Layered semiconductor structure for lateral current...

Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction

Reexamination Certificate

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C257S094000, C257S103000

Reexamination Certificate

active

06222205

ABSTRACT:

PRIORITY CLAIM
This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 197 41 609.8, filed on Sep. 20, 1997, the entire disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to a structure of semiconductor layers for conducting and laterally spreading a current between a current injection surface or current collecting surface of an electrode and an active area surface arranged parallel thereto, wherein the electrode surface has a different, e.g. smaller, surface area than the active area surface. The invention further relates to a semiconductor light emitting diode incorporating such a current spreading structure.
BACKGROUND INFORMATION
It is a well known dilemma in the field of semiconductor devices, that on the one hand a current flow should be distributed as uniformly as possible over the entire active region surface of a semiconductor device such as a light emitting diode (LED), while on the other hand, the current injecting electrode or contact should be made as small as possible, i.e. covering the smallest portion of the active region surface as possible, to avoid blocking or reflecting the light emitted from the active region and for reasons of costs and complexity. The same holds true for current injecting electrodes or contacts on the surface of, or internally within, semiconductor devices other than LEDs, and also applies to current collecting electrodes. In general, the electrode surface is any current carrying surface such as a contact surface that is connected to a bonding wire or the like.
In view of the above dilemma, certain problems arise. Namely, when using an electrode surface that is smaller than the associated active region surface, the current flow becomes “crowded”, i.e. the current density is highest, directly under the electrode surface and diminishes laterally away from the electrode surface. Moreover, since the electrode surface acts as a reflecting screen for the emitted light (e.g. in the case of an LED), it is especially important that a good current density is provided to the areas of the active region surface laterally away from the electrode surface. Therefore, it is a well known problem to provide lateral current spreading between the electrode surface and the active region surface.
Several different structural arrangements have been proposed in the prior art for achieving the desired lateral current spreading. Published European Patent Specification 0,434,233 (Fletcher et al.) discloses one possible solution for LEDs, wherein a relatively thick transparent window layer having a lower electrical resistivity than the active layers is arranged between the electrode surface and the active region surface. Particularly, the transparent window layer has a band gap greater than the band gap of the active layers and a resistivity at least an order of magnitude less than that of the active layers. A metal contact or electrode layer is formed over a portion of the surface of the transparent window layer. This published reference discloses that thicker window layers tend to be desirable, and layer thicknesses in the range from 2 to 30 &mgr;m are suitable, while thicknesses in the range from 5 to 15 &mgr;m are preferred.
Published European Patent Application 0,551,001 (Fletcher et al.) similarly discloses the use of a thick transparent layer arranged over the active region of an LED. This reference discloses the quasi-conical current spreading effect of such a uniform, thick, transparent window layer. The window layer is purposely rather thick, to avoid or minimize the internal reflection of the light emitted from the active region. The thickness is determined as a function of the width of the window layer and the critical internal reflection angle.
The article “High-Efficiency InGaAlP Visible Light-Emitting Diodes” by H. Sugawara et al., published in Jpn. J. Appl. Phys. Vol. 31 (1992), pages 2446 to 2451 also discloses an LED structure including a current spreading layer having a thickness of 7 &mgr;m (see page 2449).
FIG. 4
of this article clearly shows the improvement in total light emission or light emission efficiency when using the GaAlAs current spreading layer as compared to a structure without such a current spreading layer.
The article “Two-Fold Efficiency Improvement in High Performance AlGaInP Light Emitting Diodes . . . ” by K. Huang et al. in Appl. Phys. Lett. 61(9), Aug. 31, 1992, pages 1045 to 1047, also discloses a thick window layer for achieving current spreading, wherein the window layer thickness is particularly in the range from 9 to 63 &mgr;m, and especially 45 &mgr;m. Once again, the window layer thickness is defined in view of the light reflection effects.
A substantial disadvantage of all of the above mentioned conventional thick window layers is that the window layer has a substantially greater thickness than the active layers, which typically have thicknesses less than 1 &mgr;m. This very great thickness of the window layer leads to a high material consumption for forming the window layer, and also requires a very long time for carrying out the epitaxial growth using conventional equipment. While it is possible to use more complicated and more expensive equipment for carrying out the epitaxial growth more quickly, it is still not possible to reduce the material consumption and costs, and the equipment costs are increased even further.
The article “Highly Reliable Operation of Indium Tin Oxide AlGaInP Orange Light-Emitting Diodes” by J. F. Lin et al., published in Electronics Letters, Oct. 13, 1994, Vol. 30, No. 21, pages 1793 to 1794 discloses a current spreading window layer made of indium tin oxide (ITO). The disclosed ITO current spreading layer is 600 Å thick. Disadvantageously, the production costs and material costs of an LED incorporating such an ITO current spreading layer are even considerably higher than conventional LEDs or LEDs with the above mentioned thick window layers.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide a layered semiconductor structure for achieving lateral current spreading, which can be manufactured by relatively simple process technologies, which is cost-economical with regard to the materials and the processes, and which achieves the required current spreading or even improves the current spreading beyond that which could be achieved in the prior art, and in a more compact structure than was possible in the prior art. It is a further object of the invention to provide a semiconductor light emitting diode (LED) incorporating such a layered semiconductor current spreading structure, whereby the LED achieves a very good light power output and efficiency due to the improved lateral current spreading, and wherein the LED may be manufactured in a simple and cost-economical manner. The invention further aims to overcome or avoid the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present description.
The above objects have been achieved in a layered semiconductor current spreading structure according to the invention, wherein the current spreading structure is arranged between an electrode surface, such as a current injection surface of an electrical contact, and an active region surface, which is larger than the electrode surface and arranged parallel to the electrode surface for example. Particularly according to the invention, the layered semiconductor current spreading structure comprises at least one pair of parallel adjacent semiconductor layers that have different material characteristics so as to form a heterojunction therebetween. Namely, the two layers of each pair are made of different semiconductor materials or compound materials having the same elemental components in different composition ratios. Another related feature is that the two layers of each heterojunction layer pair respectively comprise different energy band gaps. A majority charge carrier energy band discontinuity is formed across the hete

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