Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/83—Electrodes
  • H10H20/832—Electrodes characterised by their material
  • H10H20/835—Reflective materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/81—Bodies
  • H10H20/819—Bodies characterised by their shape, e.g. curved or truncated substrates
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
  • H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
  • H01L2224/42—Wire connectors; Manufacturing methods related thereto
  • H01L2224/47—Structure, shape, material or disposition of the wire connectors after the connecting process
  • H01L2224/48—Structure, shape, material or disposition of the wire connectors after the connecting process of an individual wire connector
  • H01L2224/481—Disposition
  • H01L2224/48151—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive
  • H01L2224/48221—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked
  • H01L2224/48245—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked the item being metallic
  • H01L2224/48247—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked the item being metallic connecting the wire to a bond pad of the item
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
  • H01L2924/15—Details of package parts other than the semiconductor or other solid state devices to be connected
  • H01L2924/181—Encapsulation
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/81—Bodies
  • H10H20/822—Materials of the light-emitting regions
  • H10H20/824—Materials of the light-emitting regions comprising only Group III-V materials, e.g. GaP
  • H10H20/825—Materials of the light-emitting regions comprising only Group III-V materials, e.g. GaP containing nitrogen, e.g. GaN

Definitions

• the present invention relates to light emitting diodes (LEDs) and in particular relates to LEDs formed with active portions of Group III nitrides on silicon carbide substrates.
• Incandescent lighting is produced by heating a metal filament, usually by passing electric current through a metal filament. The heated filament emits light. Household and other indoor lighting is a common application of incandescent lighting. "Halogen" lighting works on the same general principle, but more efficiently. Fluorescent light is generated by exciting a vapor (typically mercury-containing) with an applied potential difference. Photons emitted by the excited vapor then strike phosphors that emit the visible light. Fluorescent light is also common in household, office and a variety of other applications.
• a light emitting diode is a p-n junction semiconductor diode that emits photons when forward biased.
• light emitting diodes produce light based upon the movement of electrons in a semiconductor material. Therefore, LEDs do not require (although they can be used in conjunction with) vapors or phosphors. They share the desirable characteristics of most semiconductor-based devices, including high efficiency (their emissions include little or no heat), high reliability and long life. For example, typical LEDs have a mean time between failures of between about 100,000 and 1,000,000 hours meaning that a conservative half lifetime for an LED is on the order of 50,000 hours.
• an LED's emitted light has a frequency (which in turn relates directly to wavelength and color in accordance with well-understood principles of physics) based upon the energy difference between permitted energy levels in the material, a characteristic referred to as the bandgap.
• the bandgap is a fundamentally property of the semiconductor material and its doping.
• LEDs formed in silicon Si, bandgap of 1.12 electron volts (eV)
• eV electron volts
• LEDs formed in gallium arsenide bandgap 1.42 eV
• silicon-doped aluminum gallium arsenide AlGaAs
• Such materials include diamond (5.47 eV), silicon carbide (2.99 eV) and Group III nitrides such as GaN (3.4 eV).
• wide bandgap LEDs can be combined with red and green LEDs to produce white light, or with phosphors that produce white light when excited by blue or UV light, or both.
• the Group III nitride compositions i.e., Group III of the periodic table
• GaN, AlGaN, InGaN and AImGaN are particularly useful for blue-emitting LEDs.
• they are "direct” emitters, meaning that when an electron transition occurs across the bandgap, much of the energy is emitted as light.
• indirect emitters such as silicon carbide
• Group III nitrides offer efficiency advantages over indirect transition materials.
• the bandgap of ternary and quaternary Group III materials depends upon the atomic fraction of the included Group III elements.
• the wavelength (color) of the emission can be tailored (within limits) by controlling the atomic fraction of each Group III element in a ternary or quaternary nitride.
• GaP gallium-arsenide or gallium phosphide
• blue and UV-emitting LEDs have lagged behind GaP-based LED's in their commercial appearance.
• silicon carbide is physically very hard, has no melt phase, and requires high temperatures (on the order of about 1500-2000 °C) for epitaxial or sublimation growth.
• the Group III nitrides have relatively large nitrogen vapor pressures at their melting temperatures and thus are likewise difficult or impossible to grow from a melt.
• lighting is typically quantified as to its output.
• One typical unit of measure is the lumen, defined as a unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source of one candela (cd) intensity.
