Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Reexamination Certificate
1998-08-31
2002-01-22
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S100000, C257S098000, C257S089000, C313S512000
Reexamination Certificate
active
06340824
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a semiconductor light emitting element, semiconductor light emitting device, image display device, and so on. More specifically, the invention relates to a semiconductor light emitting element, semiconductor light emitting device, image display device, and any other elements and devices configured to prevent external leakage of primary light emitted from a light emitting layer and to thereby waveform-convert it into secondary light and extract it with a remarkably high efficiency.
Semiconductor light emitting elements and various types of semiconductor light emitting devices using same have various advantages, such as compactness, low power consumption and high reliability, and are used in progressively wider applications, such as indoor and outdoor display panels, railway and traffic signals, car-borne signal illuminators, which are required to be highly luminous and highly reliable.
Among these semiconductor light emitting elements, those using gallium nitride compound semiconductors are being remarked recently. Gallium nitride compound semiconductors are direct-transitional III-V compound semiconductors which can efficiently emit light in relatively short wavelength ranges.
Throughout the present application, the “gallium nitride compound semiconductor” pertain to III-V compound semiconductors expressed by B
x
In
y
Al
z
Ga
(l−x−y−z)
N (0≦x≦1, 0≦y≦1, 0≦z≦1) and to any mixed crystal which includes phosphorus (P) or arsenic (As), for example, as group V species in addition to N in the above-mentioned chemical formula.
Gallium nitride compound semiconductors are remarked as hopeful materials of LEDs (light emitting diodes) and semiconductor lasers because the band gap can be changed from 1.89 to 6.2 eV by controlling the mole fractions x, y and z in the above-mentioned chemical formula. If highly luminous emission is realized in short wavelength ranges of blue and ultraviolet, the recording capacities of all kinds of optical discs will be doubled, and full color images will be realized on display devices. Under such and other prospects, short wavelength light emitting elements using gallium nitride compound semiconductors are under rapid developments toward improvements in their initial characteristics and reliability.
Structures of conventional light emitting elements using gallium nitride compound semiconductors are disclosed in, for example, Jpn. J. Appl. Phys., 28 (1989) p.L2112; Jpn. J. Appl. Phys., 32(1993) p.L8; and Japanese Patent Laid-Open Publication No. 5-291621.
FIG. 141
is a cross-sectional view schematically showing a conventional semiconductor light emitting element. The semiconductor light emitting element
6100
shown here is a gallium nitride semiconductor light emitting element. The light emitting element
6100
has a multi-layered structure of semiconductors stacked on a sapphire substrate
6120
, namely, a buffer layer
6140
, n-type contact layer
6160
, n-type cladding layer
6118
, light emitting layer
6120
, p-type cladding layer
6122
and p-type contact layer
6124
which are stacked in this order on the sapphire substrate
6120
.
The buffer layer
6140
may be made of n-type GaN, for example. The n-type contact layer
6160
has a high n-type carrier concentration to ensure ohmic contact with the n-side electrode
6134
, and its material may be GaN, for example. The n-type cladding layer
6118
and the p-type cladding layer
6122
function to confine carriers within the light emitting layer
6120
, and their refractive index must be lower than that of the light emitting layer
6120
. The light emitting layer
6120
is a layer in which emission occurs due to recombination of electric charges injected as a current into the light emitting element.
The light emitting layer
6120
may be made of undoped InGaN, for example, and the cladding layers
6118
and
6122
may be made of AlGaN having a larger band gap than the light emitting layer
6120
. The p-type contact layer
6124
has a high p-type carrier concentration to ensure ohmic contact with the p-side electrode
6126
, and its material may be GaN, for example.
Stacked on the p-type contact layer
6124
is the p-side electrode
6126
which is transparent to the emitted light. Stacked on the n-type contact layer
6160
is the n-side electrode
6134
. Bonding pads
6132
of Au are stacked on these electrodes, respectively, so that wires (not shown) for supplying a operating current to the element be bonded. The surface of the element is covered by the protective films
6130
and
6145
of silicon oxide, for example.
The conventional light emitting element
6100
is so configured that light emitted from the light emitting layer be directly extracted externally, and involved the problems indicated below.
One of the problems lies in variable emission wavelengths caused by structural varieties of light emitting elements. That is, semiconductor light emitting elements, even when manufactured under the same conditions, are liable to vary in quantity of impurities and in thicknesses of respective layers, which results in variety in emission wavelength.
Another problem lies in changes in emission wavelength depending upon the operating current. That is, emission wavelength of a semiconductor light emitting element may change depending upon the quantity of electric current supplied thereto, and it was difficult to control the emission luminance and emission wavelength independently.
Another problem lies in changes in emission wavelength depending upon the temperature. That is, when the temperature of a semiconductor light emitting element, particularly of its light emitting layer, changes, the effective band gap of the light emitting layer also changes, and causes an instablility of the emission wavelength.
As explained above, in conventional semiconductor light emitting elements, it was difficult to entirely control varieties in structure, temperature and electric current and to thereby limit changes in emission wavelength within a predetermined range, such as several nm(nanometers).
Conventional semiconductor light emitting devices involved an additional problem, namely, materials and structures of semiconductor light emitting elements used therein had to be determined and changed appropriately in accordance with desired emission wavelengths, such as selecting AlGaAs materials for emission of red light, GaAsP or InGaAlP materials for yellow light, GaP or InGaAlP materials for green light and InGaN materials for blue light.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a semiconductor light emitting element and a semiconductor light emitting device which are highly stable in emission wavelength and can wavelength-convert light with a high conversion efficiency in a wide wavelength range from visible light to infrared band.
According to the first aspect of the invention, there is provided a semiconductor light emitting element and a light emitting device comprising a wavelength converter located adjacent to a light extraction end of the light emitting layer to absorb the primary light emitted from the light emitting layer and to release secondary light of a second wavelength different from the first wavelength.
The first aspect of the present invention is embodied in the above-mentioned mode, and attains the effects explained below.
Light from the light emitting layer is not extracted directly but converted in wavelength by a fluorescent material. Therefore, it is prevented that the emission wavelength varies with varieties of manufacturing parameters of the semiconductor light emitting elements, drive current, temperature and other inevitable factors. That is, the invention realizes remarkable stability of emission wavelengths and makes it possible to control the emission luminance and the emission wavelength independently.
The fluorescent material may include a plurality of different materials to obtain a plurality of different emission wavelengths. For example, by appropriately mixing d
Furukawa Chisato
Komoto Satoshi
Konno Kuniaki
Nitta Koichi
Sugawara Hideto
Hogan & Hartson L.L.P.
Kabushiki Kaisha Toshiba
Tran Minh Loan
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