FIELD OF THE INVENTION

This invention relates to the field of semiconductor light emitting devices, and in particular to a light emitting device having addressable segments/pixels, with different phosphors over different segments.

BACKGROUND OF THE INVENTION

The use of semiconductor light emitting devices for general illumination purposes continues to increase, particularly for lamps that emit white light. The suitability of a lamp for a particular application is dependent upon a number of factors, including the lamp's "color temperature" and "color rendering index".

The color temperature is a measure of the apparent warmth of a white light, a white light with a high blue content being considered a 'cool' light, and a white light with a high red content being considered a 'warm' light. Color temperature is typically expressed as a "correlated color temperature" (CCT) corresponding to the light emitted by an ideal black- body radiator at a given temperature (typically in the range of 2,500- 10,000°K).

The color rendering index (CRI) is a measure of the lamp's ability to illuminate an object such that the perceived color of the object is the same as the perceived color when the object is illuminated by a reference light source, typically daylight. The CRI is dependent upon the spectrum of light produced by a light source and how well its spectrum matches the spectrum of light produced by the sun (after traveling through the earth's atmosphere).

Semiconductor white-light lamps combine a plurality of light emissions to produce a composite light output that appears to be white. Typically, emitters of light in the red, green, and blue wavelengths are used to produce this composite white light, with wavelength conversion material, such as phosphors, often being used to produce emissions of a particular desired wavelength. USPA 2012/0001555, "TUNABLE WHITE COLOR METHODS AND USES THEREOF", published 5 January 2012 for Tu et. al (hereinafter Tu), for example, discloses a white-light semiconductor lamp comprising a plurality of blue LEDs, green LEDs, and red LEDs, wherein the blue and red LEDs emit their light directly, while the green LEDs use a wavelength conversion material that converts light from blue LEDs into a green light. The current to each of the different colored LEDs is controlled to produce a composite white-light output at a desired color temperature.

The use of LEDs that emit different colors, however, typically requires the use of different semiconductor materials. For example, a conventional blue LED will use InGaN as the active region, whereas a conventional red LED will use AlInGaP as the active region. The different active region materials will provide different bandgaps, and thus different drive voltages will be required for the red and blue LEDs, leading to more complicated driver mechanisms than for a light emitting device comprising the same semiconductor active region material.

Additionally, when semiconductor materials with different bandgaps are used, the temperature dependence of these LEDs will be different. In the invention of Tu, cited above, the red LED will have a much stronger temperature dependence than the blue LED, leading to a strong color shift of the composite white LED with changing LED drive current and temperature.

The discovery of broad emission phosphors, however, has allowed for the

development of semi-conductor white-light lamps having a CRI of over 70. A mixture of phosphors that produce different shades of colors when illuminated by a common light source can be used to produce a less 'spikey' output spectrum. For example, a composite mixture of Ce-doped garnets, ECAS, (Ba,Sr) ₂ Si ₅ Ng, and so on, can be used to generate, respectively, yellow/green, green, red, amber, etc. light from a blue LED to produce a white- light lamp with a high CRI. Other phosphor compounds may include, for example, SiAlONs _, silicates, oxynitrides, sulfides. In like manner, the (Ba,Sr) ₂ Si ₅ Ng compound, above, may be generalized to M ₂ Si ₅ N8, where M corresponds to metals in addition to Ba and Sr.

FIGs. 1A-1B illustrate an example semiconductor light emitting device that is coated with a mixture of a variety of phosphors to produce a composite white light with a high CRI. Three phosphors 101 (light shaded dots), 102 (medium shaded dots), and 103 (dark shaded dots) are illustrated, although any number of phosphors may be included in the mixture.

As illustrated in FIG. IB, the example light emitting device includes an active layer 120 that is sandwiched between n-type and p-type layers 110, 130. Either type may be the 'upper' layer 110, the other being the 'lower' layer 130, but for ease of understanding and explanation, the upper layer 110 is assumed herein to be the n-type layer, and the lower layer 130 is assumed to be the p-type layer. A p-contact 135 allows and external connection to the p-layer 130, and an n-contact 115 allows an external connection to the n-layer 110 via one or more connections 117 through the p-layer 130 and the active layer 120. Other layers may also be present, as is well known in the art, but are not illustrated for ease of explanation and understanding.

