Abstract:
A single-chip integrated LED particularly adapted for direct use with a high voltage DC or AC power sources comprises a plurality of electrically isolated LEDs on a generally transparent substrate and bonded to electrically conductive elements on a thermally conductive mount. A reflective coating may be applied to the area between LEDs.

Description:
[0001]     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00014-02-C-0214 awarded by the Office of Naval Research. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a light emitting device, and more particularly, to a heterogeneously integrated light emitting device which may be directly powered by a high DC voltage or by an AC voltage for general lighting, indication or display purposes.  
         [0003]     The advances in III-Nitride semiconductors (including GaN, InN, AlN and their alloys) based light emitting diodes (“LEDs”) is dramatically changing the lighting technology with a new lighting paradigm. LEDs, which have been until recently mainly used as simple indicator lamps in electronics and toys, now have the great potential to replace incandescent light bulbs in many applications, particularly those requiring durability, compactness, and/or directionality (e.g., traffic, automotive, display, and architectural lighting). Compared with the conventional lighting, semiconductor LED based solid state lighting (“SSL”) has the benefits of being more energy efficient with less power consumption, having a longer operational life with reduced maintenance costs, being vibration-resistant, having a vivid saturation color, and the added benefit of a flexible lighting design. It has been estimated that by the year 2025 the electricity saved in the United States by using solid state lighting would be approximately 525 trillion watt hours per year, or $35 billion a year. Additionally, the human visual experience would be enhanced by independently tuning the light intensity and colors of the LEDs.  
         [0004]     The conventional LED, depending on the semiconductor materials, operates at a very low DC voltage (roughly between 1V and 5V) and a limited current (˜20 mA) with very low luminance, only suitable for indication purposes. To achieve a high luminance for general lighting applications, two methods have been adopted. In the first method the LED still operates at a low DC voltage, but with a very high DC current (&gt;100 mA) to achieve a high luminance. The so-called power LED requires a bulky voltage transformer, an electronics controller and driver to power the LED. In a second method many LEDs are integrated on the same chip with a serial interconnection to achieve one light emitting device, which can directly run under a high DC input voltage. Depending on the integrated LED numbers, the operational voltage may be 12V, 24V, 110V, 240V, or even higher. Additionally, with two current paths the high voltage light emitting device may also operate directly at 110/120V or 220/240V AC. This highly integrated high voltage LED device has a size of between hundreds of microns to tens of millimeters as disclosed in U.S. Pat. No. 6,787,999. Other devices have used serially connected packaged LEDs soldered together on a PCB board to form a bulk LED cluster for high voltage applications.  
         [0005]     The concept of an integrated single chip LED device which operates under a high DC and/or AC voltage (high voltage DC/AC LED) unfolds a new paradigm for LED applications in lighting, indication and displays. As one example, the high voltage LED may be directly powered by the 110V power grid without any voltage transformer. If the high voltage LED is packaged with a standard Edison or European screw base, it may be directly screwed into a standard light bulb fixture for indoor or outdoor lighting.  FIGS. 1 and 2  illustrate the principle to build such a device by directly integrating many LEDs together on a single chip. As illustrated, an InGaAlN LED is grown on a sapphire substrate or other insulating substrate, for example. A prior art conventional low voltage DC LED is generally indicated by reference numeral  10 . LED  10  includes a substrate  12 , an n-type semiconductor layer  14 , a light emission region  16 , and a p-type semiconductor layer  18 , a p-contact  20 , an n-contact  22 , and a current spreading layer  24 . As illustrated in  FIG. 2 , a prior art integrated high voltage LED device is generally indicated by reference numeral  26 . A number of LEDs serially connect by connecting the p-layer  18  of one LED  10  with the n-layer  14  of the adjacent LED with an interconnection metal layer  28 . The integrated LED  26  has two terminals  30  and  32  for connection to an input voltage. Light  34  is extracted from the semiconductor epilayer  18  through the semi-transparent current spreading layer  24 .  
         [0006]     Several problems with prior art integrated LEDs include inefficient light extraction, thermal dissipation, and low product yield and reliability. Each individual LED  10  has to be isolated from the others by etching through the n-type semiconductor layer  14  to the insulating substrate  12  or to an insulating growth layer (buffer, epilayer, etc.). For InGaAlN-based LEDs, this etching depth is approximately from 2 μm to 6 μm. The deep trenches  36  provide technical challenges for depositing the metal layers  28  to interconnect each LED  10 . An inconsistent or thin metal layer  28  may cause leakage or disconnection at the trench side walls  38 , which may result in product performance, yield and reliability degradation.  
