LED white light technology principle and development trend

Lighting has always been closely related to human civilization.

In 1879, Edison invented the incandescent lamp, bringing mankind from the era of flame lighting to the era of electric light sources. Over the past century, electric light source lighting technology has made great strides, and has gone through three important stages represented by incandescent lamps, fluorescent lamps, and high-intensity discharge lamps (HID). Today, 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 the fourth revolution in the development of electric light sources.

LED light sources fundamentally change the light-emitting mechanism of light sources. While improving the quality and effectiveness of lighting, they can also improve the environment, save energy, and have high economic benefits. At present, white light LED light sources are slowly swallowing up the market of traditional light sources in various fields. Its application areas mainly include: local low- illuminance lighting, backlight sources for liquid crystal ( LCD ) displays , traffic lighting, indoor lighting and special lighting. According to Toshiba's forecast, in the special lighting market, 16% will be replaced by LED in 2010 and 30% in 2012.

However, from the current situation, the main application of solid-state lighting is still in the field of color LED lighting . As the ultimate goal of the LED industry, high-power and high- brightness white light LEDs have not achieved the excellent performance expected through mechanism analysis in the current market, and the price is relatively high.

Technical principle

To understand the room for improvement of white light LEDs, let us first add a little bit about the principles of LEDs.

LED is made of III-V compounds, such as semiconductors such as gallium arsenide (GaAs) and gallium arsenide phosphide (GaAsP), and its core is an electroluminescent PN junction. One layer of the PN junction carries excess electrons, and the other layer lacks electrons to form positively charged "holes" (carriers). When voltage is applied across the PN, the electrons and "holes" combine with each other and release energy in the form of photons . If the energy of the photons is within the visible light range, light is radiated. The emission wavelength of LED is determined by the bandgap width of the semiconductor material. Different materials have different bandgap widths, and the wavelength of the light produced is also different, so the color presented is also different. Therefore, different materials can be used to make LEDs of different colors, such as red, green, blue, etc.

White light is a composite light, generally composed of two or three wavelengths of light. At present, there are three main methods for LED to achieve white light: one is to synthesize white light by combining multiple chips of red, green and blue primary colors; the second is to use blue light LED chips to excite yellow phosphors, and synthesize white light by LED blue light and yellow-green light emitted by phosphors. In order to improve the color rendering characteristics, appropriate amounts of red and green phosphors can be added; the third is to use ultraviolet LED (UVLED) to excite three primary color phosphors to synthesize white light.

Solution

The current white light LED technology has more or less some development bottlenecks. That is, no matter which white light realization method is adopted, there are many technical problems such as chip structure, drive circuit, optical optimization, packaging process, semiconductor materials, phosphor selection, etc., which are mainly manifested in insufficient brightness, poor uniformity, low color rendering , and short life.

Technical bottlenecks are also business opportunities. Currently, a large number of research institutions at home and abroad are actively conducting research to solve these problems, and many new technologies have been studied and developed. Who can take the lead?

Flip chip technology. A thick layer of silver emitter is made on the P electrode. Since the P-type electrode made of thick alloy material has good ohmic contact characteristics and current expansion performance, and has greater thermal conductivity, the chip's luminous efficiency and heat dissipation capacity are improved, solving the problems of poor current expansion performance, optical performance and heat dissipation capacity of traditional positive structure LEDs.

The University of British Columbia, Canada, and the National Key Laboratory of Integrated Optoelectronics , Department of Electronic Engineering , Tsinghua University have both achieved certain results in the research of this technology.

Surface roughening technology changes the direction of light that meets total reflection so that it does not pass through the interface due to total reflection, thereby improving light collection efficiency and reducing costs without affecting light conversion characteristics.

Osram of Germany has made the window layer surface of aluminum indium gallium phosphide (AlInGaP) based chips into a texture structure with inclined triangles, and the reflection path of photons is enclosed in this structure. This technology can achieve an external quantum efficiency of more than 50%.

Photonic crystal structure. Photonic crystal has a periodic medium structure with photon bandgap and photon localization. The photon bandgap characteristics can be used to improve the luminous efficiency. This is because the photon bandgap can suppress the radiation of a certain frequency. At the same time, when the device's luminous frequency is in the photon bandgap, more light modes can be radiated into the air.

At present, photonic crystal structure has become the main technical direction for improving the performance of white light LEDs. White light LEDs with quantum well, quantum dot and array structures of different wavelengths have been developed. The "ThinGaN " LED developed by Osram stimulates more light output through the mirror effect of the metal film formed on the indium gallium nitride (InGaN) layer.

Optimize the driving circuit. The characteristics of LED light sources also put forward high requirements for the driving power supply . At present, the low-power power supply system restricts the energy-saving characteristics of LED. High efficiency, low cost, small size and strong stability are the main directions of LED light source driving circuit design.

The Institute of Modern Physics of the Chinese Academy of Sciences has designed a set of driving circuit solutions for high-speed high-power LEDs , which has the characteristics of fast leading and trailing edges of driving pulses and large current output. In addition, great progress has been made in the design of control circuits and dimming circuits for white light LEDs with adjustable brightness and high color rendering index.

Semiconductor material technology. The main line of LED technology development is the renewal of chip semiconductor materials and the continuous improvement of 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 packaging design or materials used are 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, using a "DoubleReflection " optical structure to output the light loss caused by the wide angle of the LED to the outside, thereby improving the luminous efficiency.

In addition, other technologies such as optical design, chip structure optimization, light-emitting area improvement, and phosphor materials are being actively researched and developed.