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`US 20040206970Al
`
`(19) United States
`(12) Patent Application Publication
`Martin
`
`(10) Pub. No.: US 2004/0206970 Al
`Oct. 21, 2004
`(43) Pub. Date:
`
`(54) ALTERNATING CURRENT LIGHT
`EMITTING DEVICE
`
`(52) U.S. Cl. ................................................................ 257/98
`
`(76)
`
`Inventor: Paul S. Martin, Pleasanton, CA (US)
`
`(57)
`
`ABSTRACT
`
`Correspondence Address:
`PATENT LAW GROUP LLP
`2635 NORTH FIRST STREET
`SUITE 223
`SAN JOSE, CA 95134 (US)
`
`(21) Appl. No.:
`
`10/417,735
`
`(22) Filed:
`
`Apr. 16, 2003
`
`Publication Classification
`
`(51)
`
`Int. Cl.7 ..................................................... HOlL 33/00
`
`A plurality of semiconductor light emitting diodes formed
`on a single substrate are connected in series for use with an
`alternating current source. In one embodiment, the series
`array of light emitting diodes is directly connected to an
`alternating current source. In other embodiments, the series
`array of semiconductor light emitting diodes is mounted on
`a submount with integrated rectifying and filtering circuitry.
`The submount may be, for example, a silicon integrated
`circuit. In some embodiments a wavelength converting
`material is provided over the semiconductor light emitting
`diodes such that light emitted by the light emitting diodes
`and light emitted by the wavelength converting material mix
`to produce white light.
`
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`IPR PAGE 1
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`Patent Application Publication Oct. 21, 2004 Sheet 1 of 8
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`US 2004/0206970 Al
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`Patent Application Publication Oct. 21, 2004 Sheet 2 of 8
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`US 2004/0206970 Al
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`Patent Application Publication Oct. 21, 2004 Sheet 4 of 8
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`Patent Application Publication Oct. 21, 2004 Sheet 5 of 8
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`Patent Application Publication Oct. 21, 2004 Sheet 6 of 8
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`Patent Application Publication Oct. 21, 2004 Sheet 7 of 8
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`Patent Application Publication Oct. 21, 2004 Sheet 8 of 8
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`

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`US 2004/0206970 Al
`
`Oct. 21, 2004
`
`1
`
`ALTERNATING CURRENT LIGHT EMITTING
`DEVICE
`
`[0012] FIG. 7 is an exploded view of a packaged light
`emitting device.
`
`BACKGROUND
`
`[0001] 1. Field of Invention
`
`[0002] The present invention relates to monolithic arrays
`of semiconductor light emitting devices powered by alter(cid:173)
`nating current sources.
`
`[0003] 2. Description of Related Art
`
`[0004] Wojnarowski et al., U.S. Pat. No. 6,412,971, teach
`an array of separate, individual semiconductor light emitting
`diodes (LEDs) mounted on a conventional light bulb base
`for insertion into a residential 120 volt alternating current
`socket. Wojnarowski et al.'s devices include a rectifier and
`filter to provide direct current. Direct current provides a
`constant voltage and current to the array of LEDs, thus the
`LEDs are constantly on when the array is connected to the
`alternating current source, eliminating any visible flickering
`that may have occurred had unfiltered alternating current
`been used to power the array. Wojnarowski et al's devices
`are bulky and difficult to build and package due to the large
`number of separate LEDs used and due to the external
`filtering and rectifying circuitry. The filtering and rectifying
`circuitry also use power, create heat, and add cost.
`
`SUMMARY
`
`[0005]
`In accordance with embodiments of the invention,
`a plurality of LEDs formed on a single substrate are con(cid:173)
`nected in series for use with an alternating current source. In
`one embodiment, the plurality of LEDs are directly con(cid:173)
`nected to an unfiltered and unrectified alternating current
`source. In other embodiments, the LEDs are mounted on a
`submount with integrated rectifying and filtering circuitry.
`The submount may be, for example, a silicon integrated
`circuit. In some embodiments a wavelength converting
`material is provided over the LEDs such that light emitted by
`the LEDs and light emitted by the wavelength converting
`material mix to produce light with a wavelength distribution
`different from that emitted by the LEDs.
