Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Low workfunction layer for electron emission
Reexamination Certificate
2002-11-27
2004-05-18
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Low workfunction layer for electron emission
C257S014000, C257S103000
Reexamination Certificate
active
06737669
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light-emitting device, and in particular, to a semiconductor light-emitting device that employs an AlGaInP-based semiconductor in its light-emitting section.
In order to form a high-intensity semiconductor light-emitting device, it is required to increase the luminous efficiency of its active layer, increase the amount of injection current into the active layer and increase the efficiency of taking out the light emitted from the active layer to the outside of the device.
In order to increase the amount of injection current into the light-emitting section, a current diffusion layer and an intermediate layer or the like capable of improving the amount of injection current without increasing the operating voltage are effective. At the same time, it is required to increase the amount of electrons and holes that contribute to radiative recombination by confining the injected current (electrons and holes) without letting it escape. As a means for confining electrons and holes in the light-emitting layer, a double-hetero (hereinafter referred to as “DH”) structure is widely used.
In the DH structure, the active layer is held between semiconductor layers that have a bandgap wider than that of the active layer. Thereby, an energy barrier over which the electrons and holes hardly pass is formed on the upper and lower sides of the active layer, and therefore, the DH structure makes it difficult to let electrons and holes escape. This enables the increase of the probability that the electrons and holes may contribute to the radiative recombination.
The DH structure is widely used also for a semiconductor light-emitting device in which an AlGaInP-based semiconductor is employed in the active layer (refer to Japanese Patent Laid-Open Publication No. HEI 5-335619, page 2, paragraph 0003 and Japanese Patent Laid-Open Publication No. HEI 4-229665, page 2, paragraphs 0003 and 0004).
FIG. 10
shows a prior art semiconductor light-emitting device that has the DH structure.
According to the above-mentioned semiconductor light-emitting device, as shown in
FIG. 10
, a desired buffer layer
102
, an n-AlGaInP clad layer
103
, an AlGaInP active layer
104
, a p-AlGaInP clad layer
105
and A GaP current diffusion layer
106
are successively laminated on an n-GaAs substrate
101
. Further, on the GaP current diffusion layer
106
are successively laminated the other layers of a current blocking layer, a protective layer, an intermediate bandgap layer, a protective layer and so on that are not shown. A p-type electrode
107
is formed on the GaP current diffusion layer
106
. An n-type electrode
108
is formed under the n-GaAs substrate
101
by vapor deposition. Subsequently, the n-GaAs substrate
101
, the p-type electrode
107
, the n-type electrode
108
and so on are formed into the desired shapes so that a semiconductor light-emitting device is completed.
In the above-mentioned semiconductor light-emitting device, a semiconductor having a composition of (Al
x
G
1-x
)
y
In
1-y
P (x≈0.7 and y≈0.5) is employed for the n-type clad layer
103
and the p-type clad layer
105
. However, in the general semiconductor light-emitting device of the AlGaInP-based semiconductor, a semiconductor having a clad layer composition of (Al
x
Ga
1-x
)
y
In
1-y
P (0.7≦x≦1.0, y≈0.5) is often employed.
FIG. 11
shows a band profile in the vicinity of the active layer of the prior art semiconductor light-emitting device.
As shown in
FIG. 11
, the upper and lower clad layers have a bandgap wider than that of the active layer, and therefore, an energy barrier is formed on both outer sides of the active layer. This arrangement restrains the phenomenon that the electrons and holes injected into the active layer escape from the active layer to the outside, i.e., overflow. As a result, there increases the probability of radiative recombination of electrons and holes in the active layer, and this allows a high-intensity semiconductor light-emitting device to be obtained.
In the above-mentioned prior art example, the DH structure has been used as a method for confining a large number of electrons and holes injected from the outside of the device in the active layer. However, in a device that has a short wavelength of light emitted from the active layer, the bandgap of the active layer is widened, and the difference in the bandgap between the active layer and the clad layer is reduced.
As described above, if the bandgap difference between the active layer and the clad layer is reduced, then the energy barrier against electrons and holes is reduced. As a result, the effect of confining electrons and holes produced by the clad layer is reduced, and therefore, the electrons and holes easily escape from the active layer. That is, the electrons and holes easily overflow from the active layer. For the above-mentioned reasons, there have been the problems that the luminous efficiency has been reduced in the short-waveform semiconductor light-emitting device and a high-intensity semiconductor light-emitting device has hardly been unable to be obtained.
With regard to electron and hole, it is difficult for hole to overflow since hole has a low mobility, whereas it is easy for electron to overflow since electron has a mobility several tens of times higher than that of hole.
In concrete, with regard to the AlGaInP-based semiconductor light-emitting devices, the overflow does not matter in a device that has an emission wavelength longer than 590 nm, whereas the overflow becomes significant in a device that has an emission wavelength of not greater than 590 nm. This overflow causes a reduction in luminance.
FIG. 12
shows a graph showing the relation between emission wavelength and external quantum efficiency in the semiconductor light-emitting device.
As is apparent from
FIG. 12
, the overflow of electron becomes particularly significant in the semiconductor light-emitting device that has an emission wavelength equal to or shorter than about 590 nm, and therefore, the luminous efficiency falls with reduced luminance. For the above-mentioned reasons, the luminous efficiency falls in the short-wavelength semiconductor light-emitting device, and it is difficult to obtain a high-intensity semiconductor light-emitting device.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to improve the luminance by increasing the probability of radiative recombination of electrons and holes in the active layer of an AlGaInP-based semiconductor light-emitting device of a short wavelength.
In order to solve the aforementioned object, the present invention provides a semiconductor light-emitting device comprising:
a compound semiconductor substrate;
a first-conductive-type clad layer formed on the compound semiconductor substrate;
an active layer formed on the first-conductive-type clad layer and comprised of an AlGaInP-based semiconductor wherein light emitted from the active layer has a wavelength of not greater than 590 nm;
a second-conductive-type clad layer formed on the active layer; and
a semiconductor layer interposed between the active layer and the first-conductive-type clad layer or the second-conductive-type clad layer, wherein
an energy position at a lower end of a conduction band of the semiconductor layer is 0.05 eV to 1.0 eV higher than an energy position at a lower end of a conduction band of the second-conductive-type clad layer in a band profile before formation of a junction between the active layer and the semiconductor layer, and a junction between the semiconductor layer and the first-conductive-type clad layer or the second-conductive-type clad layer.
According to the semiconductor light-emitting device of the above-mentioned construction, since the semiconductor layer is interposed between the active layer and the first-conductive-type clad layer or between the active layer and the second-conductive-type clad layer, the semiconductor layer operates as an energy barrier against electrons to restrain the overflow of elect
Nakamura Junchi
Sasaki Kazuaki
Ho Tu-Tu
Morrison & Foerster / LLP
Nelms David
Sharp Kabushiki Kaisha
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