Abstract:
A single-chip integrated LED particularly adapted for direct use with a high voltage AC power comprises a plurality of series-connected LEDs arranged in two arrays and flip chip bonded to a transparent substrate. The opposite polarities of the arrays are connected together and then connected to the AC power source. During the positive half of the AC cycle, one array of LEDs is forward biased and energized, while the other array is reverse biased. During the negative half of the AC cycle, the other array of LEDs is forward biased and thus energized, while the first array is reverse biased and thus not energized. The arrays are alternately energized and de-energized at the frequency of the AC power source, and thus the single-chip integrated LED always appears to be energized.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation-in-part application of Ser. No. 10/279,296, filed Oct. 24, 2002, which is hereby incorporated into the present application by reference.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to single-chip light emitting diodes (LED), and more particularly to single-chip LEDs which can operate under standard alternating-current (AC) high voltage (110 V, 220 V, etc.) conditions for various applications, including general lighting.  
         [0003]     LEDs are used in displays, as indicator lights, for traffic lights, for communications, and for optical interconnects. With the realization of high brightness blue/green and violet LEDs made from the III-nitride semiconductor family InN, GaN, AlN and their alloys recently it is now possible that LEDs may be used for general lighting applications in residential houses and commercial buildings. LEDs have already found niche applications in the area of lighting, such as passenger side reading lights in vehicles. Because of the potential energy, environment and national security benefits, there is increasing national interest in creating a partnership—of industry, universities and national laboratories—aimed at accelerating the development of Solid-State Lighting science and technology. A nation-wide program called “Next-Generation Lighting Initiative” has been lunched by the Department of Energy (DOE).  
         [0004]     Several methods have been proposed and employed to achieve white light emission from LEDs. The first and the only commercial product is to use blue LEDs made of III-nitrides to generate white light by coating the blue LED chips with yellow phosphors. Phosphors down convert part of the shorter wavelength blue light to a yellow wavelength visible yellow light. Through color mixing, the eye sees white when two colors are properly balanced. Other proposed method includes using UV LEDs (more efficient sources) to pump three-color phosphors (red, blue, green, RBG) or to combine three color (RBG) LEDs to get white emission.  
         [0005]     Currently, all semiconductor LEDs are DC operated with typical operating voltages of a few volts (e.g., around 2 volts for Red LEDs and around 3.5 volts for blue LEDs). However, substantially all the houses and buildings in North America are wired with AC (60 Hz) 110 volts power sources. One way to use LEDs for general lighting applications is to convert AC 110 V to DC with a low voltage. This requires the use of power converters, which may be installed separately or built into the LED package. This approach has been utilized in LED traffic signal lights. Use of power converters have disadvantages such as added volume, added costs, and low efficiency, for example.  
         [0006]     There is also a method for achieving AC operation of LEDs by wiring two discrete LEDs connected opposite of one another (the cathode of one goes to the anode of the other). When the LEDs are connected to a low voltage AC circuit, both LEDs glow alternately; one LED is biased by positive voltage side of the AC cycle (forward biased), and the other LED is biased by the negative voltage side of the AC cycle (reverse biased). Since the AC source usually runs at 60 Hz both LEDs appear to be always on to the naked eye. However, there are no new technologies involved in this type of “AC-LEDs” by ganging together a strand of LEDs and they are not suitable for lighting applications. To achieve high voltage AC operations, one needs to connect a few dozens of LEDs in a similar fashion. Hence it would not be viable economically or physically to replace an incandescent lamp by a strand of discrete of LEDs.  
         [0007]     A need remains in the art for single-chip LEDs for standard high AC voltage (110 volts or 220 volts) operations. A need also remains in the art for integrated semiconductors optical components on a single chip; in this case it involves the integration of many LEDs.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a single-chip LED device through the use of integrated circuit technology, which can be used for standard high AC voltage (110 volts for North America, and 220 volts for Europe, Asia, etc.) operation. The single-chip AC LED device integrates many smaller LEDs, which are connected in series. The integration is done during the LED fabrication process and the final product is a single-chip device that can be plugged directly into house or building power outlets or directly screwed into incandescent lamp sockets that are powered by standard AC voltages.  