`
`[0006] Devices without external LED electrical drivers (or
`with rectifying and filtering circuitry integrated in the sub(cid:173)
`mount) offer the advantages of being small and simple to
`build and package. Such devices may also have reliability
`and cost advantages.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0007] FIG. lA is a plan view of a monolithic array of
`electrically isolated LEDs. FIG. lB is a plan view of a single
`LED in the array shown in FIG. lA.
`
`[0008] FIG. 2 is a cross-sectional view of the device
`illustrated in FIG. lB.
`
`[0009] FIG. 3 illustrates a monolithic array of LEDs
`mounted on a submount.
`
`[0010] FIGS. 4 and 5 are circuit diagrams of different
`embodiments of the device of FIG. 3.
`
`[0011] FIG. 6 illustrates an embodiment of the invention
`incorporating a zener diode.
`
`[0013] FIG. 8 is a cross section of a portion of a mono(cid:173)
`lithic array of LEDs mounted on a submount.
`
`[0014] FIGS. 9 and 10 illustrate packaged devices.
`
`DETAILED DESCRIPTION
`
`[0015]
`In accordance with embodiments of the present
`invention, semiconductor light emitting devices such as light
`emitting diodes are arranged in monolithic arrays suitable
`for use with high voltage power sources include those using
`alternating current. "High voltage" refers to a voltage greater
`than 10 times the forward voltage of a single LED. For
`example, some current III-nitride devices operate at a for(cid:173)
`ward voltage of about 3.5 Vat a current density of about 50
`Ncm 2
`. Such devices usually fail at current densities greater
`than 150 Ncm 2
`. If a single such device were connected to
`a 120 RMS V alternating current circuit, the peak voltage of
`about 180 V would far exceed the maximum operating
`current density. Accordingly, in some embodiments, mul(cid:173)
`tiple LEDs are connected in series in order to achieve an
`appropriate voltage drop and current density for each device
`when connected to a high voltage power source.
`
`[0016] FIG. lA is a plan view of a monolithic array of
`LEDs for use in an alternating current device. An array of
`individual LEDs 7 are formed on a single substrate 3. The
`individual LEDs in the array are electrically isolated from
`each other by, for example, trenches 8 etched between the
`devices down to substrate 3 or to an insulating layer such as
`an undoped semiconductor layer.
`
`[0017] FIG. lB is a plan view of an example of a single
`small junction III-nitride LED 7 (i.e., an area less than
`approximately one square millimeter) formed in the mono(cid:173)
`lithic device illustrated in FIG. lA. In one embodiment, the
`device in FIG. lB has an area greater than 300 microns by
`300 microns. FIG. 2 is a cross section of the device shown
`in FIG. lB, taken along axis CC. As illustrated in FIG. 2,
`the device includes an n-type region 11, an active region 12,
`and a p-type region 13 formed over a substrate 15. Each of
`the n-type region 11, the active region 12, and the p-type
`region 13 may be, for example, III-nitride semiconductor
`layers, and each region may contain multiple layers with the
`same or different characteristics. The substrate may be, for
`example, sapphire, GaP, Si, or SiC.
`
`[0018] The device shown in FIGS. lB and 2 has a single
`via 14 etched down to n-type layer 11. An n-contact 21 is
`deposited in via 14. N-via 14 is located at the center of the
`device to provide uniformity of current and light emission.
`A highly reflective p-contact 20 is deposited on p-type layer
`13. A thick p-metal layer 20a is deposited over reflective
`p-contact 20. N-contact 21 is separated from the p-metal
`layers 20, and 20a by one or more dielectric layers 22. A
`p-submount connection 16 connects to p-metal layer 20a,
`and an n-submount connection 17 connects to n-metal layer
`21, for connecting the device to a submount. Interconnects
`16 and 17 may be, for example, solder bumps.
`[0019] As illustrated in FIG. lB, the device is connected
`to a submount by p-submount connections 16 and n-sub(cid:173)
`mount connection 17. N-submount connection 17 may be
`located anywhere within n-contact region 21 (surrounded by
`insulating layer 22) and need not be located directly over via
`
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`Oct. 21, 2004
`
`2
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`14. Similarly, p-submount connections 16 may be located
`anywhere on p-metal layer 20a. As a result, the connection
`of the device to a submount is not limited by the shape or
`placement of p-contact 20a and n-contact 21.
