Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Reexamination Certificate
2001-07-09
2003-08-05
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S079000, C257S082000, C257S091000
Reexamination Certificate
active
06603152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a blue light emitting diode, and more particularly a blue light emitting diode, which has such an electrode structure as to prevent the concentration of current density, causative of a rapid temperature increase in the diode, thereby increasing resistance to electrostatic discharge (ESD) and lowering a deriving voltage without significantly altering the laminate structure of the electrode.
2. Description of the Related Art
Recently, a light emitting diode which is able to emit light in the region of short wavelength (ultraviolet light to green), particularly blue light has gained public popularity. Such semiconductor materials include ZnSe (II-VI), nitrides such as GaN, InN, AlN (III-V) and nitride mixtures combining these nitrides in a certain ratio and particularly GaN are widely used.
Growing of GaN crystal is effected by MOCVD (Metal Organic Chemical Vapor Deposition) method. The MOCVD method is carried out by supplying reactive gas of an organic compound into a reaction chamber at a temperature of 700 to 1200° C. to grow crystals in an epitaxial layer on a substrate, in which sapphire or SiC is used as the substrate. The reason that sapphire or SiC substrate is used is that there exist no substrates commercially available that can achieve lattice matching with the nitride crystal while having the same crystalline structure as the nitride crystal. Also, it can hardly be expected that the growth process of an epitaxial layer on such substrate would form a good quality of crystal, due to the stress resulting from lattice mis-matching. Therefore, a buffer layer is used as a low temperature-growing layer between the substrate and epitaxial layer.
Also, the blue light emitting diode with such a limited structure differs from the general light emitting diode in terms of the driving method, as explained below with reference to
FIGS. 1
a
and
1
b.
FIG. 1
schematically shows the difference in the driving manner between a general light emitting diode (for example, a class of LEDs using GaAs, GaP, etc) and a blue light emitting diode (III-nitride). In the general light emitting diode, as shown in
FIG. 1
a,
a light emitting diode chip is operated using a total structure including a wafer which acts as a substrate for crystal growth. However, as shown in
FIG. 1
b,
the blue light emitting diode is operated through a thin structure fabricated on a chip, but not through the substrate. That is, the blue light emitting diode has a planar type structure using the insulating substrate as a sapphire, unlike the general light emitting diode.
Further, the blue light emitting diode is known to need a relatively high driving voltage at a constant current, compared to the general light emitting diode.
FIGS. 2
a
and
2
b
show the voltage-current characteristics of an infrared light emitting diode (A), a red light (wavelength 635 nm) emitting diode (B) and a blue light (wavelength 450 nm) emitting diode (C) in a driving region of a forward direction. The blue light emitting diode requires a driving voltage of about two times as high as the red light emitting diode at a rated current of 20 mA. It is thought that such a high driving voltage is attributed to properties of GaN semiconductor layer and the planar type structure.
As described above, the blue light emitting diode suffers from problems in two aspects. First, the blue light emitting diode must adopt a deriving method for use in a planar type structure owing to the structural limit of growing a semiconductor layer on a sapphire substrate and a buffer layer so as to grow crystals with prevention of lattice mismatching. Another problem with the blue light emitting diode is the inherent feature of requiring a higher driving voltage as compared to general light emitting diodes. Consequently, the driving method and the high driving voltage of the blue light emitting diode may lead to reduced reliability and deteriorated quality of products.
FIG. 3
a
is a plan view of the conventional blue light emitting diode and
FIG. 3
b
is a sectional view of the diode, taken along line A—A in
FIG. 3
a.
Referring to
FIGS. 3
a
and
3
b,
problems caused by the above-described restrictive problems of the blue light emitting diode will be explained in detail. As shown in
FIG. 3
a,
the conventional blue light emitting diode include a sapphire substrate
1
, a buffer layer
2
formed on the substrate
1
, an n-type nitride semiconductor layer
3
comprising a central part R
1
in a predetermined region and a peripheral part R
2
surrounding the central part R
1
, and a laminated structure formed on the n-type nitride semiconductor layer
3
.
The laminated structure has an active layer
4
made of intrinsic nitride semiconductor crystal in the central part R
1
on the N-type nitride semiconductor
3
, a p-type nitride semiconductor layer
5
formed on the active layer
4
, a metal layer
6
atop the semiconductor crystal layer
5
, and a first electrode
7
, corresponding to a P electrode, formed in a predetermined region on the metal layer
6
. Also, the light emitting diode includes a second electrode
8
as a N electrode formed in the peripheral part R
2
over the N-type nitride semiconductor layer
3
while keeping a predetermined distance space from the central part R
12
over the N-type nitride semiconductor layer
3
.
In such conventional blue light emitting diode, current flows as injected carriers move on the surface of the diode and at the interface between the electrodes in the characteristic driving manner of the planar type structure. Also, the blue light emitting diode requires a high driving voltage across a given area for light emission, thereby forming a flow of a great quantity of injected carrier (herein electron). The current path Rp formed by the above flow of the carriers is distributed in accordance with the area of the electrode formed at an upper position. In
FIG. 3
a,
therefore, the current density distribution is very high in the region Rd defined with a dotted line, and decreases gradually toward the periphery. The higher current density at the region Rd defined with the dotted line leads the temperature of the entire chip to increase, resulting in reducing the light output.
In consequence, since the conventional blue light emitting diode of the planar type structure requires a high driving voltage, defects existing in the region where a high current density is generated causes the chip temperature to be increased as well as incurring quality deterioration, for example, weak resistance to electrostatic discharge (ESD), which cause fundamental problems in achieving the reliability and quality stabilization of products.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a blue light emitting diode with an improved electrode structure which is capable of effectively dispersing the current density concentration causative of local temperature increase in the blue light emitting diode without requiring a significant structural change.
Another object of the present invention is to provide a blue light emitting diode which is highly resistant to ESD with a resulting improvement in terms of the quality and reliability of products.
Still another object of the present invention is to provide a blue light emitting diode in which the driving voltage is reduced and the rapid increasing of a temperature occurring locally in the chip is suppressed.
In order to achieve the above object, the present invention provides a blue light emitting diode comprising an insulating substrate, typically in a square shape, and a first conductive nitride semiconductor layer formed on the insulating substrate to have a surface divided into a central part and a peripheral part. The peripheral part is provided over the surface adjacent to and along the edges of the nitride semiconductor layer and the central part surrounded by the peripheral part.
Also, the blue light emitting diod
Choi Don-Bok
Park Young-Ho
Song Kyung-Sub
Flynn Nathan J.
Lowe Hauptman & Gilman & Berner LLP
Mondt Johannes
Samsung Electro-Mechanics Co. Ltd.
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