Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular dopant concentration or concentration profile
Reexamination Certificate
1999-05-25
2001-10-23
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular dopant concentration or concentration profile
C257S103000, C257S094000, C257S096000
Reexamination Certificate
active
06307219
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light-emitting device comprising a gallium-nitride-group compound semiconductor, used in opto-devices such as a light-emitting diode, a laser diode, etc. More specifically, a semiconductor light-emitting device, with which the operating voltage can be lowered, at the same time, the efficiency of light emission can also be improved.
Gallium-nitride-group compound semiconductors have been increasingly used as semiconductor material for the visible light-emitting devices and the electronic devices of high operating temperature. The development has been significant in the fields of, in particular, blue light-emitting diodes, green light-emitting diodes and blue-purple laser diodes.
A basic method of manufacturing the light-emitting device comprising the gallium-nitride-group compound semiconductor is growing a gallium-nitride-group semiconductor film on the surface of a substrate of sapphire, SiC, etc. by means of metal organic chemical vapor deposition. In a practical process of forming a compound semiconductor layer of gallium-nitride-group, a substrate is placed in a reaction tube; metal organic compound gases, for example tri-methyl-gallium (TMG), tri-methyl-aluminum (TMA), tri-methyl-indium (TMI), as the material gas for the Group III element, and ammonia, hydrazine, etc. as the material gas for the Group V element, are supplied therein. The substrate is maintained at a high temperature 900° C.-1100° C., and an n-type layer, a light-emitting layer and a p-type layer are grown on the substrate in a stacked layer structure. After that, by using the technologies of photolithography, vapor deposition, etc., an n-side electrode and a p-side electrode are formed, respectively, on the n-type layer and the p-type layer. Thus, a light-emitting device is completed.
In the semiconductor light-emitting devices, including those of the gallium-nitride-group compound-semiconductor, a material for forming the light-emitting layer is selected in accordance with a requested wavelength of light-emission. The electron injection from the n-type layer to light-emitting layer and the hole injection from the p-type layer into the light-emitting layer cause a recommbination of the electron and the hole within the light-emitting layer to generate a light of a certain desired wavelength.
Among the gallium-nitride-group compound semiconductor light-emitting devices, those having a double hetero junction structure comprising an n-type clad layer of GaN, AlGaN, a light-emitting layer of InGaN and a p-type clad layer of AlGaN are forming the main current of products.
FIG. 5
shows the structure of a conventional gallium-nitride-group compound semiconductor light-emitting device.
As shown in
FIG. 5
, an n-type layer
13
of gallium-nitride (GaN), a light-emitting layer
14
of indium-gallium-nitride (InGaN) and a p-type clad layer
15
of aluminum-gallium-nitride (AlGaN) are stacked on a sapphire substrate
11
, with the intermediary of a buffer layer
12
, to form double hetero junction structure. Stacked over it is a p-type contact layer
16
of GaN A p-side electrode
17
is formed on the p-type contact layer
16
, and an n-side electrode
18
is formed on the surface of the n-type layer
13
, which has been exposed as a result of the removal in part of the three layers, viz. p-type contact layer
16
, p-type clad layer
15
and light-emitting layer
14
.
In the above described gallium-nitride-group compound semiconductor light-emitting device, when a voltage of negative polarity is applied on the n-side electrode
18
and a voltage of positive polarity is applied on the p-side electrode
17
, electron is injected from the n-type layer
13
into the light- emitting layer
14
, at the same time the hole is injected from the p-type clad layer
15
into the light-emitting layer
14
. Thus, the light-emitting layer
14
emits a light having an energy corresponding to the band gap energy of the semiconductor material constituting the light-emitting layer
14
. The above structure has been disclosed by, for example, Japanese Laid-open Patent No 6-268259.
The structure containing the double hetero junction provides a significant improvement in the output of the light emission and in the operating voltage, as compared with a conventional light-emitting device of metal - insulator - semiconductor (MIS) structure.
With the improved light-emitting output and the operating voltage, the light-emitting display devices comprising gallium-nitride-group compound semiconductor have now become usable in, for example, the outdoor display application.
However, in the large-size display devices, including those for outdoor use, the clear visibility is readily affected by the strong mid-day sunlight. Therefore, a further increase in the light-emitting output is requested in order to present a better image of higher visibility. Also with the view to curtailing the power consumption of the large outdoor display devices, reduction in the operating voltage of a light-emitting device is asked for.
It is known that the higher the injection efficiency of electron into the light-emitting layer the higher the output of light emission of a light-emitting device; suppressing the overflow of electron into the p-type layer is an effective measure to increase the efficiency of electron injection into the light-emitting layer. In order to suppress the electron overflow into the p-type layer, it is necessary to raise the energy barrier in the conduction band of the p-type clad layer forming the double hetero junction structure. In a gallium-nitride-group compound semiconductor light-emitting device as shown in
FIG. 5
, for example, the energy barrier in the conduction band can be raised by increasing the Al composition in the p-type clad layer
15
formed of AlGaN.
However, if the rate of Al composition in the p-type clad layer
15
is increased, the energy barrier against hole at valence band in the junction between the p-type clad layer
15
and the p-type contact layer
16
formed of GaN tends to go higher; which can be understood easily from the band structure shown in FIG.
6
. When the energy barrier against hole in the junction between p-type clad layer
15
and p-type contact layer
16
is high, an excessive voltage-drop is caused to lower the energy barrier when a driving voltage is applied to obtain a light emission. Accordingly, the operating voltage of the light-emitting device becomes high. The present invention addresses the problem, and aims to form a gallium-nitride-group compound-semiconductor light-emitting device that provides a lowered operating voltage and an increased light emission output altogether. Using such light-emitting devices enables to offer an outdoor display device of a superior visibility.
SUMMARY OF THE INVENTION
A gallium-nitride-group compound-semiconductor light-emitting device is formed by stacking an n-type clad layer, a light-emitting layer and a p-type clad layer of gallium-nitride-group compound semiconductor on a substrate in the order. The composition distribution of the gallium-nitride-group compound semiconductor in the p-type clad layer is varied along the direction of layer thickness so as the forbidden band width (same as the band gap energy) gradually decreases along with the increasing distance from the light-emitting layer.
With the above described structure, the energy barrier against electron overflow can be made high, and at the same time the excessive voltage-drop in the junction between the p-type clad layer and the p-type contact layer can be alleviated. Therefore, the light-emitting power of a light-emitting device is increased, while the operating voltage is retained low.
REFERENCES:
patent: 06268259A (1994-09-01), None
patent: 10145004A (1998-05-01), None
Kamei Hidenori
Oku Yasunari
Matsushita Electric - Industrial Co., Ltd.
Rossi & Associates
Tran Minh Loan
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