When the active layer 120 is energized, light is emitted from the active layer 120 and will enter the phosphor layer 100. Depending upon the concentration and mix of phosphors 101, 102, 103 in the phosphor layer 100, some or all of the light emitted by the active layer 120 will be absorbed by the phosphors 101, 102, 103, with a corresponding emission from these phosphors at different wavelengths from the wavelength of the light emitted by the active layer, and different from each other. The light from the active layer 120 that is not absorbed as it travels through the phosphor layer is emitted at its original wavelength.

Because a larger number of different wavelengths are emitted, compared to the tricolor emissions of conventional semiconductor white-light emitting devices, an output spectrum may be produced that is more similar to daylight, thus achieving a higher color rendering index. However, the color temperature of the composite output will be determined by the particular phosphor combination and the emissions of the active layer of the particular underlying light emitting device.

SUMMARY OF THE INVENTION

It would be advantageous to provide a light emitting device with a controllable color temperature and a high color rendering index. It would also be advantageous to provide this light emitting device at relatively low cost.

To better address one or more of these concerns, in an embodiment of this invention, a light emitting device is produced with multiple pixels, some or all of which are individually controllable. Each pixel may be coated with a different phosphor combination, or uncoated. By controlling the current to pixels that produce different colors, or bands of color, the color temperature of the composite light output can be controlled. By using a variety of different colors, or different bands of colors, a high color rendering index can be achieved. Because the individual pixels are situated on a common light emitting device, the cost of producing these light emitting devices will likely be lower than the cost of producing multiple individual light emitting devices. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein:

FIGs. 1A-1B illustrate an example prior art light emitting device with a composite of multiple phosphors.

FIGs. 2A-2E illustrate example light emitting devices with individually controllable pixels of differing colors.

FIGs. 3A-3F illustrate an example fabrication of light emitting devices with individually controllable pixels of differing colors.

FIG. 4 illustrates an example flow diagram for fabricating light emitting devices with individually controllable pixels of differing colors.

FIG. 5A-5B illustrate example lamps that incorporate a light emitting device with

individually controllable pixels of differing colors.

FIG. 6 illustrates an example multi-pixel light emitting device with selectable sub-pixels.

Throughout the drawings, the same reference numerals indicate similar or

corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. In accordance with one aspect of this invention, a light emitting device comprises a single light emitting structure having multiple light emitting areas, herein termed pixels, wherein different colors are produced by different pixels or sets of pixels. In accordance with a further aspect of this invention, some or all of the different pixels or sets of pixels are individually controllable. In the example embodiments, the choice and control of the different colors is intended to provide a white light over a wide range of color temperatures, although one of skill in the art will recognize that the principles presented herein may also be used to provide a lamp with a controlled light output of any color.

FIGs. 2A-2E illustrate example light emitting devices with individually controllable pixels of differing colors. As contrast to the prior art light emitting device of FIGs. 1A-1B having a composite of phosphor compounds 101, 102, 103, each of the phosphor compounds 101, 102, 103 of the light emitting devices of FIGs. 2A-2E is situated in one or more discrete areas of the light emitting device, each discrete area being termed a pixel.

In FIGs. 2A-2B, phosphor compound 101 is situated in pixel 201; phosphor compound 102 is situated in pixel 202; and phosphor compound 103 is situated in pixel 203. In this example, another pixel 204 is defined that does not include a phosphor compound, thereby emitting the color produced by the active layer 120 of the light emitting device, herein termed the 'base' color. Although the example illustrates the use of three phosphor compounds 101-103 to provide four different colored pixels, one of skill in the art will recognize that any number of different phosphor compounds may be used, producing, for example, different shades of colors to provide a more uniform composite bandwidth.

