White LEDs can be roughly divided into three types according to the way of illumination. Among them, blue LEDs with yellow wavelengths of 410 to 460 nm and yellow phosphors are the most common way to produce white light, and are also the best way of efficiency and mass production, but LED manufacturers. Holding a wide variety of patents, coupled with the production management of the phosphor powder itself has not been established, so it is not easy to mass-produce at low cost.
In addition, the white light of the ultraviolet light + RGB phosphor with a wavelength of 365 ~ 460nm, and the combination of three primary light colors of RGB can also obtain white light, but the ultraviolet light encapsulation material and peripheral components are exposed for a long time. In the ultraviolet light environment, the RGB mode has wavelength drift and control complexity, so foreign companies develop dual-wavelength white LEDs.
development path
Figure 1 is a conventional blue LED + phosphor, compared with the package structure of the dual-wavelength white LED. It can be seen from the figure that the dual-wavelength white LED has no need to be used for the phosphor powder distribution, and can also completely solve the complication of the RGB three primary color LED driving circuit. The obtained luminance, wavelength, and color rendering properties can also be freely adjusted within a certain range.
2 is a comparison of the principle of illumination of a conventional white LED and a dual-wavelength white LED. It can be seen from the figure that a conventional white LED uses a phosphor coated GaN-based blue wafer to obtain white light, and a dual-wavelength white LED uses a wafer alone to obtain white light.
The basic structure of the dual-wavelength white LED is on the wafer unit, and at the same time, blue and yellow-green light is generated, and the blue light and the yellow-green light are obtained in a very short distance to obtain a pseudo white light. The pseudo white light has high transparency and purity which cannot be achieved by the conventional white LED. sense.
Figure 3 is a comparison of the structure of a conventional white LED with a dual-wavelength white LED chip. Since the LED manufacturer holds a wide variety of patents, it can only be described briefly. As shown in the figure, the dual-wavelength white LED chip buffer Multi Dot Active is mainly composed of:
‧Multi Dot Active layer ‧Active layer ‧Air Ocean layer
This part plays a very important role when the two wavelengths of blue light and yellow-green light emit light at the same time. Therefore, the researchers established the crystal growth technology of each mold layer to enable the dual-wavelength white light LED to enter the practical stage smoothly.
Dual-wavelength white LED features
Figure 4 is the current LED mainstream blue LED + yellow phosphor, compared with the characteristics of the dual-wavelength LED components, the main characteristics of the dual-wavelength LED are shown in the figure, respectively:
‧Can adjust the uneven distribution of color tone ‧Can be used for white light ‧Long life
The biggest problem with the mainstream blue LED + yellow phosphor combination white LED is its yellow phosphor lifetime, which is shorter than the life of the blue LED chip. The service life of LED chips varies from 80 to 100,000 hours depending on the environment. However, the lifetime of phosphors is only 10,000 to 40,000 hours.
In contrast, dual-wavelength white LEDs use no phosphors at all, and the lifetime of the components is the same as that of LED chips. It can be used for a long time, which means that dual-wavelength white LEDs have far-reaching effects on future lighting applications.
In addition, the dual-wavelength white LED does not use phosphor at all. It can also adjust the distribution of color tone through the wafer fabrication process. Especially in the traditional white LED package , the wavelength of the blue LED and the light main force must be limited to a narrow range, and then The use of phosphor content adjustment, cumbersome and complex process control, compared to the new dual-wavelength white LED, has an absolute advantage in the process yield.
Although the absolute production quantity has not yet reached the quantification stage, the dual-wavelength white LED design adopts a larger tonal range. If the future production efficiency problem and the component supply are taken into consideration, the new dual wavelength is considered. The cost of white LEDs has an absolute competitive advantage, especially in the case of micro-packages and complex component packages, which can easily meet the optical characteristics of the harsh substrate source required by customers.
The new dual-wavelength white LED is composed of blue light and green light, so it can be used for flexible color illumination other than white. The specific method is to control the wavelengths of the water color and the blue-green color. The color tone obtained by this is more pure than the LED using the phosphor. The net color is ideal for LCD backlight modules with very strict chroma requirements.
The above luminescence luminosity has reached the practical stage. If compared with the high-luminance white LED of blue LED+yellow phosphor, the brightness of the new dual-wavelength white LED is not inferior.
