Packaging Advances High-Power White LEDs
Anne L. Fischer
High-power white LEDs are in high demand for automotive use and are a potential replacement for the incandescent lightbulb. But problems with size, heat dissipation and efficiency have taken researchers back to the drawing board to come up with new packaging designs.
White LED lamps consisting of a blue-emitting semiconductor LED chip and a yellow-emitting phosphor were examined in two configurations: one in which the phosphor is close to the chip (top) and a more favorable one in which the phosphor is removed from the chip (bottom). Different encapsulant shapes — hemispherical, flat and convex — also were examined, with the hemispherical shape proving to be the most favorable.
Recently, a group of researchers from Rensselaer Polytechnic Institute (RPI) in Troy, N.Y., and from Samsung Advanced Institute of Technology in Suwon, South Korea, developed a package for white-light LEDs that uses a diffuse reflector cup and a large separation between the LED chip and the phosphor. The scientists spent more than a year experimenting with surface roughness, placement of phosphors and the geometry of the encapsulant to determine their effect on phosphor efficiency.
E. Fred Schubert of RPI explained that there were two points to consider in the design of such emitters: the use of diffuse reflectors and the distance of the phosphor from the current-injected semiconductor chip. Using diffuse reflectors breaks the deterministic nature of conventional light-reflector cups. He noted that the angle of reflection for specular reflectors is equal to the angle of incidence, but unpredictable in diffuse reflectors. The researchers reorganized the optical rays to redistribute them in such a way that they could come out of the package.
In conventional LED packages, the wavelength-converting phosphors are placed close to the semiconductor chip, which absorbs much of the phosphorescence. Placing the phosphors farther from the chip reduces the probability of this absorption and increases the efficiency of the emitter and the surface area of the light-emitting region.
The challenges included finding the optimum phosphor and encapsulant configurations and quantifying the diffusiveness of reflectors. To attain various degrees of diffusiveness, the researchers utilized methods ranging from bead blasting to photolithographic processes that are compatible with microchip fabrication technology.
Photolithography followed by etching generally produced a lower reflector diffusivity than bead blasting. The latter, in turn, produced a greater magnitude of surface roughness.
Using a diffusive reflector, a hemispherical encapsulant shape and a remotely placed phosphor, the scientists increased the phosphorescence efficiency of dichromatic LED lamps by 50 percent in simulations and by 27 percent in experiments, compared with conventional packaging designs.
The next step will involve research into whispering-gallery modes in white LEDs with remote phosphor configurations. Schubert explained that his team found these modes to exist within LEDs as optical rays that go in a circular pattern around the perimeter of the package. Such trapped light rays have not been a problem in traditional LEDs because the light is emitted from a point source. But with remote rather than proximate phosphors, whispering-gallery modes could pose a problem, as the light rays would be unable to leave the encapsulant, presenting a new loss mechanism.
The researchers also are focused on understanding and minimizing fundamental loss mechanisms. They hope to develop a generation of omnidirectional reflectors with orders of magnitude lower mirror losses than conventional metal reflectors.
Applied Physics Letters, June 13, 2005, 243505.
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