AlInGaP LED technology has been instrumental in not only establishing the High Brightness portion of the LED market; it is fast becoming the mainstream workhorse of the industry. Indeed, AlInGaP itself is splitting into High Brightness versions, as well as a lower brightness versions (smaller die, less deposited layers, faster fab throughput, etc.). The low brightness versions are designed to replace older LED technology types with a lower cost AlInGaP derivative that would still be 2 to 3X higher in brightness than the LED style it is intended to replace at only a modest premium in price, if any. On their own, High Brightness AlInGaP LEDs, have led the industry into new and varied applications as they have emerged as the dominant technology for Red, Red-Orange, Amber, and Yellow LED applications.

In terms of High Brightness AlInGaP/GaAs LEDs, there are three basic types of implementation: 1) absorbing substrate AlInGaP (AS); 2) transparent substrate AlInGaP (TS); and 3) UNIROYAL Optoelectronics' POWER-BRite™ line of AlInGaP LEDs. POWER-BRite™ LEDs utilize an integrated, distributed Bragg Reflector (BR) or “mirror” to enhance light propagation, the results of which compares very well with the brightest AlInGaP die produced throughout the LED industry. UNIROYAL Optoelectronics is focused on High Brightness AlInGaP LEDs and does not provide or plan to provide lower brightness derivatives such as those referred to, or AS versions wherein the GaAs substrate absorbs light unnecessarily.

The Bragg Reflector (BR), which is commonly used in vertical cavity surface emitting lasers (VCSELs), has an interesting history. Prior to 1915, the problem of calculating the crystal structures based on formula was an exceedingly complicated one. Both the space lattices and the wavelengths in the spectra were unknown quantities. It was consequently a discovery of epoch-making proportions when W. L.Bragg (winner of the Swedish Royal Academy of Sciences' Nobel Prize in Physics in 1915) made a significant discovery when he determined that this phenomenon could be addressed mathematically when the successive parallel planes are positioned to cause the reflection to pass through the lattice points. In this way, the ratio between the wavelengths and the distances between the planes and from each other, can be calculated by a simple formula from the angle of reflection.

For example, if a number of cubes are laid on and beside each other in such a way that one cube face coincides in every case with the face of an adjoining cube and their eight vertices always meet in one point, those angular points give a visual picture of the lattice points in the simple cubic lattice. If again a lattice point is placed so as to coincide with the central point of each cube face, the face-centered cubic lattice is obtained, where the centered cubic lattice has one lattice point in every cube-center. With the exception of these examples, there is no cubic lattice that fulfills the condition that parallel planes placed in any direction whatever, pass through all the lattice points, and are also at a constant distance from each other. The space lattice in the regular or cubic system must therefore coincide with one of these, or constitute combinations of them. In lattice combinations in which this condition is not fulfilled, i.e., where parallel planes are positioned to pass through all the lattice points in certain directions and are not equidistant, that condition is revealed by an abnormal intensity distribution among spectra, when reflection takes place within these planes.

Thus the key to understanding the structure of crystals and their reflective properties as put forward with a modern analytical tool to design and develop this science became a reality. UNIROYAL Optoelectronics has incorporated this technology in its' AlInGaP POWER-BRite™ product line which significantly differentiates these products from AS and TS types. The UOE POWER-BRite™ technology is implemented with unique, custom reflectors that are designed for each chromatic wavelength required. At the emitted wavelength, POWER-Brite™ is >99% reflective, exhibiting excellent uniformity over the full emitting surface.

Integrating a POWER-BRite™ BR is an entirely epitaxal growth process in which all layer interfaces are intimately joined to provide a simple, very robust, monolithic structure. This structure exhibits more highly uniform and repeatable results than those associated with the TS removal and bonding system process. The POWER-BRite™ BR consists of several identical pairs of epitaxial dielectric layers each with different refractive indices. The small difference in refractive index between the layers of the “mirror” are built up according to design. In the case of TS AlInGaP, the GaAs substrate upon which the AlInGaP epitaxial layers are grown, is removed and replaced with a transparent substrate, usually GaP.

This is accomplished through a delicate wafer bonding process that removes the AlInGaP from the GaAs and then affixes the AlInGaP layers to its new host with an adhesive bonding method. The amount of adhesive, the uniformity of the bond, the yield, and the quality of the bond (mechanically, electrically and optically) require rigid process controls and sophisticated systems to implement to achieve a good degree of success. Resulting product has shown the industry that TS AlInGaP LEDs resulting from this process, to be of the highest optical efficiencies, but can experience bond joint reflectivity, and device voltage irregularities.

The resulting UNIROYAL Optoelectronics’ POWER-BRite™ technology exhibits very high luminous efficiency and is capable of producing high light propagation over a wide range of drive conditions from <500 µA to >100 mA and operation over wide temperature ranges, depending upon device component packaging. Operational external quantum efficiencies are of the highest order with exceptionally uniform electrical drive characteristics, and long life.