`
`[0020] FIG. 3 illustrates the monolithic array of FIG. 1A
`mounted on a submount. FIG. 8 illustrates a cross section of
`a portion monolithic array mounted on a submount. Array 3
`is flipped over and mounted with the contacts closest to
`submount 2. The dashed lines illustrate the location of each
`of the individual LEDs 7. The individual LEDs 7 may be
`separated by a trench 87 as illustrated in FIG. 8. LED array
`3 is mounted on submount 2 by electrically and physically
`connecting interconnects (such as solder bumps 81-84 of
`FIG. 8) of each LED 7 to submount 2. LED array 3 is
`therefore mounted in flip chip configuration, such that light
`is extracted from each of LEDs 7 through the substrate 15
`(FIG. 2). In one embodiment, each of the LEDs are con(cid:173)
`nected to each other in series by interconnects within or on
`the surface of submount 2. Interconnect 86 of FIG. 8,
`formed on the surface of submount 2, connects solder bumps
`82 and 83 of the two LEDs pictured in FIG. 8. Interconnect
`85, formed within submount 2, connects solder bump 81 to
`another LED or other circuitry (not shown). Alternatively,
`the individual LEDs can be connected by interconnects
`formed on array 3. Such interconnects are described in more
`detail in U.S. Pat. No. 6,547,249, issued Apr. 15, 2003, titled
`"Monolithic Series/Parallel LED Arrays Formed On Highly
`Resistive Substrates," and incorporated herein by reference.
`In some embodiments, the individual LEDs in array 3 may
`be interconnected by a combination of interconnects formed
`on array 3 and interconnects formed on or within submount
`2. In embodiments where all or a portion of the interconnects
`are formed on array 3, only a portion of the LEDs in array
`3 may be electrically and physically connected to submount
`3.
`
`[0021] Series interconnection reduces the voltage drop
`across each LED to a level that does not exceed the
`maximum forward voltage of each LED. Excessive forward
`voltage can damage the LEDs irreversibly. Bonding pads 4
`and 5 are electrically connected to the positive and negative
`terminals of the array of LEDs and are used to electrically
`and physically connect the submount to a package. An
`example of a package is described below in reference to
`FIG. 7.
`
`[0022] FIG. 3 illustrates a 6x7 monolithic array of LEDs
`mounted on submount 2. The number of LEDs in the
`monolithic array may be selected to achieve a particular
`voltage drop across each device. The voltage across each of
`the individual LEDs in the array is the line voltage divided
`by the number of LEDs in series. The number of LEDs is
`chosen such that the maximum voltage across each indi(cid:173)
`vidual LED during the peak in the alternating current cycle
`is low enough so as to not damage the LEDs. At 120 RMS
`volts, the peak voltage will be about 180V. If the individual
`LEDs illustrated in FIGS. lB and 2 can tolerate a maximum
`forward voltage of 4.SV, at least 38 LEDs connected in
`series are required to prevent the voltage across each LED
`from exceeding the maximum tolerable voltage at the peak
`of the alternating current cycle. Sources with 240 RMS volts
`would require twice as many LEDs connected in series,
`while 60 RMS volt sources would require half as many
`LEDs connected in series. In some embodiments, the num(cid:173)
`ber of LEDs is selected to accommodate common alternat-
`
`ing current sources, such as lOOV in Japan, 120V in the
`United States, and 240V in Europe and parts of Asia. For
`example, for a 120 V alternating current circuit, the peak
`voltage may be about 180 V. If each of the LEDs in the 6x7
`array of LEDs illustrated in FIG. 3 has a forward voltage of
`3.5 V, each device will achieve the desired current density of
`50Ncm2 at 3.5x42=147 V, without exceeding the maximum
`current density of 150 Ncm 2 at the peak voltage of 180 V.
`In the embodiment illustrated in FIG. 3, a single monolithic
`array is mounted on a single submount. In some embodi(cid:173)
`ments, more than one monolithic array may be mounted on
`one or more submounts, in order to achieve the desired
`maximum voltage drop across each LED in the arrays.
`
`[0023] FIGS. 4 and 5 illustrate two examples of circuits
`that may be implemented in the device illustrated in FIG. 3.
`FIG. 4 illustrates a device with an LED array connected in
`series with an alternating current source, without any recti(cid:173)
`fying and filtering circuitry for converting the alternating
`voltage to a direct voltage. The LEDs are only on during that
`portion of the positive voltage half of each cycle of the
`alternating current where there is enough voltage to turn on
`the LEDs. Thus, at 60 Hz, the LEDs turn on 60 times per
`second.
