Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – Plural light emitting devices
Reexamination Certificate
2003-01-08
2004-05-04
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
Plural light emitting devices
C257S089000, C257S093000, C257S622000, C257S623000, C257S625000
Reexamination Certificate
active
06730936
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a light-emitting diode array having a large light-emitting power, particularly to a light-emitting diode array suitably usable for light sources of electrophotographic printers.
BACKGROUND OF THE INVENTION
An electrophotographic printer forms electrostatic latent image on a photosensitive drum by light corresponding to an image signal, and toner is selectively attached to the electrostatic latent image for development and transferred onto a paper to obtain an image. Widely used as light sources for forming the latent image are of a laser type or a light-emitting diode array type. Particularly light sources constituted by light-emitting diode arrays are suitable for small printers and large-size printing, because they do not need long light paths unlike the laser-type light sources. Recent development of higher-speed, higher-quality printing and miniaturization of printers require light-emitting diode arrays with higher precision and higher output.
Widely known is a light-emitting diode comprising a pair of electrodes on a top surface (light-emitting side) and a rear surface sandwiching a semiconductor substrate and a light-emitting part on the substrate. When voltage is applied to electrodes provided on top and rear surfaces of a light-emitting diode having such a structure, electric current flows in a direction perpendicular to the semiconductor substrate, whereby electrons and holes are recombined in the light-emitting part to emit light. Though there is the maximum light output in the light-emitting part immediately under the top surface electrode, the light generated in this portion is reflected and absorbed by the top surface electrode, failing to be taken out efficiently. Accordingly, as light sources particularly in high-resolution printers of 600 dpi, 1200 dpi, etc., which should have light-emitting portions with small areas, conventional light-emitting diodes are insufficient in light-emitting power.
JP 2000-323750 A discloses a technology of improving the efficiency of taking out the generated light by forming cathodes and an anode on the same surface of a substrate.
FIG. 3
is a plan view showing a light-emitting diode array having such a structure;
FIG. 4
is a cross-sectional view taken along the line A-A′ in
FIG. 3
; and
FIG. 5
is a cross-sectional view taken along the line B-B′ in FIG.
3
. Incidentally, an insulating layer is omitted to clearly show the underlayers in
FIG. 3. A
plurality of light-emitting parts
2
are arranged on a p-type GaAs conductive layer
11
formed on an n-type GaAs substrate
10
at a predetermined interval. Each light-emitting part
2
is constituted by a p-type AlGaAs etched stopper layer
12
, a p-type AlGaAs clad layer
13
, a p-type AlGaAs active layer
14
, an n-type AlGaAs clad layer
15
and an n-type GaAs capping layer
16
, which are successively laminated on the p-type GaAs conductive layer
11
. The light-emitting part
2
has a double hetero structure in a light emission region, which comprises a p-type AlGaAs clad layer
13
, a p-type AlGaAs active layer
14
and an n-type AlGaAs clad layer
15
.
Each light-emitting part
2
is formed by removing an epitaxial layer by mesa etching. Mesa-etched grooves are constituted by a first mesa-etched groove
21
separating light-emitting parts
2
from bonding portions
8
and second mesa-etched grooves
23
separating light-emitting parts
2
.
Part of the top surface of each light-emitting part
2
is provided with a cathode
3
. An anode
4
formed in a strip shape on the p-type GaAs conductive layer
11
near the light-emitting parts
2
is a common electrode for operating a plurality of light-emitting parts
2
. The cathodes
3
and the anode
4
are formed on the mesa top surfaces of the light-emitting parts
2
and the p-type GaAs conductive layer
11
, respectively, by vapor deposition and alloying of metals. The light-emitting parts
2
and the exposed surfaces of the conductive layer
11
except immediately under the cathodes
3
and the anode
4
are covered by an insulating film layer
17
of phosphosilicate glass (PSG). Each Au wiring layer
5
formed with its one end connected to each cathode
3
not covered by the insulating film layer
17
extends to a surface of the bonding portion
8
, and the other end of each Au wiring layer
5
is provided with a bonding pad
6
.
In the light-emitting diode array with such a structure, an electric current path
19
from the anode
4
to the cathode
3
passes through the light-emitting part
2
, resulting in the generation of light L in the p-type AlGaAs active layer
14
. This light L is emitted outside from a light-emitting portion
9
provided by removing the n-type GaAs capping layer
16
by etching.
However, in the above conventional light-emitting diode array, as is clear from
FIGS. 3 and 4
, there is the p-type GaAs conductive layer
11
in the second mesa-etched grooves
23
separating the light-emitting parts
2
, there is an electric current path
20
that reaches the cathode
3
from the anode
4
without passing through the p-type AlGaAs active layer
14
in each light-emitting part
2
(FIG.
3
). This electric current path
20
may be called “detour electric current pass.” Because electric current passing through the detour electric current path
20
does not contribute to light emission, each light-emitting diode has low light-emitting power.
OBJECT OF THE INVENTION
Accordingly, an object of the present invention is to provide a light-emitting diode array free from a detour electric current path not contributing to light emission, thereby providing increased light-emitting power.
DISCLOSURE OF THE INVENTION
As a result of intensive research in view of the above object, the inventor has found that by removing a conductive layer on a substrate in regions corresponding to second mesa-etched grooves, the above detour electric current path can be eliminated, resulting in increase in the light-emitting power of each light-emitting diode. The present invention has been completed based on this finding.
Thus, the light-emitting diode array of the present invention comprises a conductive layer formed on a substrate, a plurality of separate light-emitting parts formed on the conductive layer, a first electrode formed on at least part of a top surface of each light-emitting part, and a second electrode formed on the conductive layer near the light-emitting part, the second electrode being a common electrode for operating a plurality of the light-emitting parts, and regions of the conductive layer between the adjacent light-emitting parts being removed.
In the light-emitting diode array of the present invention, the light-emitting parts are preferably formed by dividing an epitaxial layer formed on the conductive layer by mesa-etched grooves. The light-emitting diode array according to a preferred embodiment comprises a first mesa-etched groove separating the light-emitting parts from bonding portions for forming a plurality of separate light-emitting parts, and second mesa-etched grooves separating the light-emitting parts, portions of the conductive layer between the light-emitting parts being removed by the second mesa-etched grooves, whereby electric current does not flow between the first and second electrodes without passing through the light-emitting parts.
The mesa-etched grooves preferably are constituted by the first mesa-etched groove separating the light-emitting parts from the bonding portions and the second mesa-etched grooves for removing portions of the conductive layer between the light-emitting parts, in the shape of a comb as a whole.
In the light-emitting diode array of the present invention, electric current does not flow between the first electrodes and the second electrode without passing through the light-emitting parts, resulting in large recombination of electrons and holes in the epitaxial layer in the light-emitting parts and thus increased light-emitting power.
REFERENCES:
patent: 5386139 (1995-01-01), Idei et al.
patent:
Flynn Nathan J.
Forde Remmon R.
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