• the candela is the base unit of luminous intensity in the International System of Units that is equal to the luminous intensity in a given direction of a source which emits monochromatic radiation of frequency 540 x 10 12 hertz and has a radiant intensity in that direction of 1/683 watt per unit solid angle.
• an intensity of 1200-1800 lumens is typical of incandescent bulbs and 1000-6000 lumens (depending upon circumstances) is typical in natural daylight.
• Light emitting diodes are much less intense, for example on the order of about 10-100 lumens.
• One reason is their small size.
• applications for single (or small groups of) LEDs have historically gravitated towards indication (e.g. the register of a hand-held calculator) rather than illumination (a reading lamp).
• indication e.g. the register of a hand-held calculator
• illumination a reading lamp
• the output of LEDs is often measured in units other than lumens. Additionally, an LED's output also depends upon the applied current, which in turn depends upon the potential difference applied across the diode. Thus, the output of an LED is often referred to as its radiant flux (R f ) and is expressed in milliwatts (mW) at a standard 20 milliamp (rnA) drive current.
• R f radiant flux
• mW milliwatts
• rnA milliamp
• blue LEDs and their related derivative devices are becoming more frequently included in consumer electronic devices, particularly small displays. Common examples include items such as computer screens, personal digital assistants ("PDAs”) and cellular phones. In turn, these small devices drive demand for LEDs with reduced size (“footprint"). Such LEDs, however, must still operate at low forward voltages (V f ) and high light output. To date, however, reducing the size of the Group III nitride devices has tended to increase their forward voltage and reduce their radiant flux.
• the invention is a light emitting diode having small size, low forward voltage and high light output.
• the invention is a light emitting diode with advantageous output on a per unit area basis.
• the diode includes an area of less than 100,000 square microns, a forward voltage of less than 4.0 volts, a radiant flux of at least 24 milliwatts at 20 milliamps drive current, and a dominant wavelength between about 395 and 540 nanometers.
• the invention is a light emitting diode that demonstrates a radiant flux of at least 270 milliwatts per square millimeter, at a dominant wavelength of between about 395 and 540 nm, at a forward voltage of less than 4.0 volts and at 20 milliamps drive current.
• the invention is a light emitting diode with a die with an area of less than 100,000 ⁇ 2 , a 5 mm package encapsulating the die, a forward voltage of less than 4.0 volts, an external quantum efficiency of greater than forty five percent (45%) at wavelengths of between about 420 and 465 nm and a drive current of 20 milliamps.
• the invention is a light emitting diode that includes a 5 mm (TE 1-3/4) polymeric package, a die in the package with an area of less than 100,000 square microns, a forward voltage of less than 4.0 volts, a radiant flux of at least 24 milliwatts at 20 milliamps drive current, and a dominant wavelength of between about 395 and 540 nm.
• a 5 mm (TE 1-3/4) polymeric package a die in the package with an area of less than 100,000 square microns, a forward voltage of less than 4.0 volts, a radiant flux of at least 24 milliwatts at 20 milliamps drive current, and a dominant wavelength of between about 395 and 540 nm.
• the invention is a light emitting diode that includes a conductive silicon carbide substrate; respective p-type and n-type Group III nitride layers on the silicon carbide substrate; ohmic contacts in a vertical orientation with respect to the silicon carbide substrate and the Group III nitride layers; a dominant wavelength between 455 and 465 nm; a polymer package encapsulating the substrate, the Group III nitride layers and portions of the ohmic contacts; and a radiant flux from the package of at least 270 milliwatts per square millimeter of the emitting surface at a drive current of 20 milliamps.
• Figure 1 is a cross-sectional view of a diode in accordance with the present invention.
• Figure 2 is a cross-sectional view of an LED lamp according to the present invention.
• Figure 3 is cross-sectional view of a second embodiment of a diode in accordance with the present invention.
• Figure 4 is cross-sectional view of another embodiment of a diode in accordance with the present invention.
• Figure 5 is cross-sectional view of yet another embodiment of a diode in accordance with the present invention.
• Figure 6 is a top plan view of a prior art diode.
• Figure 7 is a combined plot of spectral radiant flux output and external quantum efficiency for a light emitting diode according to the present invention.
• the present invention is a light emitting diode having small size, a low forward voltage, and a high light output resulting in high-efficiency and high output on a per unit area basis.
• Figure 1 is a cross sectional view of a light emitting diode broadly designated at 20 having the performance characteristics of the present invention.
