Lighting is related to human civilization from beginning to end.
In 1879, Edison invented incandescent lamps, bringing humans from the era of flame lighting to the era of electric light sources. For more than a century, the electric light source lighting technology has developed by leaps and bounds, and has experienced three important stages represented by incandescent lamps, fluorescent lamps and high-intensity discharge lamps (HID). Nowadays, with the emergence of a new generation of semiconductor materials and breakthroughs in light-emitting diode (LED) packaging technology, as well as the continuous improvement of LED power levels, LED light sources are setting off a fourth revolution in the development of electric light sources.
The LED light source fundamentally changes the light-emitting mechanism of the light source. While improving the quality and utility of the light, it can also improve the environment and save energy, and has high economic benefits. At present, white LED light source is slowly consuming the traditional light source in various fields. Its application fields include: local range low illumination illumination, liquid crystal (LCD) display backlight, traffic lighting, indoor lighting and special lighting. According to Toshiba, in the special lighting market, 16% will be replaced by LEDs in 2010 and 30% in 2012.
However, from the current situation, the main application of solid lighting is still in the field of color LED lighting. As the ultimate goal of the LED industry, high-power high-brightness white LEDs have not achieved the high performance expected by mechanism analysis in today's market, and the price is relatively high.
Technical Principles To understand the progress of white LEDs, let us first add a little bit of LED principles.
The LED is made of a III-V compound such as gallium arsenide (GaAs) or gallium arsenide (GaAsP), and its core is an electroluminescent PN junction. One layer of the PN junction carries an excess of electrons, and the other layer forms a positively charged "hole" (carrier) due to the lack of electrons. When a voltage is applied across the PN, the electron and the "hole" interact with each other. Combines and releases energy in the form of photons, and if the energy of the photons is in the visible range, it radiates light. The wavelength of the LED's illumination is determined by the forbidden band width of the semiconductor material. The forbidden band widths of different materials are different, and the wavelength of the generated light is also different, so that the colors presented are also different. Therefore, different materials can be used to make LEDs of different colors, such as red, green, blue, and the like.
White light is a kind of composite light, generally composed of two-wavelength light or three-wavelength light. At present, there are three main methods for implementing white light by LED: one is to synthesize white light through a combination of red, green and blue multi-chip multi-chip; the other is to use blue LED chip to excite yellow phosphor, which is synthesized by yellow light and blue light emitted by LED blue light and phosphor. White light, in order to improve the color rendering characteristics, an appropriate amount of red and green phosphors can be added; the third is to use ultraviolet light LED (UVLED) to excite three primary color phosphors to synthesize white light.
The solution is good. You may find that the current white LED technology has more or less development bottlenecks. No matter which white light implementation is used, there are chip structure, drive circuit, optical optimization, The limitations of many technical problems, such as packaging process, semiconductor materials, and phosphor selection, are mainly manifested in insufficient brightness, poor uniformity, low color rendering, and long life.
The technical bottleneck is also a business opportunity. At present, a large number of research institutions at home and abroad are actively carrying out research work to solve these problems, and many new technologies have been researched and developed. Who can take the lead?
Flip chip technology. A thick layer of silver emitter is formed on the P electrode. Since the P-type electrode of the thick alloy material has good ohmic contact characteristics and current spreading performance, and the thermal conductivity is larger, the luminous efficiency and heat dissipation capability of the chip are improved. The problem of poor current spreading performance, optical performance and heat dissipation capability of the conventional dressing structure LED is solved.
The National Key Laboratory of Integrated Optoelectronics, Department of Electronic Engineering, University of British Columbia and Tsinghua University, has achieved certain results in this technology research.
Surface roughening technology. The light that satisfies the total reflection is redirected so that it does not pass through the interface due to total reflection, thereby improving the light extraction efficiency and reducing the cost without affecting the light conversion characteristics.
The Osram company of Germany has made the window layer surface of the aluminum indium gallium phosphide (AlInGaP)-based chip into a textured structure with a beveled triangle, and the reflection path of the photon is enclosed in this structure. Using this technique, an external quantum efficiency of more than 50% can be obtained.
Photonic crystal structure. Photonic crystals have a periodic dielectric structure with photonic band gaps and photon localities. The luminous efficiency can be improved by the photonic band gap property. This is because the photonic band gap can suppress the radiation of a certain frequency, and at the same time, when the device emits light at the photonic band gap, more optical modes can be radiated into the air.
At present, photonic crystal structure has become the main technical direction to improve the performance of white LEDs. It has been developed into white LEDs with quantum wells of different wavelengths, quantum dots and array structures. The "ThinGaN" LED developed by Osram has stimulated more light output by the mirror action of the metal film formed on the indium gallium nitride (InGaN) layer.
Drive circuit optimization. The characteristics of the LED light source also imposes high requirements on the driving power supply. At present, the low-power power supply system restricts the energy-saving characteristics of the LED. High efficiency, low cost, small size, and strong stability are the main directions for the design of the LED light source driving circuit.
The Institute of Modern Physics of the Chinese Academy of Sciences has designed a set of drive circuit schemes for high-speed and high-power LEDs, featuring fast pulse front and rear and high current output. In addition, great progress has been made in the design of control circuits and dimming circuits for adjustable brightness and high color rendering white LEDs.
Semiconductor material process. The main line of development of LED technology is the continuous improvement of wafer semiconductor materials and processing technology. Similar to Moore's Law for large-scale integrated circuits, the luminous flux of LEDs follows Haitz's law, which doubles every 18 to 24 months.
Packaging technology. Packaging is also a technology that cannot be underestimated. If the package design or the use of materials is poor, it will directly affect the effectiveness of other technologies.
Japan's OMROM has developed a new packaging technology that combines lens optics and reflective optical structures with a "DoubleReflection" optical structure that allows LEDs to be outputted out of light due to wide angles, improving luminous efficiency.
In addition, there are other technologies such as optical design, chip structure optimization, illuminating area improvement, phosphor materials, etc., which are actively researched and developed.
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