`
`[0024] FIG. 5 illustrates an LED array and a full wave
`bridge rectifier for rectifying the alternating current source.
`The full wave bridge rectifier can be an external component
`or integrated into the submount, as described below. An
`optional capacitor filters the rectified voltage to provide
`nearly direct current to the LED array. Driving the LEDs
`with a near-direct current source is common and an efficient
`drive waveform. When the line voltage drops below the
`turn-on voltage for the LEDs during the alternating current
`cycle, current is supplied from the capacitor rather than from
`the line. When the line voltage rises above the turn-on
`voltage of the LEDs, the capacitor charges. In some embodi(cid:173)
`ments of the invention, rectifying and filtering circuitry, such
`as the capacitor and full wave bridge rectifier illustrated in
`FIG. 5, is formed in submount 2 shown in FIG. 3. Sub(cid:173)
`mount 2 may be, for example, a silicon chip. The circuitry
`other than the LED array illustrated in FIG. 5 can be formed
`on and/or in submount 2 using conventional integrated
`circuit fabrication techniques.
`
`[0025]
`In some embodiments of the devices illustrated in
`FIGS. 4 and 5, one or more Zener diodes may be included
`in series with the LED array to control the voltage drop
`across the LED array, as illustrated in FIG. 6. The Zener
`diodes may be formed in submount 2. In addition, submount
`2 may contain additional circuitry such as electrostatic
`discharge protection circuitry.
`
`[0026] FIG. 7 is an exploded view of a packaged light
`emitting device. A heat-sinking slug 100 is placed into a
`leadframe 105. The leadframe 105 may be, for example, a
`filled plastic material molded around a metal frame that
`provides an electrical path. Slug 100 may include an
`optional reflector cup 102. The light emitting device array
`and submount 104, which may be any of the devices
`described herein, is mounted on slug 100. Bonding pads 4
`and 5 on submount 2 (FIG. 3) are electrically connected to
`leads 106 by, for example wire bonding. An optical lens 108
`may be added.
`
`[0027]
`In some embodiments, one or more wavelength
`converting layers are formed over the LEDs to create white
`
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`Oct. 21, 2004
`
`3
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`light. For example, blue LEDs may be used with a yellow
`wavelength converting layer, or with a red wavelength
`converting layer and a green wavelength converting layer, in
`order to create white light. Similarly, UV LEDs may be used
`with red, blue, and green wavelength converting layers to
`create white light. The wavelength converting layers may
`be, for example, any suitable phosphors, and may be depos(cid:173)
`ited over each of LEDs 7 in array 3 (FIG. 3) by screen
`printing or electrophoretic deposition, or suspended in an
`encapsulant and injected into the space between device 104
`and lens 108 in FIG. 7. Individual LEDs in the monolithic
`array may be covered with different wavelength converting
`materials.
`
`[0028] FIG. 9 illustrates a monolithic LED array in a
`package such as an Edison base, designed to be screwed into
`a conventional light bulb socket. Submount 2 is connected
`by wire bonds 92 to the contacts in base 91. Base 91 can be
`screwed into a conventional light bulb socket. A glass bulb
`90 may be positioned over base 91. FIG. 10 illustrates a
`monolithic LED array in a bi-pin base such as an MR16
`base, designed to be plugged into a conventional wall
`socket. Submount 2 is connected by wire bonds 92 to
`contacts connected to the two prongs of plug 93. In the
`devices illustrated in FIGS. 9 and 10, any necessary cir(cid:173)
`cuitry additional to array 3, such as electrostatic discharge
`protection circuitry, or rectifying and filtering circuitry, may
`be formed within submount 2.
`
`[0029] Monolithic arrays of LEDs capable of operating
`with an alternating current source may offer several advan(cid:173)
`tages. First, the use of a single monolithic array on a single
`submount simplifies building and packaging the device
`since it is only necessary to align, mount, and connect a
`single chip to the submount, rather than separate, individual
`devices. In addition, the lack of rectifying or filtering
`circuitry (or the integration of rectifying and filtering cir(cid:173)
`cuitry into the submount) makes the devices small in size
`and simple to package, since external driver circuitry is not
`required. In addition, devices without rectifying and filtering
`circuitry in particular are simple and inexpensive to fabricate
`due to the lack of additional circuitry beyond the LED array.
