Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – Plural light emitting devices
Reexamination Certificate
1999-09-21
2003-08-19
Baumeister, Bradley W. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
Plural light emitting devices
C257S103000, C257S190000, C257S015000, C257S018000, C257S022000
Reexamination Certificate
active
06608330
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Technical Field of the Invention
This invention relates to a light emitting device made of nitride compound semiconductor of III-V group (in general, expressed in a formula of In
x
Al
y
Ga
1-x-y
N, wherein 0<X, 0≦Y, X+Y≦1). And especially it relates to a light emitting device including an active layer depositing at least two kind of well layers emitting different colors of light and mixing colors thereby emitting light of another color such as white with a desired color rendering property.
2) Description of Related Art
Since light emitting devices which emit light of red, green, and blue color (so-called RGB) with quite strong intensity have been developed so far, RGB light emitting devices of high luminous intensity type are now manufactured and available. It is noted that the color of light emitted by the nitride compound semiconductor device can be varied from ultraviolet region through red region, by adjusting the composition ratio thereof.
Meanwhile, because of excellent characteristics of the light emitting devices such as high luminous intensity, downsizing, and high credibility, applications thereof have been rapidly expanding in the technical fields, for example, for light sources for indicators mounted on automobiles, light sources for backlights of liquid crystal displays, and other luminaires.
In those applications, especially since white light emitting devices are easiest and most comfortable to human eyes, such white light emitting devices are highly desired. There are basically two approaches to obtain such white light. Three of RGB or two of blue and yellow (that is complementary color of blue) color light emitting diode chips are mounted on a same stem to mix them to produce the white light. Alternatively fluorescer emitting fluorescent yellow light when absorbing blue light are applied on the blue color emitting diode chips to produce white light.
However those approaches to achieve white light have some problems. When mounting a plurality of light emitting diode chips on the same stem to produce white light, those chips have to be disposed as closely as possible each other in order to improve a color mixing property. (The color mixing property is referred herein denotes the extent to which the light from the device can be evenly seen as single white light.) But the finite sizes of the chips limit the improvement. Further when utilizing chips of different semiconductor based materials, they have different forward voltages and distinct characteristics dependent upon the temperature.
Also when utilizing fluorescer to produce the white light, a step for applying fluorescer on the chip is required, which is a complicated step. Further when combining the blue light emitting chip and fluorescer emitting fluorescent yellow light by absorbing the blue light to obtain white light, the luminous efficiency is theoretically reduced, in comparison with the combination of a plurality of different color chips.
Therefore those conventional approaches are not yet satisfactory to replace the current light sources with the semiconductor light emitting devices, and it is desired to provide the light emitting devices with higher luminous efficiency and higher luminous intensity that are capable of emitting white light. This invention is directed to providing such light emitting device that can emit white light by itself.
SUMMARY OF THE INVENTION
The first object of the present invention is to provide a light emitting device including at least two well layers made of nitride compound semiconductor emitting different color light due to different In composition ratios to so that white light can be obtained by mixing the different color light.
The second object of the present invention is to provide a light emitting device including at least one of first and second well layers where the second well layer has rougher surface than that of the first well layer to improve a luminous efficiency.
The third object of the present invention is to provide a light emitting device further including a first barrier layer containing Al and second barrier layer substantially not containing Al, which are formed on each well layer to reduce the forward voltage thereby improve the luminous efficiency.
The light emitting device according to an aspect of the present invention, comprises an active layer of a multiple quantum well structure, sandwiched between an n-type semiconductor layer and a p-type semiconductor layer; the active layer including, at least one of first well layers made of nitride compound semiconductor containing In, and at least one of second well layers made of nitride compound semiconductor containing In, the second well layer emitting light having a principal peak wavelength longer than that of the first well layer.
When the principal peak wavelength of the first and second well layers are selected such that they are complementary colors each other, and thus a white color light can be obtained by mixing the two color lights.
The light emitting device according to another aspect of the present invention, is characterized in that the second well layer is disposed between the first well layer and the p-type semiconductor layer.
In general, it is very difficult to grow the nitride compound semiconductor containing In. As the In composition ratio within the well layer is higher, the light emitted by the well layer has a longer wavelength and its crystallinity is worse so that the luminous efficiency is also reduced. This property is remarkable when the In composition ratio is not less than 0.05. In fact, the diffusion length of the hole is very short within the nitride compound semiconductor of quantum well structure. Therefore it is, preferable to deposit the second well layer adjacent to the p-type semiconductor layer that provide hole because the second well layer has less the luminous efficiency due to the more In composition ratio and worse crystallinity. In other words, this is because the hole-electron recombination possibility near the p-type semiconductor layer is higher than that far from the p-type semiconductor layer. Thus the luminous efficiency of the device can be improved by depositing the second well layer between the first well layer and the p-type semiconductor layer.
The light emitting device according to another aspect of the present invention, is characterized in that growth numbers of the first and second well layers are adjusted to control the luminous intensity ratio of light emitted by the first well layer over light emitted by the second well layer. Thus a desired color rendering property can be obtained. (The color rendering property is referred herein as to one of the effects of a light source under which an object can be seen.)
So far, most of light sources rather than light emitting semiconductor devices have been also developed to realize many kinds of color rendering property: For instance there are many types of fluorescent lamps illuminating with white light of different nuance. Therefore the light emitting device according to the present invention is desired to easily adjust its color rendering property to replace the conventional light sources.
According to the present invention, its color rendering property, in other words, the ratio of luminous intensity emitted by each well layer is adjusted by controlling the ratio of the growth numbers of the first and second well layers. Rather than this approach, as shown in Japanese Laid-Open publication 10-22525, the ratio of luminous intensity emitted by each well layer can be adjusted by controlling thickness of each well layer. However it has a problem. When its active layer is made of quantum well structure, as the well layer is thinner, although the luminous intensity is stronger to some extent, the peak wavelength shifts to shorter side due to the quantum size effect. Meanwhile as the well layer is thicker, although the luminous intensity is weaker to some extent, the peak wavelength shifts to longer side. That is to say, it is difficult to achieve light ha
Baumeister Bradley W.
Nichia Corporation
Volentine & Francos, PLLC
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