Abstract:
The present invention relates to a plurality of light emitting diodes connected in series to elevate the working voltage and to enable the devices to be connected directly to the AC voltage sources. The LED device has five pluralities of series-connected diodes. Four pluralities of series-connected diodes are arranged to at as a rectifier bridge so the fifth plurality of diodes is always forward biased and energized. The light emitting diodes in the device are arranged to accommodate various AC line voltages, diode operating voltages, and diode reverse breakdown voltages. The plurality of diodes was manufactured by first etching epitaxial layer to the insulating substrate to isolate individual diodes, and then use metal lines to interconnect them according to the layout design. The number of die-attach and wire-bonding steps used in the subsequent chip array and lamp manufacturing process is therefore greatly reduced or eliminated. This invention simplifies LED array and lamp process flow, lowers the manufacturing cost, improves product reliability and LED device efficacy.

Description:
REFERENCES CITED 
     U.S. Patent Documents 
       [0001]      
         [0000]                                                    4,271,408   June 1981   Teshima et al           4,298,869   November 1981   Okuno et al           5,091,756   February 1992   Iga et al           5,521,935   May 1996   Irikawa et al           5,936,599   August 1999   Reymond           6,547,294 B2   April 2003   Collins et al           2003/0122501 A1   July 2003   Clauberg et al           2004/0075399 A1   April 2004   Hall et al           2005/0087884 A1   April 2005   Stokes et al           6,957,899 B2   October 2005   Jiang et al           7,221,044 B2   May 2007   Fan et al           2008/0017871 A1   January 2008   Lee et al           7,474,681 B2   January 2009   Lin et al           7,531,843 B2   May 2009   Lin et al           2010/0059735 A1   March 2010   Lee et al           7,714,348 B2   May 2010   Fan et al           7,964,880 B2   June 2011   Lee et al           7,834,364 B2   November 2010   Lee et al           7,842,959 B2   November 2010   Lee et al                        
This is a continuation-in-part of U.S. application Ser. No. 61/601,698, filed on Feb. 22, 2012, now abandoned.
 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to the design of an alternating current (AC) light-emitting diode (LED) device and its manufacturing process based on GaN epitaxial growth on transparent sapphire substrate. This design incorporates a special layout of LED cells monolithically integrated to form an LED device which can be directly connected to AC power sources 120V, 220V or other voltages without AC to DC drivers. 
         [0004]    2. Description of the Related Art 
         [0005]    LEDs are semiconductor diode devices to produce light when the diodes are forward biased and the recombination of electron and hole pairs release energy in the form of photons. Traditional light sources, such as incandescent lamps, which rely on visible portion of the thermal radiation from the filament heated to very high temperatures, and compact fluorescent lamps, which use discharge from mercury vapor and argon gas to generate UV light to excite luminophore for visible light. LED, a solid lighting device, has numerous advantages over traditional lighting options, including compact size, long lifetime, high durability, low power consumption, and absence of harmful mercury. LEDs are widely used as indicator lights, vehicle signal and illuminating lights, LCD backlights for TV and mobile devices, projector light sources, outdoor display and general illumination or lighting. 
         [0006]    LEDs are low voltage DC devices that let electron pass in one direction and block electron flow when connected in the opposite polarity. They are driven by either DC voltage sources or by AC voltages with AC to DC drivers to supply the desired DC voltage. 
         [0007]    To allow LEDs to directly connect to the AC power sources without using AC to DC drivers, two sets of LEDs are connected in parallel with opposite polarity, as described by Okuno et al in U.S. Pat. No. 4,298,869 entitled “LIGHT-EMITTING DIODE DISPLAY” and Reymond in U.S. Pat. No. 5,936,599 entitled “AC POWERED LIGHT EMITTING DIODE ARRAY CIRCUITS FOR USE IN TRAFFIC SIGNAL DISPLAYS”. Their approaches were based on expensive discrete single LED emitters and also required complex wiring and assembly process. Fan et al in U.S. Pat. No. 6,957,899 B2 entitled “LIGHT EMITTING DIODES FOR HIGH AC VOLTAGE OPERATION AND GENERAL LIGHTING” proposed the design of a single high voltage LED chip, composed of two arrays of series-connected individual LEDs in opposite polarity for connection to high voltage AC sources. The design doubles the number of LED diodes in the chip to achieve the same amount of light output from similar DC driven LED device. Hall et al in U.S. Pat. No. 2004/0075399 A1 entitled “LED LIGHT ENGINE FOR AC OPERATION AND METHODS OF FABRICATING SAME” proposed a chip layout design to deal with issues related to individual open or short diode in the series-connected chains. Lee et al in the U.S. Pat. No. 7,834,364 B2 entitled “AC LIGHT EMITTING DIODE AND METHODE FOR FABRICATING THE SAME” adopted a similar approach as described by Fan et al, and described how the individual LEDs were connected to form a single AC LED chip. Lin et al in the U.S. Pat. No. 7,474,681 B2 entitled “ALTERNATING CURRENT LIGHT-EMITTING DEVICE” proposed a different design to incorporate multiple active layers with a rectifier bridge in the design so the active layers take turns to emit light with different colors when powered by AC voltage sources. There are no specifics regarding how to incorporate the rectifier bridge and how to handle specific AC line voltages. Lee et al in another U.S. Pat. No. 7,964,880 B2 entitled “LIGHT EMITTING ELEMENT WITH A PLURALITY OF CELLS BONDED, METHOD OF MANUFACTURING THE SAME, AND LIGHT EMITTING DEVICE USING THE SAME” described a lateral and a vertical chip with LED cells monolithically connected to take high DC voltage and also connected to an external predetermined rectifier bridge to handle AC voltage. However, there were no specifics regarding how to integrate the rectifier bridge into the LED chip design, and how to handle different AC line voltages. 
