IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 2, MARCH/APRIL 2002 339 High-Power Phosphor-Converted Light-Emitting Diodes Based on III-Nitrides Regina Mueller-Mach, Gerd O. Mueller, Michael R. Krames, and Troy Trottier Abstract—Phosphor conversion of light-emitting diode light for white light sources and some monochrome applications requires particular phosphor properties and has to take into account spe- cific issues if aimed at high-power output. Limitations and solu- tions will be discussed, giving special considerations to drive and temperature dependencies. Efficiencies of 32 lm/W for white with good color rendering at 4600 K and 35 lm/W for green (535 nm) have been demonstrated. Index Terms—III–nitrides, light-emitting diodes (LEDs), pcLED, phosphor, phosphor conversion, solid-state lighting, solid-state illumination. I. INTRODUCTION H IGH BRIGHTNESS light-emitting diodes (LEDs) based on the III–P and III–N materials cover the bulk of the spectral range from 380 to 780 nm, defined as visible. As any color can be mixed from three primaries, the question as to the usefulness of phosphor conversion of III–N LED pump wave- lengths into other spectral components by luminescence of spe- cial materials (phosphors) is legitimate. Taking into account, that no conversion process is loss free, justifies the question even more. However, there are at least three reasons to consider phos- phor conversion of blue or near-UV LED pump light into other wavelength bands. • Mixing of colors from LEDs with vastly different de- pendencies of basic properties upon temperature, time (aging), and drive demands active feedback control, if the color mix is wanted to remain constant. • It might be easier to achieve a good illumination “white,” if wider spectra of phosphors than those of LEDs are mixed with each other or with the pump spectrum. • As at present there are large differences in achieved effi- ciencies of LEDs with wavelength, it can be advantageous to convert a highly efficient pump into a color of the lower efficiency region. The first argument however does hold only, if the phosphor spectra are stable with time, temperature and excitation level. In this case, the only variable is the LED, and all other parts seem- ingly change proportional to it. We are going to discuss this req- uisite in more detail, and will find that eventually good approxi- mations can be found. Of the two alternative approaches—blue pump plus one or two phosphors, or UV pump plus three phos- phors—only the former will be discussed here. Manuscript received December 17, 2001; revised February 7, 2002. R. Mueller-Mach, G. O. Mueller, M. R. Krames, and T. Trottier are with the Lumileds Lighting, Advanced Laboratories, San Jose, CA 95131 USA. Publisher Item Identifier S 1077-260X(02)03765-6. Conversion efficiency will turn out to be a key issue, finally deciding on the significance of any solution. It will be factored down to some independent variables, which can be determined in separate assessments. It will be necessary to discern in many of the considerations between the intended uses of the pcLED—signaling or illumi- nation. While for signaling, meaning to look on or in to the light source, chromaticity (and brightness) is the only color prop- erty of interest, for illumination the color appearance of objects under this light, or color rendition, is of importance. It is mea- sured by international convention by the general color rendering index Ra introduced by CIE in 1976. We will not try to explain it here, but refer to [1] and text books of color science or lighting. In this paper, we are mainly discussing power LEDs, the ob- vious major application is in illumination, and therefore, solu- tions with high color rendering are the most wanted. There are different “whites,” “warm white,” and “cold white” being the most common descriptions. For almost one century, incandes- cent lamps have been the only light source. Low light levels are usually associated with “warmer” white, as the color tempera- ture decreases during dimming. Scientifically, all chromaticties corresponding to black body spectra making up the Planckian locus are “whites.” A color near to this locus, a “white,” is com- monly characterized by a CCT (correlated color temperature) and a deviation of its chromaticity from the planckian locus. This deviation is called “tint,” as it is perceived as a slight col- oration. In standards like the automobile industries’ SAE J578, the maximum tint permitting the designation “white” is speci- fied. (This range in a color diagram is illustrated in Fig. 13.) II. PHOSPHORS FOR PHOSPHOR-CONVERTED LEDS (pcLED) Obviously, the primary requisite for a phosphor to convert the radiation from a pump LED into luminescence emission is that it absorbs the pump wavelength. A strong absorption will turn out to be highly wanted. The strongest absorption or highest absorption coefficients are those of the band-to-band transitions in direct gap semiconductors; LED materials them- selves being good examples. Other well-known and used semi- conductor phosphors are ZnS, ZnSe, ZnO, and CdS. Their lumi- nescence is based on the recombination of electron hole pairs, generated by fundamental absorption, via excitons or shallow defect states, if we are interested only in emission with small Stokes’ shift. It turns out that this class of materials has not yet proven to be of real value for pcLED, the main difficulty arising from strong temperature dependencies. Strong absorption can also be expected from dipole-allowed electron transitions in dopant ions, if the dopant concentration 1077-260X/02$17.00 © 2002 IEEE