Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Reexamination Certificate
1999-05-11
2001-11-13
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S103000, C257S096000, C257S097000
Reexamination Certificate
active
06316792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to a light emitting device and, more particularly, to a structure and a method for manufacturing electrodes for such light emitting devices.
2. Description of the Related Art
Many types of light emitting elements are used in a wide range of devices such as light emitting diode (LED) displays and vehicle indicators. In particular, gallium nitride-based compound semiconductors such as GaN, InGaN, GaA
1
N have attracted a great deal of attention for use as materials for light emitting elements, LEDs, laser diodes (LDs), and the like which emit blue light. In the past, it was difficult to manufacture stable blue light emitting elements. However, these gallium nitride-based compound semiconductor materials make it possible to realize blue or green light emitting elements which have stability and strong intensity.
Generally, blue light emitting elements using a gallium nitride-based compound semiconductor are manufactured by growing a gallium nitride-based compound semiconductor layer such as GaN on a sapphire substrate. Since sapphire is an insulator, the electrodes of these elements cannot be formed on the substrate side. Therefore, the electrodes must be located on the compound semiconductor layer side. However, the electrodes block the light emitted from the compound semiconductor layers. Thus, the emitted light is taken out from the substrate side since the sapphire substrate is transparent. When the light emitting element is mounted on a lead frame, the electrodes on the compound semiconductor layer side must contact the lead frame.
The conventional structure of the light emitting elements using the gallium nitride-based compound semiconductor will be described below.
FIG. 11
is a cross-sectional view of a conventional blue light emitting element. An N-type GaN buffer layer
3
, an N-type GaN layer
5
and a P-type GaN layer
7
are grown on a sapphire substrate
1
. P-type GaN layer
7
is partially removed by a conventional etching method to expose a portion of N-type GaN layer
5
. An anode
17
is formed directly on the remaining portion of P-type GaN layer
7
and a cathode
9
is formed on the exposed portion of N-type GaN layer
5
. Anode
17
and cathode
9
are connected to lead frames
53
by conductive (e.g., silver) paste layers
55
a
and
55
b
, respectively. In this light emitting element, light is emitted from the boundary between N-type GaN layer
5
and P-type GaN layer
7
when electrons and holes recombine. The light is reflected by anode
17
and is emitted through sapphire substrate
1
.
It is difficult to reduce the size of the light emitting element of
FIG. 11
because anode
17
and cathode
9
must be spaced apart sufficiently so that conductive paste layers
55
a
and
55
b
do not contact each other and so that N-type GaN layer
5
and P-type GaN layer
7
are not electrically connected together by conductive paste layer
55
b
at the region indicated by reference number
59
in FIG.
11
. This means that the number of chips on a single wafer cannot increased and that the cost of the device can not be reduced. Further, the connections between the lead frames
53
and anode
17
and cathode
9
require high precision when the size is reduced. This inhibits the production of a large quantity of devices.
FIG. 12
is a cross-sectional view, of a conventional GaAs-based compound semiconductor. An N-type GaAs buffer layer
33
, an N-type AlGaInP cladding layer
35
, an AlGaInP active layer
37
, a P-type AlGaInP cladding layer
39
, a P-type AlGaAs current spreading layer
57
and a P-type GaAs contact layer
42
are successively formed on an N-type GaAs substrate
31
. One electrode
49
is formed on P-type GaAs contact layer
42
and the other electrode
51
is formed on the back side of N-type GaAs substrate
31
. In this structure, injected current is spread in P-type AlGaAs current spreading layer
57
. Light is emitted from the pnjunction between AlGaInP active layer
37
and P-type AlGaInP cladding layer
39
and is output via current spreading layer
57
.
Current spreading layer
57
must have considerable thickness. When current spreading layer
57
too thin, the injected current is not spread sufficiently and the light can not be emitted uniformly from the pn junction. In addition, current passes under electrode
49
and thus light is emitted from the portion of the pn junction under electrode
49
. However, this light is blocked by electrode
49
and thus this light emitting device has a lowered light intensity.
Japanese Laid Open 7-131070 discloses an electrode structure for a light emitting device. The disclosed device is LED array type and has deep grooves around each light emitting region to isolate the pn junctions. The electrode of each light emitting region has a metal layer and a transparent conductive layer so that the electrode is able to form an ohmic contact to compound semiconductors. This electrode has a high transmittance for the wavelength of the emitted light. However, this structure does not effectively protect the pn junctions from the mesa etching to form the deep grooves. Therefore, the device has a lowered quality.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a compound semiconductor light emitting element includes a light emitting region formed by a pn-junction between a first compound semiconductor layer of a first conductivity type and a second compound semiconductor layer of a second conductivity type; a first electrode connected to the first compound semiconductor layer and isolated from the second compound semiconductor layer; a current spreading layer formed on the second compound semiconductor layer; a block formed on the second compound semiconductor layer; and a second electrode formed on the block and connected to the current spreading layer.
In accordance with another aspect of the present invention, a compound semiconductor light emitting element includes a substrate; a first compound semiconductor layer of a first conductivity type formed on the substrate; a second compound semiconductor layer of a second conductivity type formed on the first compound semiconductor layer; a first electrode formed on the first compound semiconductor layer, the first electrode being isolated from the second compound semiconductor layer; a current spreading layer formed on the second compound semiconductor layer; a block formed on the second compound semiconductor layer; and a second electrode on the block and on a portion of the current spreading layer. The first and the second conductive compound semiconductor layers may be In
x
Ga
y
Al
z
N (x+y+z=1, 0≦x, y, z≦1) layers. A protective layer may be formed on the current spreading layer except for the second electrode.
In accordance with still another aspect of the present invention, a compound semiconductor light emitting element includes a conductive substrate; a first electrode formed on a first surface of the substrate; a first compound semiconductor layer of a first conductivity type on a second surface of the substrate which is opposite to the first surface of the substrate; a second compound semiconductor layer of a second conductivity type formed on the first compound semiconductor layer; a current spreading layer formed on the second compound semiconductor layer; a block formed on the second compound semiconductor layer; and a second electrode formed on the block and on a portion of the current spreading layer. A protective layer may be formed on the current spreading layer except for the second electrode.
In accordance with yet another aspect of the present invention, a method for manufacturing a compound semiconductor light emitting element includes the steps of: forming a first compound semiconductor layer of a first conductivity type on a substrate; forming a second compound semiconductor layer of a second conductivity type; forming a first electrode on the first compound semiconductor layer, the first electrode being i
Okazaki Haruhiko
Watanabe Yukio
Banner & Witcoff , Ltd.
Jackson, Jr. Jerome
Kabushiki Kaisha Toshiba
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