         [0008]    Therefore, a need exists to develop a novel alternating current light-emitting diode chip, based on flip-chip technology, to address the drawbacks of the previous art, and to provide a lower cost and a more reliable solution for the general lighting industry. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is to overcome the drawbacks of the previous arts related to the AC LED chip design and its manufacturing process. The new light-emitting device has a built-in rectifier bridge constructed using the same diodes as other arrayed light emitting cells, and a protection mechanism using resistors and capacitors embedded in the chip carrier. This design, combined with a new wafer fabrication process, enables the AC power source directly connect to the device. The monolithically integrated approach at wafer level is used to manufacture a single AC LED device with a plurality of light emitting cells connected in series. The chip design and the fabrication process minimize process steps, improve product yield, and reduce overall product cost. 
         [0010]    For the present invention, five blocks of light emitting elements connected in series, with number of cells in each block depending on the AC line voltage and individual cell diode operating and reverse breakdown voltage, on a single substrate with specially arranged layout to form an AC light emitting device. 
         [0011]    The AC light emitting device comprises the plurality of light emitting cells, each of which includes a transparent substrate, a buffer layer of undoped or silicon doped gallium nitride (GaN) stacked on the substrate, a heavily silicon doped n-type GaN layer, an active layer consisting of multiple quantum well (MQW) formed through alternating GaN and lnGaN in a pre-determined region on top of the n-GaN layer, an electron block layer consisting of magnesium doped AlGaN, and a magnesium doped p-type GaN layer formed on top of the electron blocking AlGaN layer. A stress release super lattice (SL) structure consisting of alternating very thin GaN and InGaN layers is inserted between n-GaN and multiple quantum well to release layer stress and to improve internal quantum efficiency of the light emitting cells. 
         [0012]    Each cell is isolated from adjacent cells through lithography patterning and dry or wet etch process. Then, another patterning and dry or wet etch process is used to partially expose the n-GaN layer, which is then connected to the p-GaN layer of adjacent cells. The isolation oxide or nitride is deposited and etched according to AC light emitting device layout design. The wafer is then covered with blank metal films and the metal connection lines are formed through patterning and dry etch. A subsequent passivation process is used to cover the metal lines with silicon oxide or nitride layer. A final etch of the passivation layer exposes the underneath metal to form p and n-type electrodes for connection to the external power sources. The transparent sapphire substrates then go through steps of grinding and polishing to reduce its thickness. The wafers with AC light emitting chips are cut into individual dies, whose exposed metal electrodes are connected to the metal bumpers on the chip submount carrier, and then are covered with phosphors of specific color to achieve the desired LED color and light output power when connected to the AC voltage sources. 
         [0013]    The light emitting cells inside the structure of the AC LED device are to be manufactured in flip-chip forms, so the generated light comes out from the transparent substrate side. The connection electrodes of the light-emitting device contact the metal bumps on the top surface of the submount substrate. Heat generated from the light-emitting device is dissipated through the submount substrate, thereby promoting the dissipation of heat, reducing the thermal load on the light emitting device, improving light emitting efficiency, and lengthening the lifetime of the light emitting devices. 
         [0014]    All light emitting cells in the structure of the AC LED device have the same light emitting area. They may have the same or similar wavelength, the same or similar operating and reverse breakdown voltage as those of the other LED cells in the AC LED device. 
         [0015]    Thus, the structure of the LED device may be used to generate uniform light output, and to provide stable and durable lighting solutions for a wide range of applications. 
         [0016]    The objectives, constructions, features, and functions of the invention may be better understood through the following detailed description with respect to the preferred embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1A  is a diagram to illustrate a conventional AC LED device with two sets of series-connected light emitting cells in opposite polarity.  FIG. 1B  is similar to  FIG. 1A  with addition of either an optional protective resistor or an optional capacitor or both a protective resistor and a capacitor to prevent voltage spike during device operation. 
           [0018]      FIG. 2  is a cross sectional view of a conventional light-emitting cell based on lateral chip design, which requires two wires for each device. 
           [0019]      FIG. 3  is a cross sectional view of a light emitting cell according to the embodiment of the present flip-chip design. 
           [0020]      FIG. 4A  is a top view of one type of light emitting cell according to the embodiment of the present invention.  FIG. 4B  is a top view of another type of light emitting cell according to the embodiment of the present invention.  FIG. 4C  is a variation of the cell illustrated in  FIG. 4A .  FIG. 4D  is a variation of the cell illustrated in  FIG. 4B  with different n-contact shape. 
           [0021]      FIG. 5  is a cross sectional view illustrating an AC light-emitting device according to an embodiment of the present invention. Multiple light emitting cells are connected monolithically. 
           [0022]      FIG. 6A  is a prior art to illustrate a diode interconnection schematic diagram of the AC LED device with 2 emitting cells in series mode.  FIG. 68  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 6A . 
           [0023]      FIG. 7A  is the diode interconnection schematic diagram of the AC LED device with 5 emitting cells in series mode according to the embodiment of the present invention.  FIG. 7B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 7A . 
           [0024]      FIG. 8A  is the first variation of diode interconnection schematic diagram of the AC LED device with 10 emitting cells in series mode according to the embodiment of the present invention.  FIG. 88  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 8A . 