         [0007]     For an InGaAlN based LED device sapphire is the most common substrate and is also the best option for a high voltage LED device because of its high insulation property. If SiC or Si is used as the substrate an insulation buffer layer will be required. Unfortunately, sapphire has a very low thermal conductivity and the limited thermal dissipation degrade the high voltage (and high power) LED device performance and lifetime. Another drawback for the prior art is that the light is extracted from the epilayer device side and a significant portion of the light is blocked and absorbed by the metal layers, including the p-contact  20 , n-contact  22 , metal layer  28  and the current spreading layer  24  limiting the light emitting efficiency.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides an improved III-nitride semiconductor based high voltage DC/AC light emitting device by heterogeneously integrating an array of LEDs with a passive/active submount through flip-chip bonding or other mounting method. The submount may be aluminum nitride, boron nitride, or other appropriate materials with both insulating and thermal conductivity properties. The submount may include flip-chip bumps for bonding the LED array and enhancing the thermal dissipation and light extraction. The submount may also include passive circuits to serially interconnect the discrete LED array and provide current limiting protection. Furthermore, the submount may also be silicon with an active control circuit on one side and insulating and metal layers to connect the LED array on the other side. The final device has two or more outlet connections for the supplied power (and control signals). The supplied power may be 12V, 24V or other DC voltages, or it may be an AC voltage such as 110/120V or 220/240V. The light emission may be white light, a single color, multiple colors or time-varying color.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a cross sectional view of a prior art LED.  
         [0010]      FIG. 2  is a cross sectional view of a prior art LED array.  
         [0011]      FIG. 3  is a cross sectional view of a flip-chip bonded high voltage light emitting device with the interconnection between each individual LED on the submount.  
         [0012]      FIG. 4  is a diagrammatic view of a high voltage DC LED.  
         [0013]      FIG. 5  is a diagrammatic view of a high voltage AC LED.  
         [0014]      FIG. 6  is a cross sectional view of the device of  FIG. 3  with a passive protection circuit.  
         [0015]      FIG. 7  is a cross sectional view of the device of  FIG. 3  with an integrated circuit for protection, control and driving of the LED array.  
         [0016]      FIG. 8 a  diagrammatic view of three integrated single color LED arrays connected in parallel.  
         [0017]      FIG. 9 a  diagrammatic view of three integrated single color LED arrays connected in series.  
     
    
     DETAILED DESCRIPTION  
       [0018]     Referring to  FIG. 3 , a chip-scale high voltage DC/AC light emitting device is generally indicated by reference numeral  100 . The high voltage DC/AC light emitting device  100  is built by heterogeneously integrating a laterally conducting InGaAlN LED array  102  fabricated on a substrate  104  with a submount or mount assembly  106 . The array  102  is connected to the submount assembly  106  by flip-chip bonding or other connection method. The substrate  104  may be transparent, semi-transparent, translucent or have similar properties to allow light to be extracted from the substrate. Substrate may be an insulating material such as sapphire (Al 2 O 3 ), SiC, Si, GaAs, for example. By flip-chip bonding with the semiconductor epilayers facing down to the submount  106 , the light will be extracted from the substrate  104  of the LED array  102 . By moving more metal layers from the LED array die to the submount, this invention will also improve the light extraction efficiency. It should be understood that p-n junction, heterojunction, multiple quantum well, organic electro-luminescent, polymer electro-luminescent, ultraviolet (400-300 nm), and deep ultraviolet (300-200 nm) LEDs as well as other types of light emitting diodes may be configured as described hereinabove or in other combinations.  
         [0019]     LED array  102  may be fabricated on the substrate  104  using standard integrated chip fabrication techniques. A deep trench  108  is etched between adjacent LEDs  110  down to the substrate  104  to electrically isolate the discrete LEDs  110  from each other. Each LED  110  is generally mesa-shaped with an n-layer  112 , a light emitting layer  114 , a p-layer  116 , an n-contact  118  and a p-contact  120 . Interconnection between adjacent LEDs  110  is accomplished using bonding bumps  122  connected to metal layers  124  which are secured to submount  106 . Terminals  126  and  128  provide power connection points at each end of LED array  102 . Bonding bumps may be solders such as lead/tin (Pb/Sn) or gold/tin (Au/Sn), or metals such as gold (Au) or indium (In), for example.  
         [0020]     The LED array  102  performance and life depends on the p-n junction temperature. For a high voltage application, heat dissipation may become more difficult. Failure to adequately dissipate the heat may cause the device performance to suffer and may result in a premature device failure. InGaAlN based semiconductor epilayers grown on a sapphire substrate is ideal for manufacture of a high voltage DC/AC light emitting device because of its insulating properties. However, sapphire has a very low thermal conductivity. To enhance the thermal performance of the light emitting device  100 , the LED array  102  is bonded to the submount  106  which may be aluminum nitride, boron nitride or other appropriate materials with a high thermal conductivity and a high electrical resistivity, so that the heat produced at the p-n junction of each LED  110  in array  102  may be easily transferred through the bonding bumps  122  and metal layers  124  to the submount  106  and to the outside package body (not shown).  
