Abstract
The development of wide band gap III-V nitride compound semiconductors has led to the commercial production of high-efficiency LEDs. The recent advent of blue InGaN technology has made it possible to produce a conventional white LED in which white light is obtained by coating a Y3Al5O12:Ce (or Sr3SiO5:Eu) phosphor onto a blue LED chip. In this device, known as a two-band white LED, white light is generated by additive color mixing of the blue light emitted by the blue LED and the yellow light emitted by the Y3Al5O12:Ce phosphor. The development of a white LED is important because it opens the way for LED applications such as light bulbs and fluorescent lamps with high durability and low energy consumption. However, the spectral composition of the light produced by the conventional two-band white LED differs from that of natural white light, particularly in the red region. The color properties of conventional two-band white LEDs can potentially be improved by adding another component to create a white LED based on three emission bands (a three-band white LED). Full-color fluorescent display devices have been developed by using a combination of ZnS:Ag (blue), ZnS:Cu,Al (green), and ZnCdS:Ag (red) phosphors excited by a nearUV LED. In addition, a white light source has been obtained by intergrating ZnS:Ag (blue), ZnS:Cu,Al (green), and Y2O2S:Eu (red) phosphors, and a UV-LED (350 nm). White light has also been achieved by combining a blue LED (460 nm) with SrGa2S4:Eu (green) and SrS:Eu (red) phosphors. Previously, we constructed a three-band white LED by combining a blue LED (465 nm) with SrGa2S4:Eu (green) and ZnCdS:Ag,Cl (red) phosphors. In the present work, we investigated the optical properties of a white LED which was obtained by using a blue LED (465 nm) in conjunction with SrGa2S4:Eu (green) and SrY2S4:Eu (red) phosphors. The SrY2S4:Eu phosphor was chosen over the ZnCdS:Ag,Cl phosphor used in our previous work in order to improve the red color characteristics of the white LED.
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