           [0025]      FIG. 9A  is the second variation of diode interconnection schematic diagram of the AC LED device with 17 emitting cells in series mode according to the embodiment of the present invention.  FIG. 9B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 9A . This layout design can be directly connected to AC voltage source of 55V, or two of them in series mode for connection directly to 110V AC power source, or four of them for 220V AC power source. 
           [0026]      FIG. 10A  is the third variation of the diode interconnection schematic diagram of the AC LED device with 26 emitting cells in series mode according to the embodiment of the present invention.  FIG. 106  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 10A . 
           [0027]      FIG. 11A  is the fourth variation of diode interconnection schematic diagram of the AC LED device with 37 emitting cells in series mode according to the embodiment of the present invention.  FIG. 11B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 11A . The AC device&#39;s operating voltage is between 110 and 120V AC, or two of them in series mode for connection to AC voltage between 220 and 240V. 
           [0028]      FIG. 12A  is the fifth variation of diode interconnection schematic diagram of the AC LED device with 50 emitting cells in series mode according to the embodiment of the present invention.  FIG. 12B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 12A . 
           [0029]      FIG. 13A  is the sixth variation of diode interconnection schematic diagram with 65 emitting cells in series mode according to the embodiment of the present invention.  FIG. 13B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 13A . The AC device&#39;s operating voltage is between 210 and 220V AC. 
           [0030]      FIG. 14A  is the seventh variation of the diode interconnection schematic diagram with 6 light emitting cells in each of the rectifier branch, and a total of 38 light emitting cells in series mode according to the embodiment of the present invention.  FIG. 148  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 14A . The AC device&#39;s operating voltage is between 110 and 120V AC, or two of them in series mode for 220 and 240V AC sources. 
           [0031]      FIG. 15A  is the eighth variation of the diode interconnection schematic diagram with 6 light emitting cells in each of the rectifier branches, and a total of 33 light emitting cells in series mode according to the embodiment of the present invention.  FIG. 158  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 15A . The AC device&#39;s operating voltage is between 110 and 120V AC, or two of them in series mode for 220 and 240V AC source. 
           [0032]      FIG. 16A-16E  are cross sectional view of the fabrication process flow of AC LED device according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    The embodiments of the present invention disclose the structure of the alternating current light emitting diodes and the associated manufacturing process flow in details with reference to the accompanying drawings. The present invention is not limited to the embodiments but may be implemented into other forms. The following embodiments are provided only for illustrative purposes so that those skilled in the art can fully understand the scope of the present invention. Throughout the drawings, the width, length, thickness and the like of elements may be exaggerated for convenience of illustration. Like components are designated by like reference numerals throughout the specification and drawings. 
         [0034]      FIG. 1A  illustrates a prior art of a single high voltage AC LED chip  101  connected to a high voltage AC power supply  102 . As shown, the single high voltage AC LED chip  101  consists of two arrays of series connected individual smaller LED cells in opposite polarity. The two arrays of series connected individual LEDs are then connected to the positive and negative electrodes of the high voltage AC power source  102 .  FIG. 1B  illustrates an improvement of the art with addition of an optional resistor in series mode with the single chip high voltage AC LED device  101  and another optional capacitor in parallel mode. The function of the resistor or the capacitor is to protect the high voltage AC LED device against high voltage or high current spikes during operation after they are connected to the high voltage AC power supply  102 . However, the single high voltage AC LED  101  device has a few drawbacks. One is the inefficient use of smaller LED cells, since only half of the LED cells are energized at any given moment, with the other half being negative biased and not being energized to generate light. The other drawback is that the voltage sweep is only effective about 65% of the time to turn on LED cells during positive voltage cycle when the effective AC voltage is similar to or the same as each individual LED cell&#39;s operating voltage. 
         [0035]      FIG. 2  illustrates a cross sectional view of a prior art of a standard lateral LED chip structures. The light emitting cell comprises a substrate  120 ; a buffer layer  130 ; an n-type semiconductor layer  140 ; an active layer  150  consisting of alternating quantum barrier layer  150   a  and quantum well layer  150   b  multiple times; an electron block layer  160 ; and a p-type semiconductor layer  170 ; an n-type bonding pad  190  formed on the exposed region of the n-type semiconductor layer  140 ; and a p-type bonding pad  180  formed on the p-type semiconductor layer  170 ; a metal layer and a Distributed Bragg Reflector (DBR) layer  110  on the other side of the substrate  120  to improve light extraction of the light emitting cells. 
         [0036]    In order to reduce operating voltage and further improve device performance, an ohm-contact metal alloy layer (not shown in  FIG. 2 ) is provided below both the n-type bonding pad and the p-type bonding pad. The metal alloy layer for n-contact has lower work function, while the layer for p-contact has higher work function than the underneath GaN layer. Moreover, a transparent and conductive layer may be added between the p-type semiconductor layer  170  and the p-type boding pad  180  (not shown in  FIG. 2 ) to reduce p-type semiconductor contact resistance and to reduce LED cell&#39;s operating voltage. 
         [0037]    The LED cell is energized and the light is generated when bonding wire  200  connects p-type semiconductor  170  through p-type bonding pad  180  to the positive electrode of the power source, while bonding wire  210  connects n-type semiconductor  140  through n-type bonding pad  190  to the negative electrode of the power source. The lateral chip manufacturing process flow requires fewer process steps, but the p-type bonding pad consumes effective light emitting area, generates non-uniform light output, and reduces light output efficiency. 
