In general, a light emitting diode (LED) is a device including a compound containing gallium (Ga), phosphorus (P), and arsenic (As) to emit light when a current is applied. Since the LED has a lifespan greater than that of a bulb and a fast response speed, the LED has recently come into the spotlight as a next-generation display device.
Since red, yellow, and green LEDs have been developed, a blue LED is introduced by Dr. Nakamura Shuji. In recent years, researches on a white LED using the blue LED have been actively performed.
Since white light is the most similar to natural light to relieve eye strain, efforts for realizing the white light in the LED as well as other types of display devices have been made.
As a result of such efforts, a cold cathode fluorescent lamp (CCFL) typically used for a computer, a mobile phone, and a projector has been gradually replaced by the white LED device.
In particular, the white LED device has recently been applied to a back light unit (BLU) of a liquid crystal display (LCD) and a lighting, and an application range thereof has gradually been broadened.
The above-described white LED device may be generally divided into a single-chip type and a multi-chip type according to a method of realizing white light.
First, the single-chip type includes a blue LED chip and a YAG-based yellow phosphor. In detail, the single-chip type has a structure in which an encapsulant resin containing the YAG-based phosphor surrounds the blue LED chip.
The above-described single-chip type white LED device provides white light in such a manner that a portion of blue light emitted from the blue LED chip is absorbed to the YAG-based yellow phosphor, the absorbed blue light is converted into yellow light having a long wavelength through the YAG-based yellow phosphor and emitted, and the emitted yellow light and blue light emitted from the blue LED chip, which is not absorbed, are mixed to provide the white light.
However, since the white light realized by the above-described single-chip type white LED device has a wide wavelength spacing between a blue color and a yellow color to generate the color flash effect due to color breakup, mass production of the white LED having the same color coordinates has been difficult.
Also, the color temperature (CT) and the color rendering index (CRI), which are important characteristics for an illumination light source, are extremely difficult to be adjusted. The typical white LED generally has the color rendering index of 75 to 80.
To solve the above-described limitations, the white LED device for increasing the color rendering index using one blue LED chip and a plurality of phosphors is disclosed. However, since the plurality of phosphors need to be mixed, many limitations such as a low yield, heat generation, and a low optical efficiency may occur. Thus, only two kinds of phosphors can be mixed.
Next, the multi-chip type is a method for realizing white light by mounting each of a blue, green, and red LED chips to one package to mix three primary colors of light unlike the above-described single-chip type.
However, in case of the multi-chip type, whereas a high efficiency is achieved, a manufacturing cost is high, and practical efficacy is lower than that of the single-chip type because a high efficiency green LED does not currently exist.