         [0009]     The series connected smaller LEDs are patterned by photolithography, etching (such as plasma dry etching), and metallization on a single chip. The electrical insulation between small LEDs within a single-chip is achieved by etching light emitting materials into the insulating substrate so that no light emitting material is present between small LEDs. The voltage crossing each one of the small LEDs is about the same as that in a conventional DC operating LED fabricated from the same type of material (e.g., about 3.5 volts for blue LEDs).  
         [0010]     The single-chip AC LED device is formed by depositing layers of n-type semiconductor material, optically active layers and p-type semiconductor material in succession on an insulating transparent substrate. The chip is then flipped to be bonded to a submount and light is extracted from the top transparent substrate side. By flip-chip bonding, the AC LED device has more light extraction from the transparent substrate side without any light blocking by the contacts and interconnection metals. Flip-chip bonding the AC-LED on a highly thermal-conductive submount will also enhance the heat transferred away from the LED active region to the submount and then dissipated in the environment.  
         [0011]     To account for the difference between the AC and DC current, two columns of series-connected mini-LEDs are wired in opposite polarities. At one instant, all the mini-LEDs in one of the columns are forward biased and hence are all turned-on, while the mini-LEDs in the other column are all reverse biased and hence are all turned off. However, the AC current turns on and off these two columns alternately. Since the frequency of AC power supply is 60 Hz (or 50 Hz) all these small LEDs within the single-chip appear to be on all the time to the naked eye.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a diagrammatic plan view of a single-chip high voltage AC LED device of the present invention.  
         [0013]      FIG. 2  is an equivalent circuit of the single-chip high voltage AC LED device of  FIG. 1 .  
         [0014]      FIG. 3  is a diagrammatic cross sectional illustration showing integration between adjacent LED elements within the single-chip high voltage AC LED device using standard mounting.  
         [0015]      FIG. 4  is a diagrammatic cross sectional illustration showing the single-chip high voltage AC LED device mounted using a flip-chip bonding method.  
     
    
     DETAILED DESCRIPTION  
       [0016]     Referring to  FIG. 1 , a single-chip high voltage AC LED device  10  is illustrated connected to a high voltage AC power supply  12 . As shown, the single-chip high voltage AC LED device  10  effectively consists of two arrays of series-connected individual smaller LEDs  14 . The two arrays of series-connected individual LEDs are then connected to the opposite polarities of the high voltage AC power source  12 . Each LED array could be made into one or many columns to fit the desired geometrical shape of the single-chip high voltage LED. In  FIG. 1 , each array consists of two columns for illustration.  
         [0017]     Referring to  FIGS. 1 and 2 , the first array  16  of the single-chip AC LED device  10  provides a number of series-connected smaller LEDs  14 . The cathode of one LED  14  is connected to the anode of the next LED to form the array  16 . The array  16  of LEDs  14  presents a positive terminal corresponding to the cathode of the last LED (shown at the bottom of the first column of array  16  in  FIG. 1  and shown at the top of array  16  in  FIG. 2 ), and a negative terminal corresponding to the anode of the first LED (shown at the bottom of the second column of array  16  in  FIG. 1  and shown at the bottom of the array  16  in  FIG. 2 ). The second array  18  of the single-chip AC LED device  10  provides an equal number of series-connected smaller LEDs  14 . The cathode of one LED  14  is connected to the anode of the next LED to form the array  18 . The array  18  of LEDs  14  also presents positive and negative terminals, which are connected to the opposite terminal of array  16 . When the AC cycle is positive, the LEDs  14  of array  16  are forward biased and thus energized. At the same time, the LEDs  14  of array  18  are reverse biased, and hence turned off. When the AC cycle is negative, the LEDs  14  of array  16  are reverse biased and hence turned off, while the LEDs  14  of array  18  are forward biased and thus turned on.  
         [0018]     The arrays  16  and  18  are connected to different polarities of the AC power source for high voltage AC operation. The arrays  16  and  18  of smaller LEDs  14  are alternatively turned on and off at a rate corresponding to the frequency of the AC source. Common frequencies for public utilities are 60 Hz or 50 Hz, for example. Thus for a 60 Hz AC power source, arrays  16  and  18  are alternatively energized at a 60 Hz rate. In this manner, to the naked eye, the single-chip high voltage AC LED device  10  always appears to be on.  
