Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – Plural light emitting devices
Reexamination Certificate
1994-12-23
2002-06-11
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
Plural light emitting devices
C257S090000, C257S093000
Reexamination Certificate
active
06403985
ABSTRACT:
TECHNICAL FIELD
This invention is in the field of light emitting diodes (LEDs).
BACKGROUND OF THE INVENTION
The background to the invention may be conveniently summarized in connection with four main subject matters. LEDs, LED bars, LED arrays and Lift-off methods, as follows:
LEDs
LEDs are rectifying semiconductor devices which convert electric energy into non-coherent electromagnetic radiation. The wavelength of the radiation currently extends from the visible to the near infrared, depending upon the bandgap of the semiconductor material used.
Homojunction LEDs operate as follows: For a zero-biased p-n junction in thermal equilibrium, a built-in potential at the junction prevents the majority charge carriers (electrons on the n side and holes on the p side) from diffusing into the opposite side of the junction. Under forward bias, the magnitude of the potential barrier is reduced. As a result, some of the free electrons on the n-side and some of the free holes on the p-side are allowed to diffuse across the junction. Once across, they significantly increase the minority carrier concentrations. The excess carriers then recombine with the majority carrier concentrations. This action tends to return the minority carrier concentrations to their equilibrium values. As a consequence of the recombination of electrons and holes, photons are emitted from within the semiconductor. The energy of the released photons is close in value to that of the energy gap of the semiconductor of which the p-n junction is made. For conversion between photon energy (E) and wavelength (&lgr;), the following equation applies:
E
(
e
v
)
=
1.2398
λ
(
μ
m
)
The optical radiation generated by the above process is called electroluminescence. The quantum efficiency &eegr; for a LED is generally defined as the ratio of the number of photons produced to the number of electrons passing through the diode. The internal quantum efficiency &eegr;
i
is evaluated at the p-n junction, whereas the external quantum efficiency &eegr;
e
is evaluated at the exterior of the diode. The external quantum efficiency is always less than the internal quantum efficiency due to optical losses that occur before the photons escape from the emitting surface. Some major causes for the optical losses include internal re-absorption and absorption at the surface. The internal efficiency can exceed 50% and, sometimes, can be close to 100% for devices made of a very high-quality epitaxial material. The external quantum efficiency for a conventional LED is such lower than the internal quantum efficiency, even under optimum conditions.
Most commercial LEDs, both visible and infrared, are fabricated from group III-V compounds. These compounds contain elements such as gallium, indium and aluminum of group III and antimony, arsenic and phosphorus of group V of the periodic table. With the addition of the proper impurities, by diffusion, or grown-in; III-V compounds can be made p- or n-type, to form p-n junctions. They also possess the proper range of band gaps to produce radiation of the required wavelength and efficiency in the conversion of electric energy to radiation. The fabrication of LEDs begins with the preparation of single-crystal substrates usually made of gallium arsenide, about 250-350 &mgr;m thick. Both p- and n-type layers are formed over this substrate by depositing layers of semiconductor material from a vapor or from a melt.
The most commonly used LED is the red light-emitting diode, made of gallium arsenide-phosphide on gallium arsenide substrates. An n-type layer is grown over the substrate by vapor-phase deposition followed by a diffusion step to form the p-n function. Ohmic contacts are made by evaporating metallic layers to both n- and p-type materials. The light resulting from optical recombination of electrons and holes is generated near the p-n junction. This light is characterized by a uniform angular distribution; some of this light propagates toward the front surface of the semiconductor diode. Only a small fraction of the light striking the top surface of the diode is at the proper angle of incidence with respect to the surface for transmission beyond the surface due to the large difference in the refractive indices between semiconductor and air. Most of the light is internally reflected and absorbed by the substrate. Hence a typical red LED has only a few percent external quantum efficiency, that is, only a few percent of the electric energy results in external light emission. More efficient and therefore brighter LEDs can be fabricated on a gallium phosphide substrate, which is transparent to the electroluminescent radiation and permits the light to escape upon reflection from the back contact. For brighter LEDs, AlGaAs, with the Al percentage equal to 0-38%, grown on GaAs substrates is used. The AlGAs LEDs are usually about 50 &mgr;m thick and are grown on GaAs by liquid-phase epitaxy(LPE). The p-n junctions are diffused. For even brighter LEDs, the AlGAs layers are grown even thicker (−150 &mgr;m), and the GaAs substrates are etched off. The thick AlGAs layer becomes the mechanical support. With no substrate and a reflector at the back side one can double the external efficiency.
Visible LEDs are used as solid-state indicator lights and as light sources for numeric and alphanumeric displays. Infrared LEDs are used in optoisolators, remote controls and in optical fiber transmission in order to obtain the highest possible efficiency.
The advantages of LEDs as light sources are their small size, ruggedness, low operating temperature, long life, and compatibility with silicon integrated circuits. They are widely used as status indicators in instruments, cameras, appliances, dashboards, computer terminals, and so forth, and as nighttime illuminators for instrument panels and telephone dials. Visible LEDs are made from III-V compounds. Red, orange, yellow and green LEDs are commercially available. Blue LEDs may be formed of II-VI materials such as ZnSe, or ZnSSe, or from SiC.
LEDs can also be employed to light up a segment of a large numeric display, used for example, on alarm clocks. A small numeric display with seven LEDs can be formed on a single substrate, as commonly used on watches and hand-held calculators. One of the major challenge for LEDs is to make very efficient LEDs, with high external efficiency.
LED BARS
A linear, one-dimensional array of LEDs can be formed from a linear series of sub-arrays, wherein the sub-arrays comprise a semiconductor die with several hundred microscopic LEDs. Each LED is separately addressable and has its own bond pad. Such a die is referred to as an LED bar and the individual LEDs in the array are referred to as “dots” or “pixels”.
LED bars are envisioned as a replacement for lasers in laser-printer applications. In a laser printer, the laser-printer applications. In a laser printer, the laser is scanned across a rotating drum in order to sensitize the drum to the desired pattern, which is then transferred to paper. The use of electronically scanned LED bars for this purpose can result in replacement of the scanning laser with a linear stationary array of microscopic LEDs that are triggered so as to provide the same optical information to the drum, but with fewer moving parts and possibly less expensive electric-optics.
Currently, commercial LED bars are of two types: GaAsP on GaAs substrates and GaAlAs on GaAs. The GaAsP/GaAs bars are grown by Vapor Phase Epitaxy (VPE). Because of the lattice mismatch between GaAsP and GaAs, thick GaAsP layers must be grown of about 50 microns or more thickness and growth time per deposition run is long (5-6 hours). LED bars produced in this fashion are not very efficient and consume much power, and have relatively slow response times.
The second type of LED bar, i.e. GaAlAs/GaAs is grown by Liquid Phase Epitaxy (LPE). LPE growth is cumbersome and does not lead to smooth growth, or thin uniform layers, and is not well suited to the growth of complex structures requiring layers of d
Dingle Brenda
Fan John C. C.
McClelland Robert W.
Shastry Shambhu
Spitzer Mark B.
Hamilton Brook Smith & Reynolds P.C.
Jackson, Jr. Jerome
Kopin Corporation
LandOfFree
Method of making light emitting diode displays does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of making light emitting diode displays, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of making light emitting diode displays will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2922691