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Semiconductor lighting, often referred to as the third generation of lighting technology, has found wide applications in areas such as large-screen color displays, special lighting, traffic signals, LCD backlights for multimedia devices, and optical communications. As a cold light source, it offers significant environmental benefits, producing no harmful emissions and achieving energy efficiency that exceeds 90% compared to traditional incandescent and fluorescent lamps. This makes it a sustainable and efficient alternative in modern lighting solutions.
In the industry, semiconductor lighting is commonly known as LED technology, which encompasses core LED technology, packaging techniques, and application development. In recent years, with strong governmental support, LED chip technology has advanced rapidly. The size and power of individual chips have increased significantly, allowing for the production of high-power LEDs up to 3W. Meanwhile, the design and functionality of packaged products continue to evolve, offering greater flexibility and diversity in LED lighting systems. These advancements provide ample opportunities for innovation in LED application technologies, highlighting the complexity and broad scope of the field.
Several countries, including the United States, Japan, and South Korea, have launched national initiatives to promote semiconductor lighting. Major global lighting companies like General Electric, Philips, and Osram have partnered with semiconductor firms to develop new lighting solutions. In the U.S., under the leadership of the Department of Energy and the Optoelectronics Industry Development Association, Sandia National Laboratories initiated a comprehensive roadmap for semiconductor lighting technology from 2002 to 2020. This plan aimed to lay the technical foundation for the next generation of lighting systems.
Japan has also made significant progress in this area through its "21st Century Lighting Project," organized by the Japan Research and Development Center of Metals and the New Energy and Industrial Technology Development Organization (NEDO). This five-year national initiative involved universities, companies, and associations, aiming to enhance energy efficiency by using long-lasting, lightweight GaN-based blue and ultraviolet LEDs. The goal was to double the efficiency of traditional fluorescent lighting and reduce COâ‚‚ emissions. The project had a total budget of 6 billion yen.
Today, the most widely researched semiconductor lighting technology involves III-nitride LEDs such as InGaN, AlGaN, and GaN. These LEDs emit blue-green, blue, and ultraviolet light, which can be combined with red and green LEDs to produce white light. They can also directly excite phosphors to generate white illumination. As a result, nitride LEDs are the preferred choice for white light sources, gradually replacing traditional incandescent and fluorescent lamps. Their rapid development has led to significant improvements in luminous efficiency, with a tenfold increase every decade. In particular, GaN-based blue LEDs, which emerged in the early 1990s, saw their efficiency increase by 100 times within 10 years, enabling full-color displays and practical white-light illumination.
The evolution of LED technology has transformed it from simple indicators (with injection currents around 20 mA) to high-power devices (now typically operating at 350 mA). Applications have expanded from basic signaling to more complex uses such as night-time lighting, traffic signals, automotive lighting, and large-scale color displays. GaN-based power blue LEDs are now considered the third-generation lighting source, surpassing traditional lamps in efficiency, durability, and environmental friendliness.
LEDs work based on the principle of forward-biased p-n junctions, where electrons are injected into the active region and recombine, emitting light in the process. Since the 1950s, LED wavelengths have extended from infrared to visible and even ultraviolet ranges. The wavelength of an LED is determined by the bandgap energy of the material used. Gallium nitride (GaN)-based materials are direct bandgap semiconductors, covering visible, ultraviolet, and deep ultraviolet regions. This versatility makes them ideal for various lighting applications.
There are three main methods to achieve white solid-state lighting:
1. **RGB LED Mixing**: Using red, green, and blue LEDs to synthesize white light, as shown in Figure 2(a). This method allows for precise control of the color spectrum but requires complex driving circuits and high-quality components, making it suitable for high-end applications.
2. **UV LED Excitation of Phosphors**: Using ultraviolet LEDs to stimulate trichromatic phosphors, which then emit white light, as shown in Figure 2(b). While promising, this method currently faces challenges due to the lack of high-power UV LEDs and efficient, reliable phosphors.
3. **Blue LED Excitation of Yellow Phosphor**: A widely used method where blue LEDs excite yellow phosphors to create a binary mix of white light, as shown in Figure 2(c). This approach is mature, cost-effective, and offers high luminous efficiency, although it may slightly compromise color rendering. It remains the most common technique in commercial LED lighting unless otherwise specified.