Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Utility Patent
1998-09-08
2001-01-02
Carroll, J. (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S099000, C257S103000, C257S621000, C257S622000
Utility Patent
active
06169294
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to semiconductor light emitting diodes—in particular to light emitting diodes fabricated with III-V compound semiconductor.
Among all the III-V compound semiconductor family, nitride material has the highest energy bandgap. The light emitting wavelength ranges from violet color to yellow color. It is most suitable for short wavelength, high efficiency devices. Although researchers have devoted a great deal of effort to develop this device, the product has not been commercialized due to the following reasons:
1. No suitable substrate material to match the crystalline structure.
2. Difficulty to grow Indium-gallium nitride (InGaN), especially that with high indium content.
3. Difficulty to grow high concentration p-type gallium nitride (GaN); and
4. Difficulty to form good electrode.
In 1993, the Nichia Chemical Industries Ltd. in Japan announced success in making a first commercial blue color light emitting diode using gallium nitride material. Subsequently, the company further develop a green color light emitting diode. At present, many research organizations have devoted a great deal of resources to develop such a product. Due to the foregoing difficulties, only very few have obtained breakthroughs and commercial success.
The structure used by the Nichia Chemical Industries Ltd. is shown in
FIG. 1. A
sapphire substrate is sequentially grown with gallium nitride (GaN) nucleation layer, an n-type gallium nitride (GaN) buffer layer, an n-type aluminum gallium nitride (AlGaN) cladding layer, an undoped quantum well light emitting layer, a p-type AlGaN cladding layer, and a p-type GaN contact layer. The structure is coated with nickel-gold (NiAu) light transmitting electrode and p-type NiAu. Due to the fact that the sapphire is an insulator, it is necessary to etch a section of the light emitting diode to contact the n-type cladding layer and to form an n-type titanium aluminum electrode. Such a traditional technique has the following shortcomings:
1. The carrier concentration of the p-type gallium nitride contact layer is less than 1*10
18
cm
−3
after thermal annealing. The resistivity is high around 1 ohm-cm. Due to such a high resistivity, the current flowing down from the electrode cannot distribute evenly to the die. The uneven current distribution causes current crowding and lowers the light emitting efficiency.
The prior art shown in
FIG. 1
uses a very thin nickel-gold layer of less than few hundred angstroms (A) as a current spreading layer to effectively spread the current over the entire die. However, such a current spreading layer has a transmittance of less than 50%. The major portion of the light emitted from the light emitting diode is absorbed by the current spreading layer to lower the light emitting efficiency.
2. In a conventional structure, the p-type and n-type electrodes both lie on the same side of the die and bonding pad must have a diameter lager than 100 &mgr;m.
The light intensity of an LED is directly related to the operation current density and to the emitting area. Furthermore a lower current density increases the reliability of the LED. For InGaN light emitting diode using sapphire substrate, the die size is 350×350 &mgr;m. For high intensity aluminum-gallium-indium light emitting diode, the die size is between 225×225 &mgr;m and 300×300 &mgr;m. The reason why the InGaN die occupies a larger area is that a portion of the area is used for wire bonding. For a 2 inch wafer with a yield of 100%, the number of InGaN light emitting diode dice is 16500. On the other hand, the number of AlGaInP light emitting diode dice is between 22500 and 40000. If the die size of the InGaN light emitting diodes can be reduced, the productivity is increased and the cost can be reduced.
3. A traditional InGaN light emitting diode using an insulating substrate has the electrodes formed on the same plane. When packaged, the two bonding wires must be wire bonded on the same side. Such a bonding method is incompatible to conventional bonding method and is not cost effective.
SUMMARY OF THE INVENTION
An object of this invention is to avoid the use of the light-transmitting layer of an InGaN light emitting diode, thus increasing the light efficiency of the LED. Another object of this invention is to provide single wire bonding contacts to the light emitting diode. Still another object of this invention is to reduce the manufacturing cost by using a small die size.
These objects are achieved by inverting an LED fabricated on a transparent sapphire substrate and mounting the inverted structure on a conductive silicon substrate. The cathode of the LED is connected to the top electrode of the silicon substrate. The surface of the silicon substrate is partially etched where a top anode is formed. In comparison with a conventional InGaN LED with both electrodes on the same side of the LED chip forming electrodes on two sides of the silicon substrate allows more light to be emitted through the sapphire. In addition, by coating a reflector layer underneath the LED which reflects light incident toward the bottom, the light efficiency can be increased. Since the cathode of the LED chip does not serve as a wire bonding pad, its area can be reduced, resulting in a reduction of the LED chip size.
REFERENCES:
patent: 5744857 (1998-04-01), Yamamoto
patent: 5925898 (1999-07-01), Spaeth
patent: 2-290084 (1990-11-01), None
F. F. Fang et al., “Merging Semiconductor Optoelectronics with Silicon Technology” IBM Technical Disclosure Bulletin, vol. 19, No. 10 (Mar. 1977) pp. 3959-3960.*
Biing-Jye Lee
Chang Chuan-Ming
Chia-Cheng Liu
Ming-Jiunn Jou
Tarn Jacob C.
Carroll J.
Epistar Co.
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