         [0038]      FIG. 3  illustrates a cross sectional view of a flip-chip light emitting cell structure according to the embodiment of the present invention. 
         [0039]    Referring to  FIG. 3 , the light emitting cell comprises a transparent substrate  120  such as sapphire, a low temperature undoped GaN buffer layer  130 , a silicon doped n-type GaN layer  140 , a stress release super lattice layer  145  which consists of alternating thin layers of undoped InGaN  145   a  and undoped GaN  145   b  multiple times, an active layer  150  which consists of alternating undoped InGaN quantum well layer  150   a  and undoped GaN quantum barrier layer  150   b  multiple times, an electron block layer  160  which consists of magnesium doped p-type AlGaN, a magnesium doped p-type GaN layer  170 , a p-type metal contact layer  180  formed on the p-type GaN layer  170 , and an n-type metal contact layer  190  formed on the exposed region of the n-type GaN layer  140 . The p-type metal contact layer and the n-type metal contact layer are isolated by layer  185 , which is made from silicon oxide or silicon nitride or a combination of silicon oxide and nitride. The isolation between adjacent light emitting cells is achieved through complete removal of materials between them through dry or wet etching and then filling with silicon oxide or silicon nitride or a combination of both silicon oxide and silicon nitride layer  195 . The light emitting device surface is then covered with passivation silicon oxide or silicon nitride layer  220 . 
         [0040]    The p-type and n-type metal contact layers are made from tungsten, aluminum or copper. A diffusion barrier made from titanium or tantalum nitride, nickel titanium or titanium tungsten alloy (not shown in  FIG. 3 ) separates the p-type and n-type metal contact layers from the isolation oxide or nitride layers. The diffusion barrier also provides initial contact to the p-type GaN and n-type GaN layers. For operating voltage reduction and further performance improvement, a thin layer of chromium or nickel layer is deposited between diffusion barrier and the p-type and n-type metal contact layers (not shown in  FIG. 3 ), respectively. Moreover, a transparent and conductive layer made from indium tin oxide may be provided between the p-type GaN layer  170  and the p-type metal contact layer  180  (not shown in  FIG. 2 ) to reduce p-type GaN contact resistance and LED cell&#39;s operating voltage. The positive electrode of the light emitting cell is connected through the opening  200  on the passivation layer and the negative electrode is connected through the opening  210 . The LED cell is energized and light is generated when p-type GaN  170  through p-type metal contact layer  180  is connected to the positive electrode of the power source, while n-type GaN  140  through n-type metal contact layer  190  is connected to the negative electrode of the power source. 
         [0041]      FIG. 4A  is a top view of one type of light emitting cells according to the embodiment of the present invention.  FIG. 46  is a top view of another type of light emitting cells according to the embodiment of the present invention. The light emitting cells in  FIG. 4A  and  FIG. 4B  have the same area of p-type contact  180  and n-type contact  190 . The layer  185  isolates n-type contact metal  190  from the stack of the p-type semiconductor GaN layer, the electron block AlGaN layer, the active multiple quantum well (MOW) layer, and the optional stress release super lattice layer which is not shown in  FIG. 4A  and  FIG. 46 . The light emitting cell is separated from its adjacent cells through isolation layer  195 .  FIG. 4C  and  FIG. 4D , having the same area of p-type and n-type metal contact, illustrated another variation of the device cell design from the embodiment of the present invention. The n-contact metal layer in  FIG. 4C  has the shape of an “L”, instead of the square as illustrated in  FIG. 4A . Square-shaped light emitting cells are used here for illustration purposes only. Other cell shapes include triangles, rectangles, diamonds and other parallelograms. In the following drawings, only the unit cells from  FIG. 4A  and  FIG. 46  are used for illustration purposes. However, the cell layout is not limited to the embodiments described here but may be implemented in other forms by those skilled in the art to meet light emitting cell&#39;s monolithic interconnection requirement. 
         [0042]      FIG. 5  is a cross sectional view illustrating an AC light-emitting device with light emitting cells monolithically connected in series mode according to the embodiment of the present invention. 
         [0043]    Referring to  FIG. 5 , the AC light emitting device of the present invention has a plurality of light emitting cells  100 - 1  to  100 - n  connected to one another in series mode. The p-Page type electric contact  180  of the light emitting cell  100 - 1  is connected to the positive electrode of the external AC power sources through metal bump  200 , which itself is pre-formed and attached to the light emitting cell before the assembly process, and the specially designed positive electrode contact on the submount carrier of the AC light emitting device (not shown in  FIG. 5 ). The n-type electric contact  190  of the light emitting cell  100 - 1  is connected to the p-type electric contact  180  of the adjacent light emitting cell  100 - 2 , until the n-type electric contact  190  of the light emitting cell  100 - n −1 is connected to the p-type electric contact  180  of the last light emitting cell  100 - n  for a total of n cells connected in series mode. The n-type electric contact  190  of the light emitting cell  100 - n  is then connected to the negative electrode of the external AC power sources through metal bump  210 , which is pre-formed and attached to the light emitting cell, and the specially designed negative electrode contact on the submount carrier of the AC light emitting device (not shown in  FIG. 5 ). 
