The future of high-efficiency lighting rests in the use of light emitting diodes (LEDs) or other semiconductor devices, specifically those used to emit white light. The actual semiconductor elements, however, produce light of specific limited spectral characteristics. There are several techniques for creating white light using LEDs or the like. The most efficient technique involves combining individual light from LEDs of different wavelength (color) outputs, for example from Red, Green and Blue LEDs, in a diffusely reflective cavity. A variety of such techniques and structures using optical integrating cavities are described in commonly assigned copending U.S. application Ser. Nos. 10/832,464 (filed on Apr. 27, 2004) and 10/601,101 (filed on Jun. 23, 2003), the disclosures of which are incorporated herein entirely by reference.
Phosphor doping techniques for generating white light from LEDs, currently favored by LED manufacturers, include Blue LED pumped with phosphors and Quantum dots pumped with UV LEDs. The macro integration by a diffusely reflective cavity, as in the above-cited applications is more efficient, however, the color rendering index (CRI) of the white light output is typically less desirable than that provided by phosphor-doped LEDs.
Although there are a variety of structures and techniques to fabricate phosphor-doped LEDs, such devices typically operate in one of two ways, as summarized below. In a UV LED pumped with RGB phosphors, non-visible UV light excites the mixture of red-green-blue phosphors doped at some point within the LED package to emit light across the visible spectrum. There is no direct contribution of visible light from the UV LED semiconductor chip within the package. In the other typical approach, a Blue LED is pumped with one or more phosphors doped at a point within the package. Some of the blue light from a blue LED chip (460 nm) excites the phosphor to emit yellow light and then the rest of the blue light is mixed with the yellow to make white light. Additional phosphors can be used to improve the spectral characteristics. In either case, the phosphor doping has been integrated directly into the LED and/or its package, for example by doping a portion of the package or by coating the portion of the package through which the light emerges. Dopants have also been used on reflectors or transmissive layers inside of the package containing the actual LED chip.
However, there are limits to the amount of phosphors that can be integrated into the LED die by such techniques. As a result, the performance of the phosphors degrades over a period of time much shorter than the operational life of the semiconductor LED chip. Epoxy degradation can affect the efficiency of the light created. In addition, there are thermal and sizing issues that must be considered.
Hence a need exists for more effective techniques to use light emitting diodes (LEDs) or other semiconductor devices to produce white light of high quality (e.g. desirable color rendering index) without significant reliance on phosphor doping within the LED die package.