Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
1999-03-01
2001-12-11
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S098000, C257S088000
Reexamination Certificate
active
06329676
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a flat panel light emitting devices, and more particularly relates to flat panel light emitting devices having substantially uniform light emission and capable of varying both luminance and chrominance.
BACKGROUND OF THE INVENTION
Visible light sources—and usually white or near-white light sources—are demanded for illumination throughout the world. Fluorescent, incandescent, and other lamps and bulb have become ubiquitous for illumination. In addition, light-emitting diodes, or LEDS, have become very common for single or limited wavelength forms of illumination such as laser pointers, indicator lamps and so on.
Recently, considerable attention has been given to the potential for using LEDs as a white light source because the power consumption of LEDs is, generally, lower than that of fluorescent lamps or bulb lamps. However, it has been difficult to use such LEDs as sources for white light due to the unavailability highly efficient blue LEDs, Recent progress in group III-nitride semiconductor systems has enabled the development of the InGaN/GaN DH (
D
ouble
H
eterojunction) structure, which does permit the high quantum efficiency emission of blue light. As a result, blue LEDs are now commercially available.
Obtaining white light from blue LEDs has been problematic. One effective approach to obtain white light is to mix blue and yellow light. Referring first to
FIGS. 1 and 2
, there is first shown in schematic form the emission of white light using a prior art blue LED combined with a yellow phosphor (S. Nakamura, SPIE, Vol. 3002, pp26-35, 1997.) In the prior art arrangement shown in
FIG. 2
, YAG (yttrium aluminum garnet) phosphor
1
is formed on the top of a blue LED
2
having an InGaN/GaN DH structure. In this structure, current is injected from the lead frame
3
with a reflector cup
4
to the LED chip
2
through a conductor
5
. Then the current flows from the LED chip
2
to the lead frame
6
. The LED chip
2
is activated by the injected current and emits blue light. The YAG phosphor
1
is excited by the blue light from the LED chip
2
and emits yellow fluorescence. The mixture of the blue emission from the LED chip
2
and the yellow emission from the phosphor
1
results in a white emission. In order to improve the external emission efficiency of the white light, an epoxy lens
7
is used. Thus white light is obtained by this structure.
However, to obtain a white light source panel having a large area size, it is necessary to use large number of LEDs. For the prior art design shown in
FIGS. 1 and 2
, the driving circuit becomes complicated and the fabrication cost becomes high. Moreover, as shown in
FIG. 3
, the luminescence pattern of the panel becomes dot-like when a large number of conventional LEDs are gathered into a single panel. This results from the “point source” nature of conventional LED's. Therefore, it is difficult to use a large number of prior art LEDs to provide a white light panel having uniformly distributed emission power. Further, while a thin lighting panel is preferred, in the art shown in
FIGS. 1-3
the thickness of the panel becomes large because the thickness of the panel must be larger than the height of the LEDs. These problems must be solved to obtain a thin white light source panel having a large area with uniform white light emission.
SUMMARY OF THE INVENTION
The present invention overcomes many of the limitations of the prior art by providing a new and novel LED structure which permits the fabrication of a thin, large area flat panel polychromatic light source having substantially uniform emission power distribution. Various alternative embodiments will be described, including some in which the LED structure of the invention may be combined with phosphors to generate polychromatic light. While some aspects of the invention are intended to provide polychromatic light emissions, the emission of white light will hereinafter generally be used as exemplary of the polychromatic emissions achievable with the present invention.
In general, the LED structure of the present invention involves a pn-junction LED fabricated from a plurality of cladding layers and an active layer deposited above either a conductive or insulating substrate. Each pn-junction LED is formed as a ridge or strip which essentially extends outward in a direction substantially orthogonal to the substrate. A plurality of such ridges are fabricated in parallel on a single substrate, with phosphor deposited in the intermediate channels formed between the ridges. With proper placement of a pair of electrodes, the LED emits light continuously along the entire ridge/channel boundary, rather than in the point source arrangement of the prior art.
In some embodiments, the light may be generated as a combination of a blue LED and a yellow phosphor. In such embodiments, the LED array typically comprises a plurality of ridges where InGaN material is used for the active layer, together with a YAG phosphor deposited in the intermediate channels between the InGaN material. In other embodiments, the polychromatic light may be generated by means of a pluraltiy of ultraviolet LEDs such as those resulting from AlGaN, formed into ridges, together with appropriate phosphors that emit red, green and blue light, respectively, formed alternately in the intermediate channels between the LED ridges. The proportion of red, green and blue components may be varied to permit polychromatic emissions. In addition, by selective driving of the electrodes, certain of the LED ridges may be turned on or off to permit dimming of the panel as a whole or polychromatic variation. As will be appreciated from the foregoing, typically although not necessarily, the active layer is formed as a quantum well.
By providing continuous emission along the entirety of each of the channels, a continuous and uniform emission of polychromatic light is provided. By adjusting the thickness of the active layer, the emission efficiency of the LED may be optimized. Further, by varying the ratio of the active layer window region relative to the phosphor, the color of the light may be adjusted for the particular application. In this instance, “window region” refers to the opening through which electrons from a first electrode enter the active layer.
The structures of the various embodiments may be fabricated in a number of ways, depending on whether a conductive or insulative substrate is preferred, and also depending on whether insulating structures are to be formed to control the sizes of the window regions. Various substrates may be used: gallium nitride (GaN), silicon (Si) and silicon carbide (SiC) are examples of conductive substrates, while sapphire is one example of an insulating substrate. The invention is not limited to any particular substrate.
In a first fabrication process involving a conductive substrate, a GaN substrate is provided. A GaN buffer layer is formed thereon, and above that is formed a first and a second GaN cladding layer. An active layer, typically of InGaN material, is formed above the second cladding layer, and a third cladding layer is thereafter formed above the active layer. Each of these layes is typically formed by metalorganic chemical vapor deposition (“MOCVD”) growth, In a typical arrangement, the substrate and the first and second cladding layers are n-type material, while the third cladding layer is p-type material. The use of n-type or p-type material may be switched if desired as long as layers of like type remain of like type.
Following the MOCVD or MBE growth of the various layers, the structure is dry etched to form ridges. Phosphor is then deposited in the channels between the ridges. Electrodes are then formed by evaporation on the bottom of the substrate and at the top of the ridges. Depending on the particular effects desired, the electrodes at the top of the ridges may be all connected, or may be selectively connected. In the event an insulating substrate is used, the first cladding layer may be revealed by etching, an
Baba Takaaki
Takayama Toru
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