Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
1999-01-12
2001-03-27
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S096000, C257S097000, C257S101000, C438S046000, C438S047000
Reexamination Certificate
active
06207972
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor light emitting diode (LED). The present invention is particularly applicable to a semiconductor LED having high efficiency and reliability and comprising a heterostructure light generating region and a transparent window layer.
BACKGROUND ART
Conventional LEDs comprise a semiconductor light generation region on a light absorbing substrate. Such LEDs enjoy various industrial applications, as in optical communication systems, optical information processing and as a light source due to their low power consumption, efficiency and reliability. Efficient operation of an LED requires uniform lateral spreading of current injected by a front electrical contact, so that the current uniformly enters the light generation region, thereby generating light with uniformity. However, as a result of current crowding, current tends to concentrate under the front electrical contact, thereby preventing uniform light generation. Industry efforts have focused upon reducing the current crowding problem as well as increasing the brightness of emitted light.
A traditional semiconductor LED is schematically illustrated in FIG.
1
and comprises a back electrical contact
10
, an n-type substrate
20
, a double heterostructure
30
(light generation region) which includes an undoped active layer
3
b
positioned between doped confinement layers
3
a
and
3
c
, and a front contact
70
. It is in such a structure that current crowding typically occurs between the light generation region
30
and front contact
70
, thereby preventing uniform light generation.
A prior effort to alleviate the current crowding effect and maximize light output is disclosed by Fletcher et al. in U.S. Pat. No. 5,233,204 and schematically illustrated in
FIG. 2
, wherein elements similar to those depicted in
FIG. 1
bear similar reference numerals and, hence, are not described in detail to avoid repetition. The improvement disclosed by Fletcher et al. comprises positioning a relatively thick transparent semiconductor window layer
40
, e.g., about 10 microns to about 50 microns, between the light generation region
30
and the front metal contact
70
. Window layer
40
is desirably selected from materials having a high conductivity to enable rapid current spreading from front contact
70
, thereby minimizing the current crowding effect. In addition, window layer
40
should have a higher bandgap than that of the light generation region
30
so that window layer
40
is transparent to emitted light. There are, however, drawbacks attendant upon the semiconductor LED illustrated in FIG.
2
. For example, semiconductor window layer
40
can not include material systems having lattice constants which are not compatible with light generation region
30
, thereby limiting design flexibility. In addition, the growth of a thick layer is time consuming.
Another prior approach to the current crowding problem is disclosed by Lin et al. in U.S. Pat. No. Re. 35,665 and schematically illustrated in
FIG. 3
, wherein elements similar to those in
FIGS. 1 and 2
bear similar reference numerals. The semiconductor LED illustrated in
FIG. 3
basically differs from that of
FIG. 2
in that the thick semiconductor window layer
40
(
FIG. 2
) is replaced by transparent conductive oxide window layer
50
and an ohmic contact layer
51
which is typically a semiconductor material having a relatively high impurity concentration, e.g., greater than about 1×10
18
atoms cm
−3
. Ohmic contact layer
51
is provided so that window layer
50
can be formed on a p-type confinement layer (
3
c
), thereby expanding utility to n-type gallium-arsenide (GaAs) substrate-based LEDs. The transparent conductive oxide
50
comprises tin oxide, indium oxide, or indium-tin oxide, which are conductive materials, relatively inexpensive and relatively easier to grow than semiconductor compound transparent window materials for window layer
40
(FIG.
2
).
With continued reference to
FIG. 3
, the utilization of a transparent conductive oxide layer
50
could reduce the current crowding effect, reduce manufacturing time, improve efficiency and expand applicability to LEDs with n-type GaAs substrates. Such oxides are suitable window materials for LEDs employing aluminum-gallium-indium-phosphorous (AlGaInP) material systems, i.e. for the light generation region, which emit light having wavelengths ranging from about 570 to about 680 nm. However, semiconductor LEDs based upon
FIG. 3
are also problematic. For example, tin oxide, indium oxide and indium tin oxide exhibit poor optical transmission at longer wavelengths and, hence, are not particularly suitable for use in semiconductor LEDs at wavelengths of about 1.3 or about 1.5 &mgr;m. Such oxides are also toxic, and do not exhibit long term chemical stability. In addition, semiconductor LEDs based upon
FIG. 3
exhibit an undesirably high contact resistance between light transmission region
30
and ohmic contact layer
51
, which unnecessarily squanders electricity and increases the operating temperature, e.g., above room temperature, thereby decreasing device reliability, i.e. longevity.
There exists a need for a semiconductor LED which exhibits improved light brightness, reduced crowding effect and increased longevity. There also exists a need for such a semiconductor LED which can be manufactured efficiently and economically.
DISCLOSURE OF THE INVENTION
An advantage of the present invention is a semiconductor LED exhibiting improved light brightness and reduced current crowding.
Another advantage of the present invention is a semiconductor LED exhibiting long term stability and reduced toxicity.
A further advantage of the present invention is a semiconductor LED exhibiting improved light brightness, reduced current crowding, long term stability and reduced toxicity which can be manufactured economically and efficiently.
Additional advantages and other features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a semiconductor LED comprising: a semiconductor substrate having a back electrode contact; a light generation region on the substrate; a transparent current diffusion layer on the light generation region; a dual transparent layer window on the current diffusion layer; and a front contact on the dual layer window.
Embodiments of the present invention include a dual layer window comprising a transition layer on the current diffusion layer and a transparent window layer on the transition layer, wherein the transition layer has a bandgap selected to reduce contact resistance between the current diffusion layer and the window layer. Embodiments of the present invention include forming the transition layer at a thickness of about 10 to about 100 nm and forming the window layer at a thickness of about 0.5 micron to about 2 microns. Embodiments of the present invention also include forming the transition layer of a doped semiconductor material, and employing a highly doped semiconductor material as the current diffusion layer at a thickness of about 5 microns to about 10 microns.
Another aspect of the present invention is a semiconductor LED comprising: a semiconductor substrate having a back electrical contact; a light generation region on substrate; a transparent window layer comprising zinc oxide; and a front contact on the window layer.
Embodiments of the present invention include forming a highly doped transparent current diffusion layer on a double heterostructure light generation region and forming a doped semiconductor transition layer between the zinc oxide window layer and current diffusion layer, such that the transition
Chen Jyh-Chia
Huang Zhenchun
Kuo Li-Hsin
McDermott & Will & Emery
Mintel William
Super Epitaxial Products, Inc.
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