In addition, through the improvement of the surface processing method of the component, the brightness of the new dual-wavelength LED is better than that of the conventional white LED. Regarding the color rendering of the novel dual-wavelength white LED, since it is composed of blue light + yellow-green light, there is almost no trouble with red light.
Wavelength distribution characteristics
The novel dual-wavelength white light LED as described above obtains white light by simultaneously emitting light at two different wavelengths of blue light and yellow-green light. Fig. 5 is a wavelength characteristic of a novel dual-wavelength white light. It can be seen from the figure that the peak wavelength of blue light is about 405 nm, and the peak wavelength of yellow-green light is about 570 nm, so that there is almost no red light component.
The future of the new dual-wavelength white LEDs is to expand the wavelength range of yellow-green light. In contrast, the current combination of white LEDs with yellow LEDs and yellow phosphors contains red light components in the yellow phosphor composition, so its color rendering is inferior to that of dual-wavelength white LEDs.
Figure 6 is a wavelength characteristic of a near-yellow-green color-tone LED. It can be seen from the figure that the blue-light peak wavelength of the novel dual-wavelength white LED is almost fixed. The peak wavelength of yellow-green light is different from that of a blue LED that emits blue LED+yellow phosphor, which is very close to 530 nm, which is also yellow-green. When combined with Blu-ray, the main reason for soft-toned color can be achieved.
Another feature of the new dual-wavelength white LEDs is the wavelength distribution characteristic of the fused yellow-green water color system. As shown in Fig. 7, since the blue light becomes a strong hue, the peak wavelength of the blue light end is almost fixed, and the peak wavelength of the yellow-green light shifts toward the short wavelength end, and the blue end and the green end output of other hue are almost constant.
In other words, the new dual-wavelength white LED mainly reduces the output of the green end and achieves the purpose of soft-toning color. Here, the new dual-wavelength white LED should be emphasized, and the wavelength of yellow-green and blue light can be freely adjusted within a certain range according to actual needs. Get an unprecedented LED color.
The above introduces the advantages of the new dual-wavelength white LED. At the same time, the research personnel make full use of the traditional packaging technology to try to improve the shortcomings of the new dual-wavelength white LED, thereby establishing a high-luminance, low-cost technology.
In addition to the high-efficiency packaging method, the brightness can be increased by epitaxial encapsulation, and the added value of the new dual-wavelength white LED can be increased.
Dual-wavelength white LEDs, like blue LEDs, are InGaN-based semiconductor components that are very fragile to static electricity. Therefore, when using a multi-chip package that improves luminance, it is necessary to package the electrostatic protection devices together (Figure 8).
Generally, the AlGanInP-based LED has a VF value of about 2V at 20mA, and an InGaN-based LED of up to 3.3V. In other words, an InGaN-based dual-wavelength white LED, when used in a portable electronic device, it is necessary to use a dedicated driver IC.
The dual-wavelength white LEDs using the new package do not adjust the current value of the LEDs, so researchers are developing packaging technologies that use constant voltage to drive the light-emitting components and improve the color rendering of the LEDs.
The dual-wavelength LED chip is packaged with the red LED chip and turned into white light (Fig. 9). However, the VF values ​​of the wafers are completely different from each other. As the luminance and wavelength distribution of the LED chips are different, it is necessary. Perform complex current limit adjustments.
In addition, heat dissipation and silicone after packaging are also issues to be overcome, so researchers are developing new countermeasure technologies.
At present, ψ3 and ψ5 cannonball type dual-wavelength white LEDs have been mass-produced, and dual-wavelength white LEDs for 3mm and 5mm square SMD will be introduced in the future.
Figure 10 is an ultra-thin LCD backlight module that suppresses the color distribution and component height. As shown in the figure, the white LED is directly fixed on the surface of the thin substrate, and then with a special hood, and the side of the ultra-thin light guide plate. With a total brightness of only 0.25mm, it is ideal for portable electronic devices such as mobile phones.
Conclusion
The above describes the new dual-wavelength white LED. Conventional ultraviolet light + RGB phosphors, and white LEDs of RGB multi-chip type, have problems in that the surrounding components are easily deteriorated, or wavelength drift and control are complicated.
In addition to completely solving the above problems, the new dual-wavelength white LEDs provide ultra-short color distance and pure color to provide another choice for LED downstream application manufacturers. In the future, if the packaging technology is successfully improved and the heat dissipation problem after packaging, the dual-wavelength white LED is expected to play a very important role in the thinning of the electronic machine system. (small soup)
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