         [0044]    As shown in  FIG. 5 , each light emitting cell comprises a transparent substrate  120 , a low temperature GaN buffer layer  130 , a n-type GaN layer  140 , an optional stress release super lattice layer  145 , an active multiple quantum well (MQW) layer  150 , an electron block layer  160 , a p-type GaN layer  170 . The p-type metal contact layer  180  is formed on the p-type GaN layer  170 , and an n-type metal contact layer  190  is formed on the exposed region of the n-type GaN layer  140 . The p-type metal contact layer and the n-type metal contact layer are separated by isolation layer  185 . The separation of adjacent light emitting cells is through another isolation layer  195 . The light emitting device surface is then covered with passivation layer  220 . The positive and negative electric contacts are formed by etching through the passivation layer, through which electric contacts are produced using p-type metal contact bump  200  and n-type metal contact bump  210 . 
         [0045]      FIG. 6A  illustrates a diode interconnection schematic diagram of four light emitting cells formed into the shape of a square or rectangle in the AC LED device. The AC light emitting device has four rectifier branches C 1  to C 4 , each branch consisting of one light emitting cell. The equivalent circuit diagram is similar to the AC light emitting chip illustrated in  FIG. 1 .  FIG. 66  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 6A . 
         [0046]    Referring to  FIG. 6B , each light emitting cell has a positive metal contact  180  and a negative metal contact  190 . The negative electrodes  190  of C 1  and C 3  are connected through a negative-to-negative bridge  260 . The positive electrodes  180  of C 2  and C 4  are connected through a positive-to-positive bridge  250 . The positive-to-negative bridge  240  is used to connect the positive metal contact of C 1  to the negative contact of C 4 . Another bridge  240  (not labeled in  FIG. 68 ) is used to connect the positive metal contact of C 3  to the negative contact of C 2 . In the flip chip design, the bridge  250  and  260  are connected monolithically during wafer fabrication process. Both  250  and  260  are also connected together for the four-cell AC light emitting device. The positive electrode of the external AC voltage source is connected to the positive metal contact  200  of C 1  and the negative electrode of the external AC voltage source is connected to the negative metal contact  210  of C 2  through a positive to negative bridge  240 . 
         [0047]    Due to the flip chip design, the negative metal contact of cell C 2  is electrically connected to the positive metal contact of cell C 3 , making it possible for all four light emitting cells in the AC LED device to have the same structure and the same light emitting area. Each of the two light emitting cells C 1  and C 2  or C 3  and C 4  in the AC LED device are connected to each other in series mode. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 5.8 and 7V. During LED operation, only two of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other two are negatively biased and do not generate any light. The light emitting cell utilization rate is exactly 50%, make this prior art the most expensive AC light emitting chip design. 
         [0048]      FIG. 7A  illustrates the diode interconnection schematic diagram of nine light emitting cells formed into the shape of a square or rectangle in the AC LED device with five light emitting cells in series mode according to an embodiment of the present invention. FIG.  7 B is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 7A . 
         [0049]    Referring to  FIG. 7A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of two light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell, and a single light emitting cell L 1 . 
         [0050]    Referring to  FIG. 7B , each light emitting cell has a positive metal contact  180  and a negative metal contact  190 . The positive-to-negative bridge  240  is used to connect the positive metal contact of one cell to the negative contact of an adjacent cell in each rectifier branch. The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the negative-to-negative bridge  260 , which is then connected to the positive contact of the light emitting cell L 1 . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through a positive-to-positive bridge  250 , which is then connected to the negative contact of the light emitting cell L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All nine light emitting cells in the AC LED device have the same structure and the same light emitting area. The contacts  220  and  230  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell L 1 . 
         [0051]    Since five light emitting cells are connected in series mode in the AC LED device, for a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 14.5 and 17.5V. During LED operation, five of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other four are negatively biased and do not generate any light. The light emitting cell utilization rate of present invention is therefore 55.6%, make this layout design better then the prior art, as illustrated in  FIG. 6A  and  FIG. 6B . 
         [0052]      FIG. 8A  illustrates the diode interconnection schematic diagram of sixteen light emitting cells formed into the shape of a square or rectangle in the AC LED device with ten light emitting cells in series mode according to another embodiment of the present invention.  FIG. 8B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 8A . 
         [0053]    Referring to  FIG. 8A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of three light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell, and an array of four light emitting cells L 1  connected in series mode. 
         [0054]    Referring to  FIG. 8B , each light emitting cell has a positive contact  180  and a negative contact  190 . The positive-to-negative bridge  240  is used to connect the positive contact of one cell to the negative contact of an adjacent cell in each rectifier branch C 1  to C 4  and in the fifth cell array L 1 . The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the negative-to-negative bridge  260 , which is then connected to the positive contact of the first light emitting cell in array L 1 . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through a positive-to-positive bridge  250 , which is then connected to the negative contact of the last light emitting cell in array L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All sixteen light emitting cells in the AC LED device have the same structure and the same light emitting area. The contacts  220  on the positive electrode of the first cell in the array L 1  and the contact  230  on the positive electrode of the first cell of branch C 2  or C 4  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell array L 1 . 
         [0055]    Ten light emitting cells are connected in series mode in the AC LED device. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of device is between 29 and 35V. During LED operation, ten of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other six are negatively biased and do not generate any light. The light emitting cell utilization rate is 62.5% for the layout design of the invention illustrated in  FIG. 8A  and  FIG. 8B . 
         [0056]      FIG. 9A  illustrates the diode interconnection schematic diagram of twenty-five light emitting cells formed into the shape of a square or rectangle in the AC LED device with seventeen light emitting cells in series mode according to another embodiment of the present invention.  FIG. 9B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 9A . 
         [0057]    Referring to  FIG. 9A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of four light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell, and an array of nine light emitting cells L 1  in series mode. 