         [0019]     The number of series-connected smaller LEDs  14  in each array  16  and  18  depends on the operating voltage of the individual LEDs  14 . The operating voltage of an LED depends on the type of the LED, which is around 2 volts for red LED and around 3.5 volts for blue LEDs. The typical variation in the operating voltage among individual smaller LEDs may be approximately 0.1-0.3 V depending on the type and manufacturer of the LED. For example, using LEDs having a typical operating voltage of 4.0 volts, the number of the LEDs “n” in each array  16  and  18  is approximately 28 for a 110 volt AC power source  12 .  
         [0020]     For a 220 volt AC application, as is commonly used in European and Asian countries, for example, approximately 55 LEDs would be integrated into each array. Thus, the number of LEDs is dependent on the voltage characteristics of the LEDs used or formed on the single-chip, and the application voltage of 110 volts AC or 220 volts AC. For a forward voltage of 3.5 volts for an individual LED  14 , for example, the number of LEDs “n” in each column  16  and  18  is approximately 31 for a 110 volt AC power source. The number of LEDs is dependent on the voltage characteristics of the type of LED used. For example, the forward voltage for a red LED may be approximately 2 volts and 3 to 4 volts for a blue LED. If the AC voltage is 220 volts, the number of LEDs in the columns  16  and  18  will be approximately double that of the 110 volt application.  
         [0021]     Referring to  FIG. 3 , a diagrammatic cross-sectional view of the single-chip AC high voltage LED device  10  is illustrated showing the details of integration and connection of two adjacent smaller LEDs  14 . The single-chip AC LED device  10  is formed by depositing layers of n-type semiconductor material  20 , optically active layers  22  and p-type semiconductor material  24  in succession on an insulating substrate  26 . In  FIG. 3 , n-type gallium nitride (n-GaN)  20 , indium gallium nitride/gallium nitride multiple quantum wells (InGaN/GaN MQW) optically active layers  22  and p-type gallium nitride (p-GaN)  24  in succession on a sapphire substrate  26  are being used as an illustration. Substrate  26  may be made of an insulating material as shown in  FIG. 3  or may be a layer of insulating film deposited on a conducting substrate or other material.  
         [0022]     The electrical insulation between the two adjacent LEDs  14  is accomplished by etching into the insulating substrate  26  so that a trench is formed and no light emitting material is present between the two LEDs  14 . An insulating film  28  such as silicon dioxide, silicon nitride, or other oxides, nitrides or polymides materials, for example, is subsequently deposited on the etched surface  26 . An n-type ohmic contact (cathode)  30  is deposited on the exposed n-type layer  20 . A transparent p-type metal film  32  is deposited on the p-type layer  24  upon which a p-type ohmic contact (anode)  34  is deposited. A conductive metal wire  36  connects the n-type ohmic contact  30  of one LED  14  to the p-type ohmic contact  34  of the next LED  14 . If the trench sidewalls are very steep, the deposition of the insulating film  28  and the interconnecting metal wires  36  may not be conformal on the steep sidewalls, which may degrade the device performance, yield and reliability. Using isotropic etching, the trench sidewalls are sloped. As a result, the insulating film  28  and metal wire  36  may be deposited with a conformal profile.  
         [0023]     The anode and cathode are formed by depositing different metals and then thermally annealing in different temperatures and ambient environments. To simplify the processing steps, a tunneling junction consisting of heavily doped n +  semiconductor and p +  semiconductor layers grown on the p-type semiconductor layer  24  may also be used. In this case, both the anode and cathode are formed in the same process step by the same metals on the n +  and n semiconductors respectively.  
         [0024]     It should be understood that p-n junction, heterojunction, multiple quantum well, organic electro-luminescent and polymer electro-luminescent LEDs as well as other types light emitting diodes may be configured as described hereinabove or in other combinations. Additionally, LEDs may be configured for 110 volt operation and 220-volt operation using switches. When configured for 110-volt operation, the arrays  16  and  18  may be connected as described hereinabove. Using a pair of switches, the 110-volt configuration may be converted to 220-volt operation wherein both arrays are series-connected and forward biased simultaneously. In this way, all of the LEDs are energized or on for half of the AC cycle and all of them are off for the other half of the AC cycle.  