• the diode 20 includes a transparent silicon carbide substrate 21 which is preferably a single crystal and has a polytype selected from the 3C, 4H, 6H, and 15R polytypes of silicon carbide with 4H often being preferred in the context of the present invention.
• Figure 1 illustrates the diode 20 in the "flip chip” orientation (i.e., mounted for use with the active layers below the substrate), the substrate 21 appears at the top of the diode 20 rather than the bottom. In this orientation, the SiC substrate becomes the primary emitting surface of the LED. It will be understood of course, that light emitting diodes can be placed in a number of different positions and orientations in end use. Thus with respect to the elements of the diode 20, the terms, “top,” and “bottom,” are relative and generally indicate the orientation of the device in a structural sense. The use of such terms is common and well understood in this art and will also be clear from the context herein.
• the diode includes at least one, and preferably several, layers that form the light emitting ("active") portions. Two layers are shown in Figure 1, an n-type layer 22 and a p-type layer 23. These opposite conductivity type layers provide the opportunity for current to flow through the diode and the resulting combination of electrons and holes that generate the emitted photons. Although only two Group III nitride layers are illustrated in Figure 1, it will be understood that in other contexts, additional layers can be used including superlattice structures and multiple quantum wells. Such structures are well-understood in this art and can be practiced in the context of the present invention without undue experimentation.
• the embodiment illustrated in Figure 1 also includes a mirror layer 24 which is typically formed of silver (Ag) or a silver-platinum (Ag/Pt) alloy.
• the silver-based layer also provides electrical contact to the active layers 22, 23.
• a barrier layer 25 typically formed of a titanium tungsten (TiW) alloy, or platinum, or both, or of titanium tungsten nitride (TiWN), encloses the silver-based layer 24 in order to prevent undesired migration and reaction of silver with other portions of the device.
• a solder layer 26 is attached to the barrier layer 25 typically, but not exclusively, based upon the method of making the diode.
• the metal or semiconductor support layer 27 can be omitted, with or without the solder layer 26. hi such embodiments, the backside ohmic contact 30 is positioned against the mirror and barrier metals 24, 25.
• the active layers are typically Group III nitrides, with gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN), and aluminum indium gallium nitride (AlhiGaN) being appropriate selections.
• the Group III nitrides offer the opportunity to change the dominant wavelength by changing the atomic fraction of the Group III element in the ternary and quaternary combinations.
• these formulas can be more exactly expressed as Al x IQyGa 1 -x-y N in which x and y can range from 0 to 1, and inclusive of either, with the qualification that x + y must always equal 1 or less.
• FIG. 2 is a cross-sectional view of a lamp that incorporates diodes according to the present invention in standard packaging.
• the lamp is broadly designated at 32 and includes elements that are well understood by those of ordinary skill in this art. Indeed, one of the advantages of the diode disclosed therein is that its size and orientation permit it to be mounted in standard packaging with associated benefits to the manufacturer and the end-user.
• the lamp 32 includes the diode which is again indicated at 20.
• the diode rests in a reflective cup 33 which in the type of diode illustrated in Figure 2 is sometimes referred to as the anvil 34 and also forms one of the electrical contacts to the lamp 32.
• the anvil is conductive and typically formed of metal. Although for clarity's sake Figure 2 shows the diode 20 as resting in the die cup 33, in actual practice it is typically fixed in place with a conductive glue or solder (not shown).
• the anvil 34 makes electrical contact with the backside ohmic contact 30 ( Figure 1) of the diode 20 and a top bond wire 35 makes electrical contact to the top ohmic contact 28 (also in
• the entire package includes the polymer housing or lens 37 the size and shape of which, along with the material, are chosen for specific output purposes.
• the active layers 22, 23 are adjacent the floor of the die cup 33 with the silicon carbide substrate 21 facing upwardly away from the cup.
• Figure 2 is an exemplary and to some extent schematic diagram and very similar packages can include a surface mount, a metal can, or a metal header, as well as the epoxy lens illustrated in Figure 2.
• the invention is not limited to use in a 5 mm package
• the 5 mm package also known as the T-1 3 A type
• T-1 3 A type is commonly understood, widely available and frequently used in the industry. It thus provides an appropriate expression of the invention in terms of its packaging and use.
• the invention is a light emitting diode with advantageous output on a per unit area basis that comprises an area of less than 100,000 ⁇ 2 , forward voltage of less than 4.0 volts, a radiant flux of at least 24 milliwatts at 20 milliamp drive current, and a dominant wavelength between about 395 and 540 nm.