         [0058]    Referring to  FIG. 9B , each light emitting cell has a positive contact  180  and a negative contact  190 . The positive-to-negative bridge  240  is used to connect the positive contact of one cell to the negative contact of an adjacent cell in each rectifier branch C 1  to C 4  and in the fifth cell array L 1 . The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the negative-to-negative bridge  260 , which is then connected to the positive contact of the first light emitting cell in array L 1 . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through a positive-to-positive bridge  250 , which is then connected to the negative contact of the last light emitting cell in array L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All twenty-five light emitting cells in the AC LED device have the same structure and the same light emitting area. The contacts  220  on the positive electrode of the first cell in the array L 1  and the contact  230  on the positive electrode of the first cell of branch C 2  or C 4  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell array L 1 . 
         [0059]    Seventeen light emitting cells are connected in series mode in the AC LED device. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 49.3 and 59.5V. Two AC light emitting units in series mode are suitable for AC line voltage between 110 and 120V. Four AC light emitting units in series mode are suitable for AC line voltage between 220 and 240V. During LED operation, seventeen of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other eight are negatively biased and do not generate any light. The light emitting cell utilization rate is 68% for the layout design of the invention illustrated in  FIG. 9A  and  FIG. 9B . 
         [0060]      FIG. 10A  illustrates the diode interconnection schematic diagram of thirty-six light emitting cells formed into the shape of a square or rectangle in the AC LED device with twenty-six light emitting cells in series mode according to another embodiment of the present invention.  FIG. 108  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 10A . 
         [0061]    Referring to  FIG. 10A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of five light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell, and an array of sixteen light emitting cells L 1  connected in series mode. 
         [0062]    Referring to  FIG. 10B , each light emitting cell has a positive contact  180  and a negative contact  190 . The positive-to-negative bridge  240  is used to connect the positive contact of one cell to the negative contact of an adjacent cell in each rectifier branch C 1  to C 4  and in the fifth cell array L 1 . The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the negative-to-negative bridge  260 , which is then connected to the positive contact of the first light emitting cell in array L 1 . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through a positive-to-positive bridge  250 , which is then connected to the negative contact of the last light emitting cell in array L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All thirty-six light emitting cells in the AC LED device have the same structure and the same light emitting area. The contacts  220  on the positive electrode of the first cell in the array L 1  and the contact  230  on the positive electrode of the first cell of branch C 2  or C 4  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell array L 1 . 
         [0063]    Twenty-six light emitting cells are connected in series mode in the AC LED device. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 75.4 and 91V. During LED operation, twenty-six of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other ten are negatively biased and do not generate any light. The light emitting cell utilization rate is 72.2% for the layout design of the invention illustrated in  FIG. 10A  and  FIG. 10B . 
         [0064]      FIG. 11A  illustrates the diode interconnection schematic diagram of forty-nine light emitting cells formed into the shape of a square or rectangle in the AC LED device with thirty-seven light emitting cells in series mode according to another embodiment of the present invention.  FIG. 11B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 11A . 
         [0065]    Referring to  FIG. 11A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of six light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell, and an array of twenty-five light emitting cells L 1  connected in series mode. 
         [0066]    Referring to  FIG. 11B , each light emitting cell has a positive contact  180  and a negative contact  190 . The positive-to-negative bridge  240  is used to connect the positive contact of one cell to the negative contact of an adjacent cell in each rectifier branch C 1  to C 4  and in the fifth cell array L 1 . The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the negative-to-negative bridge  260 , which is then connected to the positive contact of the first light emitting cell in array L 1 . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through a positive-to-positive bridge  250 , which is then connected to the negative contact of the last light emitting cell in array L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All forty-nine light emitting cells in the AC LED device have the same structure and the same light emitting area. The contacts  220  on the positive electrode of the first cell in the array L 1  and the contact  230  on the positive electrode of the first cell of branch C 2  or C 4  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell array L 1 . 
         [0067]    Thirty-seven light emitting cells are connected in series mode in the AC LED device. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 107.3 and 129.5V. One AC light emitting unit is suitable for AC line voltage between 110 and 120V. Two AC light emitting units in series mode are suitable for AC line voltage between 220 and 240V. During LED operation, thirty-seven of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other twelve are negatively biased and do not generate any light. The light emitting cell utilization rate is 75.5% for the layout design of the invention illustrated in  FIG. 11A  and  FIG. 11B . 
         [0068]      FIG. 12A  illustrates the diode interconnection schematic diagram of sixty-four light emitting cells formed into the shape of a square or rectangle in the AC LED device with fifty light emitting cells in series mode according to another embodiment of the present invention.  FIG. 12B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 12A . 
         [0069]    Referring to  FIG. 12A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of seven light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell, and an array of thirty-six light emitting cells L 1  connected in series mode. 
         [0070]    Referring to  FIG. 12B , each light emitting cell has a positive contact  180  and a negative contact  190 . The positive-to-negative bridge  240  is used to connect the positive contact of one cell to the negative contact of an adjacent cell in each rectifier branch C 1  to C 4  and in the fifth cell array L 1 . The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the negative-to-negative bridge  260 , which is then connected to the positive contact of the first light emitting cell in array L 1 . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through a positive-to-positive bridge  250 , which is then connected to the negative contact of the last light emitting cell in array L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All sixty-four light emitting cells in the AC LED device have the same structure and the same light emitting area. The contacts  220  on the positive electrode of the first cell in the array L 1  and the contact  230  on the positive electrode of the first cell of branch C 2  or C 4  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell array L 1 . 
