Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2002-06-13
2003-06-17
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S098000, C257S099000, C257S103000, C257S613000, C257S615000, C257S201000, C257S076000, C257S094000
Reexamination Certificate
active
06580096
ABSTRACT:
The present invention relates to a window for a light emitting diode formed of an aluminum-gallium-indium-phosphate (AlGaInP) compound.
BACKGROUND OF THE INVENTION
A semiconductor light emitting diode (LED) includes a substrate, a light emitting structure, and a pair of electrical contacts for powering the diode, i.e., a substrate contact and a window contact. The substrate may be transparent or opaque; and the “substrate” contact, is formed on a surface thereof. The window contact is formed on a window surface.
LED structures composed of AlGaInP compounds can be designed to emit any selected one of a range of colors by tailoring the amount of Al in the compound. However, where the substrate and lower cladding layer are of n type AlGaInP compounds, it is difficult to achieve a low resistance p type AlGaInP compound for the upper cladding layer. Unfortunately, a relatively high resistance upper cladding layer does not provide full use of the surface of the light emitting structure. That is, current flowing between the window and substrate contacts tends to concentrate in a narrow “favored” path, which lies directly under the window contact. Thus, only that portion of the light emitting surface which lies in the favored path is activated.
A number of conventional arrangements provide a “window” which is interposed between the light emitting structure and the window contact to more fully utilize the light emitting surface. The conventional windows range from a single, thick layer of compounds other than AlGaInP, to a variety of multi-layer structures which “spread” the energizing current across the face of the light emitting surfaces.
Light generated by an LED exits directly from the outer face of the light emitting surface or via the window. The “window” contact is formed on the outer face of the light emitting surface, or is part of the window.
In any event, the window contact covers a substantial surface area and blocks emission of light generated directly thereunder. For example, in the case of an LED having a 10 mil by 10 mil square window, a four mil round metal contact will obscure about 12.2% of the window surface. However, the window contact cannot be measurably reduced in diameter, since the contact must be large enough to ensure its adhesion to the window surface.
For example,
FIG. 1A
represents a top view of a conventional LED having a 10 mil by 10 mil square window with a top layer
105
, the window having a 4 mil circular metal contact
106
. Typically, the window contact is made of gold (Au). The conventional LED of
FIG. 1B
includes a metal substrate contact
101
, an “n” GaAs substrate
102
, an “n” cladding layer
103
, an active region
104
, a p cladding layer
105
, and a metal window contact
106
. As explained earlier herein, current which flows between the window contact
106
and the substrate contact
101
concentrates in a “favored” path directly under the window contact
101
. Since, only a small area of the active layer lies in that current path, much of the light emitting potential of the LED is dormant. Additionally, most of the light emitted through layer
105
is intercepted by the opaque contact
106
. In routine observations, it is noted that under the stated conditions, the light which is emitted by the LED appears as a thin halo surrounding contact
106
.
Thus, an improved window structure which allows increased emission of light therethrough when used with AlGaNP compounds, is needed.
SUMMARY OF THE INVENTION
An apparatus consistent with the present invention includes a semiconductor light emitting diode having a substrate, a substrate electrical contact, a light emitting structure, and an improved window. The window interfaces directly with the light emitting structure, and, in the following stated order includes a lightly doped p GaP layer, a low resistance p GaAs layer, a transparent, amorphous conducting window layer, and a metal window contact. The conducting layer, by way of example, may be formed of indium tin oxide (ITO), tin oxide (TO), or zinc oxide (ZnO). Layers of other amorphous, conductive and transparent oxide compounds also may be suitable for construction of the window layer.
In a first embodiment consistent with the present invention, the metal contact passes through both the conducting layer and the GaAs layer to: (a) form an ohmic contact with those layers, and (b) contact the GaP layer and form a Shottky diode connection therewith.
In a second embodiment consistent with the present invention, the metal contact passes only through the conducting layer and contacts an insulator which is formed in the GaAs layer to isolate the metal contact from the GaP layer. As in the first embodiment, the metal window contact forms an ohmic contact with the conducting layer and the GaAs layer.
Advantageously, in both embodiments, the current path lying directly under the metal contact is eliminated and the current is widely spread over the face of the light emitting structure.
With elimination of the “favored” path, less light is generated from the portion of the diode interface which is directly under the window contact; and more light is generated from the remaining surface of that interface. The net result is an increase in the total light emitted through the window layer.
Advantageously, in accordance with this invention, all of the diode, other than the metal contacts, and the conducting layer, is grown in a continuous process.
REFERENCES:
patent: 5157468 (1992-10-01), Matsumoto
patent: 5329135 (1994-07-01), Terakado
patent: 5717226 (1998-02-01), Lee et al.
patent: 5789768 (1998-08-01), Lee et al.
patent: 5821569 (1998-10-01), Dutta
patent: 6072203 (2000-06-01), Nozaki et al.
patent: 6225648 (2001-05-01), Hsieh et al.
Chen John
Liang Bingwen
Shih Robert
AXT, Inc.
Edwards Jean C.
Lee Eddie
Nguyen Joseph
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