         [0021]     Light  130  is extracted from the substrate layer  104 . The p-contact  120  may be either Ni/Au metal stacks or may incorporate a highly reflective metal layer to reflect light  132  emitted toward the submount  106  back to the substrate layer  104 . For example, a thin, transparent Ni/Au metal layer less than 10 nm may be first deposited and annealed to form the ohmic contact to the p-GaN layer, and then a thick layer (greater than 100 nm, for example) of silver or other metal may be deposited on the Ni/Au layer to form a highly reflective mirror. Within the trenches  108  a transparent dielectric/silver stack layer may be deposited as a reflective mirror with the transparent dielectric layer also acting as a passivation layer for the surface of the trench  108 . If a reflective metal is not used in the trench area  108 , a low refractive index dielectric material such as silicon oxide may be used for a surface passivation.  
         [0022]     Referring to  FIG. 4 , for a DC power input source, the number of serially connected LEDs  110  in array  102  will depend on the input DC voltage. For example, if the operational voltage of each LED  110  is three volts and the input DC voltage is 12 volts, four LEDs  110  may be connected in series across the DC input. To increase the luminance two or more LED arrays  102  may be connected in parallel across the DC input.  
         [0023]     Referring to  FIG. 5 , for an AC input power source the number of serially connected LEDs  110  in arrays  102  and  103  will depend on the input AC voltage. For example, if the operational voltage of each LED  110  is three volts and the input AC voltage is 120 volts, 40 LEDs  110  may be connected in series across the AC input for each array  102  and  103 . As shown, LED array  102  will be turned on for approximately half of the AC cycle and LED array  103  will be turned on for the other half of the AC cycle. The current flows in direction  105  for array  102  and in direction  107  for array  103 . The second array  103  of serially connected LEDs may be connected in parallel in the opposite direction to the first array  102  across the AC input. The arrays of LEDs will be turned alternately on and off 60 times per second for a 60 Hz AC input voltage and 50 times per second for a 50 Hz AC input voltage.  
         [0024]     Referring to  FIG. 6 , an LED array  102  may be integrated with a passive protecting circuit  200 . Since LEDs have a very low dynamic resistance input voltage variations, such as spikes, can overdrive the LED array  102  degrading its performance and reducing its operational life. The integrated passive protecting circuit  200  may be used to reduce or soften voltage variations. Passive protecting circuit  200  may include a current-limiting resistor directly deposited on the submount  106  or may be a surface mounted resistor assembled on the submount. Passive protecting circuit  200  may include a positive temperature coefficient (“PTC”) thermistor to protect the LED array  102  from over-current conditions. During normal operating conditions the PTC thermistor remains in a low resistance state resulting in a negligible attenuation in current flow through the device. When an over-current condition occurs the PTC thermistor switches into a high resistance state thereby limiting the current flow through the LED array  102  to a normal operating level. When the high-current condition is removed, the PTC thermistor resets to its low resistance state and permitting a normal operating current to flow through the LED array  102 .  
         [0025]     Referring to  FIG. 7 , another embodiment of a chip-scale high voltage DC/AC light emitting device is generally indicated by reference numeral  300 . Components similar to those shown in  FIG. 5  are indicated by the same reference numeral. High voltage DC/AC light emitting device  300  is built by heterogeneously integrating a laterally conducting InGaAlN LED array  102  fabricated on an electrically insulating layer  302  on a substrate  303  with a submount assembly  304  by flip-chip bonding or other mounting method. Insulating layer  302  may include GaN, AlN, InGaAlN, Al 2 O 3 , Si, or GaAs, for example. Substrate  303  may be made of a thermally conductive or electrically conductive material or an insulating material. Bonding bumps  122  are connected to metal layers  124  which are bonded to thin insulating layers  306 . Insulating layers  306  may be silicon oxide or silicon nitride, for example. Submount assembly  304  may be copper, aluminum or silicon, for example, and may include a control and driving circuit  308  to control the LED array  102  through interconnections  310 . Reflective layers  312  may be deposited in the trench areas  108  and on the p-contact layer  120  to improve the light extraction efficiency of the device  300 .  
         [0026]     Referring to  FIGS. 8 and 9 , multiple LED array emitters  102  may be integrated on a single submount  320 . Different spectrally distinguishable LED dies  322 ,  324  and  326 , such as blue, green and red, may be connected and controlled by a circuit mounted or integrated on submount  320 . LED array emitters  102  may be connected in parallel ( FIG. 8 ) or in series ( FIG. 9 ). To construct a white color DC/AC light emitting device, light from the blue  322 , green  324  and red  326  LED dies may be mixed. Each of the LED dies may be independently controlled to achieve a desired luminance and color mix. The mixed light may be balanced to create a white light or a colored light depending on the mixing parameters and control. The integrated controls may produce a time-varying colorful light. When connected in series ( FIG. 9 ), the number of individual LEDs on each of the three spectrally distinguishable emitter dies may be varied to achieve the desired white color or temperature of the white light.  
         [0027]     Another method to achieve white light emission from LEDs is to use blue LEDs made of III-nitrides to generate white light and then coating the substrate layer or the inside of the device packaging such as the inside surface of a glass bulb in which the device is mounted, 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. Another method includes using UV or near UV LEDs to pump three-color phosphors (red, blue, green, RBG) or to combine three color (RBG) LEDs to get a white emission.  
         [0028]     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.