         [0071]    Fifty light emitting cells are connected in series mode in the AC LED device. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 145 and 175V. During LED operation, fifty of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other fourteen are negatively biased and do not generate any light. The light emitting cell utilization rate is 78.1% for the layout design of the invention illustrated in  FIG. 12A  and  FIG. 12B . 
         [0072]      FIG. 13A  illustrates the diode interconnection schematic diagram of eighty-one light emitting cells formed into a square or rectangle in the AC LED device with sixty-five light emitting cells in series mode according to another embodiment of the present invention. 
         [0073]      FIG. 138  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 13A . 
         [0074]    Referring to  FIG. 13A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of eight light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell and an array of forty-nine light emitting cells L 1  connected in series mode. 
         [0075]    Referring to  FIG. 13B , each light emitting cell has a positive contact  180  and a negative contact  190 . The positive-to-negative bridge  240  is used to connect the positive contact of one cell to the negative contact of an adjacent cell in each rectifier branch C 1  to C 4  and in the fifth cell array L 1 . The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the negative-to-negative bridge  260 , which is then connected to the positive contact of the first light emitting cell in array L 1 . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through a positive-to-positive bridge  250 , which is then connected to the negative contact of the last light emitting cell in array L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All eighty-one light emitting cells in the AC LED device have the same structure and the same light emitting area. The contacts  220  on the positive electrode of the first cell in the array L 1  and the contact  230  on the positive electrode of the first cell of branch C 2  or C 4  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell array L 1 . 
         [0076]    Sixty-five light emitting cells are connected in series mode in the AC LED device. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 188.5 and 227.5V. One AC light emitting device is suitable for 220V AC line voltage. During LED operation, sixty-five of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other sixteen are negatively biased and do not generate any light. The light emitting cell utilization rate is 80.2% for the layout design of the invention illustrated in  FIG. 13A  and  FIG. 13B . 
         [0077]      FIG. 14A  illustrates the diode interconnection schematic diagram of fifty light emitting cells formed into the shape a rectangle in the AC LED device with thirty-eight light emitting cells in series mode according to another embodiment of the present invention.  FIG. 14B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 14A . 
         [0078]    Referring to  FIG. 14A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of six light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell, and an array of twenty-six light emitting cells L 1  connected in series mode. 
         [0079]    Referring to  FIG. 14B , each light emitting cell has a positive contact  180  and a negative contact  190 . The positive-to-negative bridge  240  is used to connect the positive contact of one cell to the negative contact of an adjacent cell in each rectifier branch C 1  to C 4  and in the fifth cell array L 1 . The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the positive metal contact of the first cell in array L 1 . The last cell in array L 1  is different from the other cells in the AC device and has the n-contact shape of the cell illustrated in  FIG. 4B . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through the n-type GaN of the last cell in array L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All fifty light emitting cells in the AC LED device have the same light emitting area and one of them has a different n-type contact structure. The contacts  220  on the positive electrode of the first cell in the array L 1  and the contact  230  on the positive electrode of the first cell of branch C 2  or C 4  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell array L 1 . 
         [0080]    Thirty-eight light emitting cells are connected in series mode in the AC LED device. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 110.2 and 133V. One AC light emitting unit is suitable for AC line voltage between 110 and 120V. Two AC light emitting units in series mode are suitable for AC line voltage between 220 and 240V. During LED operation, thirty-eight of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other twelve are negatively biased and do not generate any light. The light emitting cell utilization rate is 76% for the layout design of the invention illustrated in  FIG. 14A  and  FIG. 14B . 
         [0081]      FIG. 15A  illustrates the diode interconnection schematic diagram of forty-five light emitting cells formed into a rectangle in the AC LED device with thirty-three light emitting cells in series mode according to another embodiment of the present invention.  FIG. 15B  is an illustration of a plurality of structures of AC LED cells connected in a matrix form according to the equivalent circuit diagram of  FIG. 15A . 
         [0082]    Referring to  FIG. 15A , the AC light emitting device has four rectifier branches C 1  to C 4 , each consisting of six light emitting cells connected in series mode with one cell&#39;s positive contact being connected to the negative contact of the other cell, and an array of twenty-one light emitting cells L 1  connected in series mode. 
         [0083]    Referring to  FIG. 15B , each light emitting cell has a positive contact  180  and a negative contact  190 . The positive-to-negative bridge  240  is used to connect the positive contact of one cell to the negative contact of an adjacent cell in each rectifier branch C 1  to C 4  and in the fifth cell array L 1 . The negative contact of the last cell in branch C 1  is connected to the negative contact of the last cell in branch C 3  through the negative-to-negative bridge  260 , which is then connected to the positive contact of the first light emitting cell in array L 1 . The positive contact of the first cell in branch C 2  is connected to the positive contact of the first cell in branch C 4  through a positive-to-positive bridge  250 , which is then connected to the negative contact of the last light emitting cell in array L 1 . The positive electrode of the external AC voltage source is connected to the positive contact  200  of the first light emitting cell in the rectifier branch C 1 , and the negative electrode of the external AC voltage source is connected to the negative contact  210  of the last cell in the rectifier branch C 2 , which is electrically connected to the positive contact of the first cell in the rectifier branch C 3  through a positive-to-negative bridge  240 . All forty-five light emitting cells in the AC LED device have the same structure and the same light emitting area. The contacts  220  on the positive electrode of the first cell in the array L 1  and the contact  230  on the positive electrode of the first cell of branch C 2  or C 4  provide electric connection for an optional capacitor to reduce voltage ripples across light emitting cell array L 1 . 