`
`[0030] Having described the invention in detail, those
`skilled in the art will appreciate that, given the present
`disclosure, modifications may be made to the invention
`without departing from the spirit of the inventive concept
`described herein. For example, though some examples
`describe III-nitride devices, devices made from other mate(cid:173)
`rials systems, such as III-phosphide, III-arsenide, or II-VI
`materials may be used. Therefore, it is not intended that the
`scope of the invention be limited to the specific embodi(cid:173)
`ments illustrated and described.
`
`1. A device comprising:
`
`a submount; and
`
`a plurality of light emitting diodes formed on a single
`substrate, the plurality of light emitting diodes being
`mounted on the submount and connected in series;
`
`wherein the plurality of light emitting diodes are directly
`connected to an alternating current source.
`2. The device of claim 1 wherein the plurality of light
`emitting diodes are connected in series by interconnects
`formed on a surface of the plurality of light emitting diodes.
`
`3. The device of claim 1 wherein the plurality of light
`emitting diodes are connected ill series by interconnects
`formed on or in the submount.
`4. The device of claim 1 wherein the submount comprises
`a silicon integrated circuit.
`5. The device of claim 1 wherein at least one of the light
`emitting diodes is a III-nitride light emitting diode.
`6. The device of claim 5 wherein the III-nitride light
`emitting diode comprises a portion of the substrate and a
`plurality of semiconductor device layers, and wherein the
`III-nitride light emitting diode is mounted on the submount
`such that the plurality of semiconductor device layers are
`disposed between the submount and the portion of the
`substrate.
`7. The device of claim 1 further comprising at least one
`wavelength converting material overlying at least one of the
`light emitting diodes.
`8. The device of claim 7 wherein light emitted from the
`light emitting diodes mixed with light emitted from the at
`least one wavelength converting material appears white.
`9. The device of claim 1 further comprising:
`
`a plurality of leads electrically connected to the sub(cid:173)
`mount; and
`
`a lens overlying the plurality of light emitting devices;
`
`wherein the alternating current source is connected to the
`plurality of leads.
`10. The device of claim 1, wherein alternating current
`source has a peak voltage of at least 100 volts.
`11. The device of claim 1, wherein the alternating current
`source has an rms voltage of at least 120 volts.
`12. The device of claim 1, wherein the submount is
`connected to a base capable of being connected to a light
`bulb socket.
`13. The device of claim 12, wherein the base is an Edison
`base.
`14. The device of claim 1, wherein the submount is
`connected to a base capable of being connected to an
`alternating current outlet socket.
`15. The device of claim 14, wherein the base is an MR16
`base.
`16. A device comprising:
`
`a submount; and
`
`a plurality of light emitting diodes formed on a single
`substrate, the plurality of light emitting diodes being
`mounted on the submount and connected in series;
`
`a rectifying and filtering circuit formed in the submount
`and connected to the plurality of light emitting diodes.
`17. The device of claim 16 wherein at least one of the light
`emitting diodes is a III-nitride light emitting diode.
`18. The device of claim 17 wherein the III-nitride light
`emitting diode comprises a portion of the substrate and a
`plurality of semiconductor device layers, and wherein the
`III-nitride light emitting diode is mounted on the submount
`such that the plurality of semiconductor device layers are
`disposed between the submount and the portion of the
`substrate.
`19. The device of claim 16 further comprising at least one
`wavelength converting material overlying at least one of the
`light emitting diodes.
`
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`Oct. 21, 2004
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`4
`
`20. The device of claim 19 wherein light emitted from the
`light emitting diodes mixed with light emitted from the at
`least one wavelength converting material appears white.
`21. The device of claim 16 wherein the rectifying and
`filtering circuit comprises a capacitor.
`22. The device of claim 16 wherein the rectifying and
`filtering circuit comprises a full wave bridge rectifier.
`23. A method of operating a light emitting device, the
`method comprising:
`
`providing a plurality of semiconductor light emitting
`diodes formed on a single substrate and connected in
`series; and
`
`supplying an alternating current source to the plurality of
`semiconductor light emitting diodes.
`24. The method of claim 23 wherein the plurality of
`semiconductor light emitting diodes are physically mounted
`on and electrically connected to a submount, and supplying
`an alternating current source to the plurality of semiconduc(cid:173)
`tor light emitting diodes comprises supplying an alternating
`current source to the submount.
`
`* * * * *
`
`IPR PAGE 13