In the examples presented herein, each pixel is similarly sized, for ease of illustration and understanding. One of skill in the art will recognize, however, that different sized pixels may also be used. For example, in each of the phosphor coated pixels 201-203, some amount of the base color may not be absorbed by the phosphors, and consequently emitted from these pixels 201-203. In the case wherein the amount of light of a particular color is related to the size of the pixel, the size of pixel 204 may be decreased based on the amount of light of the base color that is emitted through the other regions 201-203. In like manner, in conventional white-light emitting lamps using a combination of red, blue, and green LEDs, the proportion of red, blue, and green LEDs is typically 1:1:2; that is the quantity of green light required to create a white-light output is typically equal to the quantity of red and blue light combined. Accordingly, in an embodiment of this invention using red, blue, and green pixels, the green pixel may be twice the size of each of the red and blue pixels; or, twice as many equal- sized pixels may be configured to emit green light. One of skill in the art will recognize that a ratio of 1:1:2 may not give a color point that is exactly on the Blackbody locus (BBL) or at the desired color temperature. To adjust the color, the drive current of each pixel can be independently adjusted. This can be done by a variety of methods known to those skilled in the art, including, for example, individually driving each pixel at a different current, or adjusting a series resistance of each pixel. Such adjustments may be static for a device of fixed color temperature, or dynamic for a device in which the user is able to adjust the color temperature.

As illustrated in FIG. 2B, the light emitting device includes a p-type layer 130 and an active layer 120, both of which are common to all n-type regions. The n-type layer is partitioned into four discrete regions, two 210A, 210B of which are visible in the cross- section view of FIG. 2B. These discrete n-type regions 210A, 210B are electrically isolated from each other using any number of techniques common in the art. For example, photolithographic techniques may be used to distinguish between the regions 210A, 210B in which the n-type layer is to be situated from non-conducting areas 240. These non- conducting areas 240 may include a dielectric material, or may merely be voids between the areas 21 OA, 210B. Optionally, the non-conducting areas 240 may be formed by selective ion implantation of lanes that create the isolated regions 210A, 210B in a continuous n-type layer.

N-contacts 215A, 215B allows an external connection to the n-layer regions 210A, 210B respectively via one or more insulated connections through the p-layer 130 and the active layer 120.

FIG. 2C illustrates an example light emitting device that is partitioned into a 4x4 array of sixteen pixels. These sixteen pixels may include a plurality of pixels containing each of the phosphor compounds 101, 102, 103, or pixels containing other varieties of phosphor compounds, to provide an even greater distribution of color components forming the resultant light output of the light emitting device. In the case of multiple pixels of the same color, the example structure of FIG. 2C may provide a better 'blending' of the individual colors due to the spatial distribution of each color. Also in the case of multiple pixels of the same color, these pixels may be commonly connected to allow for a single control of all of the pixels of the same color. Optionally, in each of the example embodiments, optical structures may be provided that serve to disperse and intermix the individual colors. U.S. patent 6,547,416, "Faceted Multi-chip Package to Provide a Beam of Uniform White Light From Multiple Monochrome LEDs", issued 15 April 2003 to Michael Pashley and Thomas Marshall, for example, discloses a reflective structure that provides a mixing of colors based on reflections from multiple surfaces, and is incorporated by reference herein.

FIGs. 2D-2E illustrate another example embodiment comprising four pixels 201, 202, 203, 204 arranged in bars, rather than squares as illustrated in FIG. 2A. Such a configuration of pixels may be easier to fabricate than the configuration of FIG. 2A, as detailed further below with regard to the example fabrication of FIGs. 3A-3F, although the light output may require a more complex optical arrangement to blend the 'bars' of different color light.

Because the configuration of FIGs. 2D-2E allows for the structure of each of the pixels 201-204 to be illustrated in a single cross-section view (FIG. 2E), this configuration is used hereinafter as the example embodiment for further explanation and illustration of further aspects of this invention.

As in the example of FIG. 2B, the light emitting device includes a p-type layer 130 and an active layer 120 that are common to all n-type regions, and each of the pixels 201-204 include isolated n-type regions 210A, 210B, 210C, and 210D, as illustrated in the cross- section view of FIG. 2E. One or more contacts 235 provide for external connection to the common p-type layer 130, and individual contacts 215A, 215B, 215C, and 215D provide for external connection to each of the n-type regions 210A, 210B, 210C, and 210D, respectively. Because each of the pixels 201-204 include a different phosphor composition (including a null composition), the current provided to each of these pixels 201-204 will determine the luminance produced at the wavelengths associated with the base color and the phosphor combinations 101, 102, 103. Contacts 235 may be a single contact arranged to allow placement of individual contacts 215A-215D.