         [0084]    Thirty-three light emitting cells are connected in series mode in the AC LED device. For a light emitting cell with an operating voltage between 2.9 and 3.5V, the operating AC voltage of the device is between 95.7 and 115.5V. One AC light emitting unit is suitable for AC line voltage between 110 and 120V. Two AC light emitting units in series mode are suitable for AC line voltage between 220 and 240V. During LED operation, thirty-three of the light emitting cells are energized by the positive voltage sweep of the AC voltage source, while the other twelve are negatively biased and do not generate any light. The light emitting cell utilization rate is 73.3% for the layout design of the invention illustrated in  FIG. 15A  and  FIG. 15B . 
         [0085]    The AC LED cell layouts are not limited to the aforementioned embodiments, but may be implemented in other forms or used with a combination of the layouts in  FIG. 6  to  FIG. 15  by those skilled in the art to match light emitting cell&#39;s operating voltage to the external AC power voltage. 
         [0086]    The present invention further provides a new fabrication process flow for the AC light emitting device as shown in  FIG. 16A  through  FIG. 16E . To correspond with the aforementioned embodiments, the fabrication method thereof is exemplified by the drawing to illustrate the fabrication process of the AC light emitting device based on GaN on transparent sapphire substrate. 
         [0087]    Referring to  FIG. 16A , the fabrication method comprises the steps of preparing the substrate  120  and forming a low temperature undoped GaN buffer layer  130 , a silicon doped n-type GaN layer  140 , a stress release super lattice layer  145 , an active layer  150 , a magnesium doped p-type AlGaN layer  160 , and a magnesium doped p-type GaN layer  170 , wherein the active layer  150  is composed of multiple undoped InGaN well layer and GaN barrier layer stacked on top of each other and located between the n-type GaN and p-type AlGaN and GaN layers. 
         [0088]    A plurality of openings are then formed to isolate light emitting cells from their neighbors through the first photo-lithography patterning and dry or wet etch process to remove GaN and other layers on the substrate so the substrate is exposed without residual as-deposited materials. The n-type GaN surface is then partially exposed through the second photo-lithography patterning and dry or wet etch process to remove all the stack of layers above n-type GaN layer and also some of the n-type GaN layer. The final cross sectional view of the wafer and layer stack is shown in  FIG. 16B . 
         [0089]    The substrate with a plurality of openings are then covered with a layer of insulation and protective materials, consisting of dielectrics, such as silicon oxide or silicon nitride layers or a combination of silicon oxide and silicon nitride layers, to prevent leakage of current between n-type and p-type GaN layers and also to stop current leakage between adjacent light emitting cells. Subsequently, both p-type and n-type GaN surface are exposed through the third lithography patterning and dry or wet etch of the protective layer process, as shown in  FIG. 16C . The isolation layer  195  is formed on the side wall of the light emitting cell with exposed substrate surface to isolate each light emitting cell from its adjacent cells. The isolation layer  185  is formed on the side wall of the light emitting cell with exposed n-type GaN layer to prevent n-type electrode metal to short all other layers located above the n-type GaN layer. 
         [0090]    After the isolation layer  185  and  195  are patterned and prepared, a thick layer of conductive materials, being a stack of metal layers made of, such as tungsten, titanium, or aluminum, are then deposited through sputtering of the metal target, or evaporation of individual metals or wet solution plating, or a combination of the techniques aforementioned, in order to establish electric connection from negative electrode of one light emitting cell to the positive electrode of another cell, and also to provide p-type and n-type electric connection to external power sources. The as-deposited metal layer then undergo the fourth lithography patterning and dry etch process for the individual light emitting cells to be connected and isolated according to the pre-designed AC light emitting device layout of the aforementioned embodiments of the present invention. As shown in  FIG. 16D , the etch of metal layer down to isolation layer  185  separates one light emitting cell from its adjacent cells so the p-type metal electrode  180  and n-type electrode  190  are formed, while the metal layer above isolation layer  195  is not etched. It acts as series connection bridge  240  for the electric connection of positive electrode of one cell to the negative electrode of another cell, and establishes a chain of series connected light emitting cells. 
         [0091]    As shown in  FIG. 16E , the light emitting device surface is then covered with a layer of passivation silicon oxide or silicon nitride layer  220 , followed by the fifth lithography patterning and dry or wet etch process to open the passivation layer for both positive and negative electrode connection to the external AC voltage source. Lastly, the wafer goes through grinding, polishing, and breaking process to form individual AC light emitting device, which will be ready for packaging process. 
         [0092]    In summary of the aforementioned embodiments, the AC light emitting device disclosed by the present invention is characterized in that each device has light emitting cells forming four rectifier bridges with light emitting cells in each branch connected in series mode for operations under high voltage external power sources, and a fifth positive biased cell arrays in series mode to generate light during both positive and negative cycles of external AC voltage sweep. Each AC light emitting device takes the shape of a square or a rectangle and is formed from the stacked GaN and other active layers on the transparent sapphire substrate through etching and deposition to isolate and interconnect light emitting cells according the appropriate layout design. Furthermore, the application of the layout design disclosed by the present invention can be modified by those skilled to meet the requirement of different external AC voltages. 
         [0093]    The present invention has been described and illustrated using the exemplary preferred embodiments. All modifications and variations of equal functions without violating the principle and technology of the present invention should be included in the scope of the claims as described below.