• V f the forward voltage
• Radiant flux is measured by placing an encapsulated lamp of the Tl- 3 A type in an integrating sphere attached to a spectrometer, with the Labsphere Omni LTS spectrometer for visible LEDs being an exemplary measurement device. Radiant flux is measured in units of power (Watts).
• the term "dominant wavelength" describes a measure of the hue sensation produced in the human eye by a light emitting diode.
• the dominant wavelength is determined by drawing a straight line through the color coordinates of a reference illuminant and the measured chromaticity coordinates of the LED in the International Commission on Illumination (CIE) 1931 chromaticity diagram. The intersection of the straight- line on the boundary of the chromaticity diagram gives the dominant wavelength.
• CIE International Commission on Illumination
• the peak wavelength is the wavelength at the maximum spectral power.
• the peak wavelength may have less significance for practical purposes because two different light emitting diodes may have the same peak wavelength, but different color perception.
• the lens 37 is used to change the direction and distribution of the light rays (i.e., the spatial distribution pattern) and for some light emitting diodes is colored to serve as an optical filter. Because diodes according to the present invention are frequently used in conjunction with the production of white light, colored lenses are less commonly used.
• the radiant flux of a light emitting diode is typically measured using an integrating sphere, of which the device referenced above is entirely appropriate, but not limiting.
• Radiant flux which is also referred to as the radiant power, is the rate (d ⁇ /dt) at which the radiation field transfers radiant energy from one region to another. As noted above, if theta ( ⁇ ) is the radiant energy, the unit of radiant power is the watt.
• Diodes according to the present invention typically have a dominant wavelength between about 395 and 540 nm, placing them in the green, blue, violet and ultraviolet portions of the electromagnetic spectrum. In particular, diodes according to the invention have dominant wavelengths of between about 450 and 480 nm with preferred output wavelengths being between about 455 and 465 nm. This places their output squarely in the blue portion of visible spectrum which offers a number of advantages for full-color and white light for displays and related purposes.
• Diodes according to the invention can have a radiant flux of at least 24 milliwatts at the 20 milliamp drive current which is a typical standard in measuring the performance of light emitting diodes of all types, and thus provides a useful comparison for diodes according to the present invention.
• Blue light emitting diodes according to the present invention can be incorporated into pixels, particularly in combination with those that form other primary colors (red and green) to provide the opportunity for full-color displays.
• a light emitting diode produces a radiant flux of at least 270 milliwatts per square millimeter at a dominant wavelength of between about 395 and 540 nm at a forward voltage of less than 4.0 volts and at 20 milliamps drive current.
• the invention is a light emitting diode with an advantageous output on a per unit area basis that comprises a die with an area less than 100,000 ⁇ 2 , a 5 mm package encapsulating the die, a forward voltage of less than 4.0 volts, and an external quantum efficiency of greater than 45% at wavelengths of between about 420 and 465 nm and a drive current of 20 milliamps.
• the invention can be described as a light emitting diode with advantageous output on a per unit basis comprising a 5 mm polymeric package; a die in the package with an area of less than 100,000 ⁇ 2 , a forward voltage of less than 4.0 volts, a radiant flux of at least 24 milliwatts at 20 milliamps drive current, and a dominant wavelength of between about 395 and 540 ran.
• the dominant wavelength is preferably between about 450 and 480 nm and most preferably between about 455 and 465 nm.
• Diodes according to the invention have also demonstrated radiant flux of at least 27 milliwatts at 20 milliamps drive current.
• the invention comprises a light emitting diode with a conductive silicon carbide substrate, respective p-type and n-type Group III nitride layers on a silicon carbide substrate, ohmic contacts in a vertical orientation with respect to the silicon carbide substrate and the Group III nitride layers, a dominant wavelength between 455 and 465 nm, a polymer package encapsulating the substrate, the Group III nitride layers and portions of the ohmic contacts, and a radiant flux from the package of at least about 270 milliwatts per square millimeter of the emitting surface at a drive current of 20 milliamps.
• the diode is a 5 mm or surface mount package and the silicon carbide substrate has a polytype selected from the group consisting of the 3C,
• the light emitting area or surface is defined as the "footprint" of the device.
• the term "area” means the largest area of semiconductor or substrate material within the die or chip, because this largest dimension is the one that the circuit or device designer must deal with in using the individual light emitting diode.
• FIG. 6 is a top plan schematic view of a prior art diode that illustrates this point, hi Figure 6 the overall diode is broadly designated at 40 and includes a sapphire (Al 2 O 3 ) substrate 41.
