A methodological study of the best solution for generating white light using nitride-based light-emitting diodes

In search of suitable white-LED for general illumination. we fabricated various types of white-LEDs using different methods. As the first method, we used the multichip method in which multiple emitters were mounted in one package. This type showed a good general color-rendering index (Ra) = 90 by the optimizing the emission wavelength of each LED chip. However, the electric driving circuitry was too complex for use in general illumination. Secondly, we used a monolithic white-LED by using the multicolor emitting multiple-quantum well (MQW) for the active layers, which consisted of quantum wells (QWs) with different In compositions. A high: Ra = 80.1 was obtained in the three-color-emitting white-LED but the luminous efficacy (170 was only 8.11 Im/W. As the third method, we used the color conversion method using phosphors. We fabricated a white-LED which consisted of a near-UV-LED chip and blue/yellow phosphors in order to improve the luminous efficacy of the white-LED under high forward-bias current. At 100 mA. the luminous flux (10 was estimated to be 7.6 Im. However, this white-LED degraded quickly, because the epoxy resin used for package was the general purpose one and deteriorated under the UV-light from the n-UV-LED, Next. we improved the Ra and eta(L) of a traditional white-LED which consisted of blue-LED chip and yellow phosphor. In order to improve the Ra, we added a newly developed red phosphor. We obtained a Ra = 87.7 at low-color-temperature. Then, in order to improve the efficiency of the white-LED, we improved the extraction efficiency (eta(EX)) of the blue-LED by using a patterned sapphire substrate and a high reflection Rh-mesh-patterned p-electrode. Then, we obtained a 62.0 Im/W at 20 mA. As a result, we concluded that the color conversion method of using a blue-LED for general illumination has advantages in efficiency, color-rendering, cost and lifetime. It also has simpler electric driving circuitry.