1. Field of the Invention
The present invention relates, generally, to Ce3+-activated silicate phosphor and a white light emitting diode (LED) using the same, more particularly, to a phosphor composition showing a wider emission band than the existing phosphor and capable of obtaining a white light using an ultraviolet LED with a long wavelength as well as a blue LED, and a LED including the same and a fabrication method thereof.
2. Description of the Related Art
A white LED is one of the next generation light emitting devices which may replace the existing general lightings. The white LED is advantageous in showing less power consumption than the conventional light source and a high light emitting efficiency and a high brightness and further having a long lifetime and a rapid response compared to conventional light source.
Prior methods for fabricating a white LED are roughly divided into three: mixing red, green and blue LEDs with high brightness; coating red, green and blue light emitting phosphors on an ultraviolet LED with a long wavelength; and coating a yellow light emitting phosphor on a blue LED.
The first method for mixing red, green and blue LEDs is realized by using three different semiconductor thin films, i.e., one light emitting device using the three chips. It is disadvantageous since the process for manufacturing such a LED requires a great investment costs and the production cost is high.
The second method for coating red, green and blue light emitting phosphors on an ultraviolet LED with a long wavelength is disclosed in the International Patent Laid-open Publication No. WO98/039805. This is the best method for penetrating an ultraviolet light through a three coloring phosphor to produce a white light with three wavelength (red, green and blue light). However, heat is emitted from such a LED severely so that the light emitting efficiency is not good and a phosphor with good light emitting efficiency under irradiation of an ultraviolet light with a long wavelength has not been available yet. Nichia and Toyota Gosei just shows outputs with 2 to 3 mW. The reason is that a transparent resin to cover an ultraviolet LED with a long wavelength has not been developed yet and an organic resin is mainly used. But it absorbs the ultraviolet light and degrades the same, resulting in deteriorating the lifetime and the quality of LED.
The third method for fabricating a white LED by coating a yellow light phosphor on a blue LED has been the most widely researched. The structure of the white LED is simple and it can be easily fabricated. It is advantageous in that it is possible to obtain a white light with high brightness. This method is disclosed in International Patent Laid-open Publication No. WO 98/05078 filed by Nichia of Japan in detail and in “the Blue Laser Diode” by S. Nakamura (Springer-Verlag, P 216-219, 1997) in detail. The blue light emitted from the blue LED is absorbed by a phosphor of Ce3+-doped aluminum garnet (Y2Al5O12:Ce3+;YAG:Ce) and the phosphor emits a yellow light in other words a blue light is combined with a yellow light to produce a white light. However, the YAG:Ce-based light emitting phosphor has a relatively weak light emitting intensity in a red spectral region and it is difficult to obtain good color rendering characteristics. It is also sensitive to a color temperature and is not appropriate as a backlight unit for LCD color backgrounds.
An Eu2+ activated strontium silicate phosphor has been patented by Korea Research Institute of Chemical Technology (in Korea Patent Laid-open Publication No. 2004-0085039) but it has a peak wavelength of 570 nm and emits an orange light with a rather narrow emission bandwidth without a yellow light. Therefore, a white light cannot be obtained when coupled with a blue LED.
It is difficult to find a proper phosphor excited by the blue LED in order to emit a white light using the blue LED besides YAG:Ce or Sr3SiO5:Eu2+. In other words, in the conventional method, YAG;Ce-based phosphor mainly realizes a white LED by combining with the blue LED. In order to solve the above problems, a new yellow light emitting phosphor besides YAG:Ce is desperately required.