• Sapphire is used as the substrate in a number of types of light emitting diodes because of its generally rugged physical characteristics and satisfactory optical properties. Its crystal lattice is also a satisfactory, although not necessarily optimal, match with trie crystal lattice of the Group III nitrides typically used to form blue light emitting diodes. Sapphire, however, cannot be conductively doped and thus always functions as an electrical insulator. As a result, a structure such as the one illustrated in Figure 6 must necessarily be adopted when sapphire is selected as the substrate.
• the active layers are grown on the sapphire substrate 41 to form the p ⁇ n junction for current injection purposes. Because the sapphire substrate 41 is insulating, however, the wire bond pads 44 and 45 must both face the top of the device in the manner illustrated in Figure 6.
• the area of the diode 40 (for purposes of measuring and expressing output per unit area) is the area of the sapphire substrate rather than merely the area of the active layer 42 or the smaller active layer 43. [0063] Expressed in alternative fashion, but with the same meaning, the area is the larger of either (i) the largest semiconductor area in the diode or (ii) the substrate area of the diode that must or will be packaged. In almost all circumstances, area (ii) is greater or equal to area (i).
• Figures 3, 4 and 5 are additional structural embodiments of diodes with performance output characteristics according to the present invention.
• the diode is broadly designated at 50 and is shown in a "flip chip: orientation; i.e., the silicon carbide substrate 51 is in a top or upper arrangement when the diode 50 is mounted for use.
• the substrate 51 includes the oblique surfaces 52 that can increase the light output from the device by reducing total internal reflection.
• the active layers are, for purposes of simplicity, illustrated as the single layer 53 but it will be understood, that the active structure usually includes at least one p- type and one n-type layer and potentially more sophisticated structures such as quantum wells, multiple quantum wells, and superlattice structures.
• the diode 50 preferably includes the mirror layer 54 which, if formed of silver, is preferably partially isolated in the manner previously discussed with respect to Figure 1. If formed of a less reactive metal, or one less likely to migrate, the mirror layer 54 can be a simple coating as illustrated in Figure 3. In Figure 3, the mirror layer 54 also forms the ohmic contact.
• Figure 4 illustrates a device otherwise similar to Figure 3 and broadly designated at 60.
• the only relevant difference between the diode 60 of Figure 4 and the diode 50 of Figure 3 is in the shape of the side ⁇ valls 62 which are perpendicular, or nearly so, to the top surface of the device 60.
• the slightly differently shaped substrate is designated at 61, but all of the other elements are otherwise identical in structure and function to the same elements in Figure 3, and thus carry the same reference numerals.
• the active layer is designated at 53, the mirror- contact at 54, the top ohmic contact at 56 and the " bonding metal at 55.
• the light emitting surfaces of the diode 60 can be formed into lenticular shapes or patterns that can help increase the light extraction from the diode.
• the use of lenticular surfaces for this purpose is set forth in the parent application herein, and other means of varying the surface for light extraction are likewise set forth in commonly-assigned U.S. Patent No. 6,791,119, the contents of which are incorporated entirely herein by reference.
• Figure 5 illustrates another embodiment of the invention in which the active layer(s) 66 is positioned at the top of the device and in direct contact with an ohmic contact 67.
• the mirror-ohrnic contact 70 is beneath the active layer 66 to increase the light extraction and a metal bonding layer 71 is optionally included to attach the active layer 66 and the mirror 70 to the substrate 72 which can be a semiconductor such as silicon carbide or potentially a conductor.
• An okniic contact 73 to the substrate 72 completes the device. If the mirror-contact 70 is silver or silver-based, a barrier structure (e.g. as described with respect to Figure 1) would also be included.
• Figure 7 is a combined plot of the spectral radiant flux output (mW, lower curve) and the external quantum efficiency (%, upper curve) for a light emitting diode according to the present invention. As illustrated therein, the relative luminous flux of the diode depends upon the wavelength. Accordingly, it will be understood that the diodes according to the present invention, although expressed in terms of performance at certain wavelengths, incorporate and include the corresponding proportional output at other wavelengths even if less than the maximum output at the most favorable wavelength for the particular diode.
• the descriptions herein of diodes as having a certain output per unit area at a given wavelength do not limit the diodes described or claimed herein to that output at that wavelength. Instead, in a manner analogous to plotting points on an algebraic curve, the output per unit area at a given wavelength is indicative of the entire curve rather than the single point recited.

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• 2005
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