It is noted that each of the phosphor combinations 101, 102, 103 may comprise a single phosphor material or multiple phosphor materials. For example, one of the

combinations may comprise a mix of phosphors that provide various shades of green, another mix providing various shades of red, and so on. A single phosphor material per pixel may provide a more precise control of the color temperature, but may correspondingly increase the number of pixels required to achieve a high color rendering index, thereby increasing the costs associated with the fabrication and subsequent control of this increased number of pixels. FIGs. 3A-3F illustrate an example fabrication of light emitting devices with individually controllable pixels of differing colors.

FIGs. 3A-3B illustrate an example 4x4 array of light emitting devices 310, each device 310 including a set of four n-type regions 21 OA, 21 OB, 2 IOC, 210D. These devices 310 may be formed on a common growth substrate (not illustrated), then 'flipped' and adhered to a support substrate 350, after which the growth substrate is removed, as illustrated in FIG. 3B.

The cross section of FIG. 3B illustrates a void 320 between each of the devices 310, for ease of illustration and understanding, although one of skill in the art will appreciate that material, such as a dielectric compound, may extend between the devices 310, some or all of the material optionally being removed when the array is sliced/diced (singulated) to produce the individual devices 310.

FIG. 3C illustrates a cross-section of one of the devices 310, corresponding to the device illustrated in FIG. 2E situated on a support substrate 350 before the application of phosphor compounds 101, 102, 103. In some embodiments, the support substrate 350 is designed to be removed after singulation of the devices 310, exposing the contacts 215A-D and 235. In other embodiments the support substrate 350 may include circuitry (not illustrated) that is coupled to the contacts 215A-D and 235, this circuitry and support substrate 350, collectively termed a 'sub-mount', providing the particular form factors and arrangements of contacts for different lamp structures or lighting applications after singulation.

FIGs. 3D and 3E illustrate the array of light emitting devices 310 of FIGs. 3 A and 3B after application of phosphor coatings 301, 302, and 303 over select pixels of each of the devices 310. The coatings 301, 302, 303 may be, for example, compositions that include the aforementioned phosphor compounds 101, 102, 103. The phosphor coatings 301, 302, 303 may be applied using any of a number of methods common in the art, including but not limited to electrophoretic deposition, sedimentation, dispensing of phosphor mixed in a binder, screen printing of phosphor mixed in a binder, molding of phosphor mixed in a binder, or lamination of a phosphor-containing silicone film.

In the example embodiment of FIGs. 3D-3E, for ease of manufacture, wider bands of coatings 302 and 303 are illustrated, each wider band extending over two pixels of two adjacent devices 310. When wider bands of phospors are used, adjacent devices may be mirror images of each other with regard to the order of the phosphor coatings 301, 302, 303, and the contacts 215A-D. Assuming symmetry, when the substrate is sliced along slicing lines 330 to create singulated devices 300 and 300', for example, singulated device 300' is virtually indistinguishable from device 300. As illustrated in FIG. 3F, when the devices are singulated, each device 300 will have a single width band of each coating 301, 302, 303 over respective pixels 210C, 210D, and 210A, with pixel 210B being uncoated.

FIG. 3F illustrates a singulated light emitting device 300 with phosphor coatings after the substrate 350 is removed. In this embodiment, each of the pixels (n-type regions above a common active layer 120) 210A-210D are individually controllable by controlling the current provided to the corresponding contacts 215A-D to the n-type regions, with the common p- type region being coupled to one or more contacts 235. Typically, the contact 235 will be a continuous metal layer on the bottom surface of the device, with isolation areas surrounding each of the contacts 215A-D. Depending on the size of the device, multiple contacts 215A-D may be used to contact each of the n-type regions 210A-D. FIG. 4 illustrates an example flow diagram for fabricating light emitting devices with individually controllable pixels of differing colors.

At 410, a plurality of multi-pixel light emitting devices is provided on a substrate. This substrate may be temporary or permanently affixed to the light emitting devices. For example, the substrate may be a conventional or stretchable dicing tape that is used to support the devices through the singulation/dicing process, then removed; or, it may be a ceramic substrate with circuitry that facilitates connection to the devices after singulation.

At 420, the pattern for placement of the multiple phosphor compounds over the light emitting devices is defined, and at 430, the compounds are applied, as detailed above. At 440, the light emitting devices are singulated, and the temporary substrate, if any, is removed.

