White light LED

Details White light LED White light LED

2003-06-26

2004-11-30

Ngô, Ngân V. (Department: 2814)

Active solid-state devices (e.g., transistors, solid-state diode

Incoherent light emitter structure

With heterojunction

C257S096000, C257S102000

Reexamination Certificate

active

06825498

ABSTRACT:

This application claims the benefit of Taiwan application Serial No. 092104599, filed Jan. 16, 2003.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a light emitting diode (LED), and in particular, to an LED having a BP (boron phosphide) buffer layer of single crystal structure on a ZnTe or ZnSe substrate to facilitate the epitaxy of a blue light LED of cubic crystal structure on the BP buffer layer.
2. Description of the Related Art
The light emitting diode (LED), with the characteristics of small volume, light weight, high efficiency, and long life, has seen great advances in different monochromatic color output, such as red, blue, and green. Single color LED's can be used as a backlight in a special display, for instance, mobile phones and light crystal displays (LCDs). Nowadays, many researches are being carried out on white LEDs in order to develop illumination apparatus with higher efficiency than traditional incandescent and fluorescenct devices.
FIG. 1
shows the cross section of an LED of lateral-electrode type, wherein the P-type electrode and N-type electrode are positioned on the same side of the substrate. A first cladding layer, such as N-type GaN layer
11
, is on a substrate of sapphire layer
10
. The LED further comprises a buffer layer, not shown in the figure, between the substrate
10
and the first cladding layer
11
. An active layer, such as GalnN layer
12
, is on the first cladding layer
11
. A second cladding layer of P-type GaN layer
13
is on the active layer
12
. An N-type electrode
14
and a P-type electrode
15
are respectively on the first cladding layer
11
and the second cladding layer
13
.
U.S. Pat. No. 5,998,925 discloses a white color LED, wherein the stack structure described in the previous paragraph is packaged with a phosphor, like a YAG phosphor
16
. The blue light emitted from the active layer is partially absorbed by the YAG phosphor
16
. The unabsorbed blue light is mixed with yellow light emitted from YAG phosphor
16
, and thus the white light is produced with this mixing.
However, the quality of the white light produced is not satisfactory if the intensity ratio between the blue light and the yellow light is not equal. To form the phosphor layer covering the stack structure greatly increases the cost of the packaging. Moreover, the cost also increases because the substrate of sapphire layer
10
is insulation and the N-type electrode has to be formed on the first cladding layer by an additional etching step.
To resolve the issues described above, an LED of vertical-electrode type has been developed and is shown in
FIG. 2
, wherein the P-type electrode
28
and N-type electrode
21
are positioned on the different side of substrate. Moreover, the substrate, such as an N-type SeZn substrate
22
of
FIG. 2
, is capable of transferring the wavelength of the light source. On the N-type SeZn substrate
22
, there are an N-type ZnSe buffer layer
23
, an N-type SeSMgZn cladding layer
24
, a ZnCdSe active layer
25
, a P-type SeSMgZn cladding layer
26
, and a P-type contact layer
27
in order. The N-type ZnSe buffer layer
23
is mainly used to match the lattice between the N-type SeZn substrate
22
and the N-type SeSMgZn cladding layer
24
. Both the N-type SeSMgZn cladding layer
24
and the P-type SeSMgZn cladding layer
26
have a broader band gap then the ZnCdSe active layer
25
. Therefore, the electrons produced in the ZnCdSe active layer
25
are limited therein.
The LED of
FIG. 2
further comprises an N-type electrode
21
formed on the N-type SeZn substrate
22
and a P-type electrode
28
formed on the P-type contact layer
27
. When appropriate voltages are applied to the N-type electrode
21
and the P-type electrode
28
, the ZnCdSe active layer
25
located on the P-N junction will emit blue light. The blue light is partially absorbed by the N-type SeZn substrate
22
, from which yellow light is emitted. Part of the blue light, unabsorbed by the N-type SeZn substrate
22
, is mixed with the yellow light, and thus white light is produced.
Compared with the LED of lateral-electrode type, the LED of vertical-electrode type requires a simpler manufacturing process, wherein the steps of etching for electrodes and forming phosphor on the stack structure can be omitted. However, the LED of vertical-electrode type has a lower efficiency of emitting white light than the LED of lateral-electrode type.
Therefore, the present invention provides another LED structure for the vertical-electrode type by using a P-type ZnTe layer or a ZnSe layer as a substrate. To ensure lattice match between the substrate and the blue light LED of cubic crystal, a buffer layer of single crystal is firstly formed on the substrate. Thus the efficiency of white light emission is greatly improved. Moreover, the present invention improves the quality of white light because blue light of wavelength from 450 nm to 470 nm is emitted from the blue light LED of cubic crystal on the buffer layer, and yellow light of 550 nm is emitted from the substrate after the substrate absorbs the blue light.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a light emitting diode (LED) structure of vertical-electrode type.
Another object of the present invention is to provide an LED structure comprising a P-type ZnTe layer or a ZnSe layer as a substrate. A BP buffer layer of single crystal is positioned on the substrate, such that the lattice match between the substrate and the blue light LED of cubic crystal can be improved.
On the substrate, a BP buffer layer, a first type GaN cladding layer, an active layer and a second type GaN cladding layer are stacked in order. A first type electrode and a second type electrode are positioned below the substrate and on the second type GaN cladding layer respectively. The first type GaN cladding layer is opposite the second type GaN cladding layer in conducting type, the first type electrode is also opposite the second type electrode in conducting type. The substrate, the BP buffer layer, the first type electrode and the first type GaN cladding layer are of the same conducting type.


REFERENCES:
patent: 5998925 (1999-12-01), Shimizu et al.
patent: 6528395 (2003-03-01), Nakamura

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