         [0025]     Referring to  FIG. 4 , an alternate mounting method using a flip-chip bonding technique for the single-chip AC LED device  10  is generally indicated by reference numeral  50 . By flip-chip bonding, the AC-LED device has more light extraction from the transparent substrate side without any light blocking by the contacts and interconnection metals. Flip-chip bonding the AC-LED on a highly thermal-conductive submount will also enhance the heat transferred away from the LED active region to the submount and then dissipated in the environment. The single-chip AC LED device  10  is formed by depositing layers of n-type semiconductor material  20 , optically active layers  22  and p-type semiconductor material  24  in succession on an insulating transparent substrate  52 . An n-type gallium nitride (n-GaN)  20 , indium gallium nitride/gallium nitride multiple quantum wells (InGaN/GaN MQW), optically active layers  22  and p-type gallium nitride (p-GaN)  24  in succession on the transparent substrate  52  are being used as an illustration.  
         [0026]     The electrical insulation between two adjacent LEDs  14  is accomplished by etching (dry or wet chemical etching) into the transparent substrate  52  so that no light emitting material is present between adjacent LEDs  14 . An insulating film  28  such as silicone dioxide (SiO 2 ) is subsequently deposited on the etched surface. An n-type ohmic contact  30  is deposited on the exposed n-type layer  20 . A p-type ohmic contact  34  is deposited on the p-type layer  24 . A conductive layer  36  connects the n-type ohmic contact  30  of one LED  14  to the p-type ohmic contact  34  of the adjacent LED  14 . A passivation layer  54  forms over all of the LEDs  14 . The passivation layer  54  is removed from the conductive layer  36  and the chip  10  is flipped to be bonded to submount  56 , and light is extracted from the top transparent substrate side. The p-contact  34  may be the commonly used Ni/Au metal stack; on the other hand, to enhance the light extraction from the top substrate  52  side, the p-contact  34  may incorporate a highly reflective metal layer to reflect the light emitting toward the bottom submount  56  back to the substrate  52  side. For example, thin and transparent Ni/Au metal layer less than 10 nm may be first deposited and annealed at high temperature to form ohmic contact to p-GaN  24 , and then a thick (&gt;100 nm) silver or other highly reflection metal may be deposited on the Ni/Au to form a highly reflective mirror.  
         [0027]     Submount  56  includes bonding bumps  58  which are in spaced alignment with the LEDs  14 . Bonding bumps  58  may be made of different solders such as PbSn and AuSn alloy, or other conductive material such as Au, In and Cu. Conductive terminal pads  60  and  62  are mounted at opposite ends of the submount  56  with a metal film or leads  64  and  66  to the adjacent bonding bumps  58 . Terminal connections  68  and  70  are fixed to each terminal pad  60  and  62  respectively. The entire flip chip assembly  50  is heated until the bonding bumps  58  begin to melt and then cooled to bond the chip  10  to the submount  56 . The bonding bumps  58  serve to bond the chip  10  to the submount  56 , the outside bonding bumps  58  also provide an electrical connection to the chip  10 , and the bonding bumps  58  provide a thermal conduction path from the chip  10  to the thermal-conductive submount  56  to dissipate heat more quickly. Light  72  is extracted from the transparent substrate  52 . Furthermore, if underfill is incorporated to fill the space between the LED and submount, the underfill should have a small value of refractive index, so that the light extraction downward to the submount  56  is minimized.  
         [0028]     The emitted light color from the AC-LED will depend on the bandgap energy of the semiconductor materials. By varying the semiconductor alloy composition or using different semiconductor materials, the AC-LED can emit different colors covering infrared, visible and ultra-violet (UV) light. For example, if InGaN alloy is used as the LED active layer, varying In composition in InGaN alloy, a light spectrum from UV, purple, blue and green can be covered. If a layer of suitable phosphors is incorporated to cover the AC-LED, i.e, the device side as illustrated in  FIG. 3 , or the substrate side as illustrated in  FIG. 4 , an AC-LED with white color can be formed. The selection of phosphors depends on the intrinsic color of LED itself. If AC-LED has an intrinsic color of blue, then yellow phosphor can be used. The blue light from the LED will activate the phosphor to emit yellow light. The combination of the transmitted blue light with the yellow light from phosphor gives a white light. Other phosphors with different emission wavelength can also be used. Besides, several AC-LED devices with different color emission, red, green and blue, can be packaged in a same packaging house, and the color mixture will be white emission.  
         [0029]     It should be understood that while a certain form of this invention has been illustrated and described, it is not limited thereto except insofar as such limitations are included in the following claims.