At 450, the singulated LED is mounted in the corresponding lamp system, with controller, as detailed further below. The controller may then be adjusted to provide a combination of color intensities that produce a composite light output having the desired light output characteristics, at 460. In some embodiments, this adjustment is performed during the manufacturing process, and the lamp is provided with this fixed adjustment; in other embodiments, the lamp system may allow for further adjustment by the user of the lamp.

FIGs. 5A-5B illustrate example lamp systems that facilitate the use of a multiple-pixel LED 510 to provide a selective mix of colors, including a white-light output at a selective color temperature. Terminals 520, 521 are provided for coupling the lamp to an external power source; optionally, an AC-DC converter may be included within the lamp system to facilitate use of the lamp in AC applications. One pole of the power source is coupled to the terminal 520 of the LED that is common to each of the pixels, and the other pole is coupled to a terminal 521 that is coupled to the LED via a means for controlling current to each pixel or set of pixels. In FIG. 5A, a controller 530 can be adjusted to control the current flow through each of the pixels of the LED 510, and in FIG. 5B a set of resistor elements 531-534 can be adjusted to control the current flow through each of the pixels of the LED 510.

Although the controller 530 of FIG. 5A may be configured to enable independent control of each pixel, alternative control strategies may be used. For example, a primary control may be provided that applies current to all of the pixels, to set the overall luminence output, with secondary controls of the individual pixels to 'fine tune' the output color. In like manner, the current may be controlled using a 'balance' control that distributes a constant total current among two or more pixels. Similarly, one of the pixels, such may receive a constant, non-adjustable current, and the other pixels are adjusted in relation to the light provided by this constant current pixel.

Depending upon the structure of the controller 530, a memory 540 may be provided for storing parameters based on the adjustment. For example, if the controller 530 uses a digital-to-analog converter to control the current to each pixel, the memory 540 may be configured to store the digital values that provide the desired color combination.

Alternatively, if the controller 530 includes mechanically adjustable elements that control the current to each pixel, the friction associated with the adjustable element may constitute a 'mechanical memory'.

In FIG. 5B, the current to each pixel may be controlled by varying the series resistance elements 531-534 in the current path of the pixel. In an embodiment of this invention, the resistor elements 531-534 may be sheet resistors, wherein the resistance of each resistor element 531-534 is adjusted by trimming the width of the element, a narrower width providing higher resistance. In an example embodiment, this trimming is performed during manufacture of the device, based on a comparison of the composite light output and the desired light output, using techniques common in the art. As with the controller 530 of FIG. 5A, alternative arrangements may be provided to change the relative luminance provided by each pixel of the LED 510. For example, one pixel may have a fixed series resistance, and the series resistance of the other pixels are adjusted relative to the luminance provided by the fixed series resistance. Depending upon the arrangement of the pixels of the LED, and the particular intended use of the lamp, an optical mixer 550 may be provided to enhance the mixing of the light produced by each of the pixels. As noted above, US patent 6,547,416 describes a multi- faceted reflector housing that serves to provide a collimated mix of light from multiple light emitting devices. Other techniques for enhancing the mixing of light from multiple sources are common in the art.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

For example, it is possible to operate the invention in an embodiment wherein a primary pixel is coated with a conventional multi-phosphor composite such as described with respect to FIGs. 1A-1B, and secondary pixels may be coated with the different phosphor compounds as detailed in this disclosure, and used to provide a 'fine tuning' of the light provided by the primary pixel.

In another embodiment, multiple sub-pixels may be associated with each particular color, and the light output intensity may be controlled by selective enabling/disabling of each sub-pixel, as illustrated in FIG. 6. In this example, the surface areas of three sub-pixels A, B, C of each pixel 610, 620, 630, 640 are in a ratio of 1:2:4. By selectively enabling the sub- pixels A, B, C of a particular pixel 610, 620, 630, 640, eight different intensities may be produced by that pixel. In like manner, four sub-pixels per color with an area ratio of 1:2:4:8 will allow for enabling 16 different intensities of each color, and so on. In such embodiments, the controller 530 may be embodied as a digital switch, eliminating the need for providing an adjustable analog current source for each color.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.