . Next, an etching operation is performed upon a portion of each of the p-type light-emitting layers  63   a  and  63   b , and a corresponding portion of each of the n-type light-emitting layers  62   a  and  62   b  is thus exposed, as shown in  FIG. 12B . Next, an insulating layer  64  is formed, as shown in  FIG. 12C . The insulating layer  64  may be an oxide layer, for example. Thereafter, specific portions defined for formation of pads in the n-type light-emitting layers  62   a  and  62   b  and p-type light-emitting layers  63   a  and  63   b  are etched, as shown in  FIG. 12D . Then, n-type pads  67   a  and  67   b  and p-type pads  66   a  and  66   b  are formed at their defined regions as mentioned, as shown in  FIG. 12E . Finally, a conductive bridge  65  is formed and connected between the n-type pad  67   a  and p-type pad  66   b , as shown in  FIG. 12F . 
     In addition, the structure of AC LED dies may be covered by a glue as a packaged structure and fixed on a sub-mount  69 , wherein the glue may be a heatsink glue and the sub-mount  69  may be formed with a surface that acts as a reflective layer to reflect light. Alternatively, bumps  72  may be formed over the sub-mount  69 . Trace  71  are used to connect the n-type pad  67   a  with the p-type pad  66   b , and the n-type pad  67   b  and the p-type pad  66   a  are also electrically connected with each other (not shown in the figure) as shown in  FIG. 7 . 
     In addition, the structure of AC LED dies may be connected with a third LED micro-die  23  in parallel as shown in  FIG. 8 , and an asymmetric structure of AC LED dies is thus formed. 
       FIG. 9A  illustrates another embodiment of the structure of AC LED dies. In this embodiment, a first LED micro-die  21  is connected with a third LED micro-die  23  and a second LED micro-die  22  is connected with a fourth LED micro-die  24 , and the same result as provided by the above mentioned embodiment of the structure of AC LED dies is obtained. Alternatively, the structure of AC LED dies may be further connected with a fifth LED micro-die  25  and a sixth LED micro-die  26  in parallel, similar to that shown in  FIG. 8 , as shown in  FIG. 9B . In the above embodiments, each of the LED micro-dies may emit light with a single wavelength or multiple wavelengths when a power supply is supplied, such as wavelengths corresponding to red, green and blue lights. 
     In practical usage, a first pad  41  and a second pad  42  are formed on a substrate  40 , and a plurality of units  50  of AC LED dies are coupled therebetween, as shown in  FIGS. 10A and 10B . Each unit  50  of AC LED dies comprises a first LED micro-die  21  and a second LED micro-die  22 , as shown in  FIG. 3 , and has an equivalent circuit shown in  FIG. 11 . Seen from  FIG. 11 , it may be readily known that the first and second LED micro-dies  21  and  22  are arranged in mutually reverse orientations and connected in parallel, and a plurality of thus formed units  50  is connected in series. Similar to the description in  FIG. 3 , the first LED micro-die  21  in the unit  50  emits light when a positive-half wave voltage is in the AC power supply, while the second LED micro-die  22  in the unit  50  emits light when a negative-half wave voltage is in the AC power supply (see  FIG. 10B ). Since the voltage of the AC power supply is varied between a positive peak and a negative peak with a high frequency, light emitted alternatively from the LEDs  21  and  22  is continuous. Generally, AC voltage has a large swing or a large amplitude. Even if the voltage on such a unit  50  connected at the downstream of a wire connecting a plurality of units is slightly dropped, the range of reduction is relatively small, unlike the prior art (only several volts is provided) in which slight changes over the voltage fed into the LED cause a remarkable difference of luminance of the LED. Since the LED has a fast response speed, the AC power supply may have a frequency up to 50-60 kHz. In addition, any waveform of the AC power supply may be used, provided that the waveform is symmetrical. 
     As shown in  FIGS. 10A and 10B , at least four adjoining light-emitting units  50  are on the substrate, and each unit  50  comprises at least one micro-die ( 21  or  22 ). Moreover, as shown in  FIG. 5  or  FIG. 12 , each micro-die ( 21  or  22 ) in  FIG. 10A  comprises a lower semiconductor layer of first conductivity ( 62   a  or  62   b ) on the substrate  61 , an upper semiconductor layer of second conductivity ( 63   a  or  63   b ) on the lower semiconductor layer ( 62   a  or  62   b ), and two connecting portions  55  on the lower semiconductor layer ( 62   a  or  62   b ) and the upper semiconductor ( 63   a  or  63   b ) respectively. Moreover, the four adjoining light-emitting units  50  comprise a first group of connecting portions  55  comprising four connecting portions disposed substantially at the center of the four adjoining light-emitting units  50 , and a second group of connecting portions comprising another four connecting portions  55  disposed substantially at the periphery of the four adjoining light-emitting units  50 . A crossing  56  is surrounded by four micro-dies ( 21  or  22 ) on the substrate, and each of the four micro-dies further comprises an anode and a cathode, wherein either an anode or a cathode of each of the micro-die is nearby the crossing  56 . 
     While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.