Ziming Semiconductor, a venture company in Kyoto, has developed GaN-based light-emitting diode (LED) chips on silicon substrates. Among them, the output power of the blue LED is at most 10mW, which is almost the same as the power of the popular blue LED chip. "The manufacturing cost is less than half compared to the blue LED chip fabricated on the sapphire substrate" (the company). According to reports, the reason why the manufacturing cost can be reduced is that the price of the silicon substrate is only about 1/10 compared with the sapphire substrate, and the silicon substrate is softer than the sapphire substrate and easier to process. The new product has an emission wavelength of 450 nm. The chip size is 0.3mm square. The luminosity when driving at 20 mA is 1.5 to 2 cd. In addition to the blue LED, Ziming Semiconductor has also produced green LEDs on the silicon substrate. In general, when a GaN film is formed on a silicon substrate, since the difference in thermal expansion coefficient and lattice constant between GaN and silicon is large, defects such as cracks are likely to occur. This time, the occurrence of defects is suppressed by alternately stacking a plurality of layers other than AlInGaN and AlInGaN as a buffer layer on the silicon substrate. Using the same approach, Sanken Electric and Nagoya Institute of Technology are jointly developing GaN-based LEDs fabricated on silicon substrates. Sanken Electric acts as a buffer layer by stacking multiple layers of AlN and GaN alternately on a silicon substrate. Since silicon absorbs light more easily than sapphire, a reflective structure is placed on the blue LED buffer layer. Compared with sapphire, silicon has the advantage of high thermal conductivity, and the heat dissipation characteristics of GaN-based LEDs fabricated on silicon substrates are improved. Unlike the insulated sapphire, since silicon can conduct electricity, it also has the advantage that the n-type electrode can be mounted on the lowermost portion of the LED chip. Generally, an LED made of a sapphire substrate has a sapphire as an insulator, so it is necessary to cut a part of the upper portion of the LED to mount an n-type electrode. With a silicon backplane, the cutting process can be eliminated, so the yield is expected to increase. In addition, it is said that since silicon has a lower hardness than sapphire and SiC, it is easy to cut, so it is also easy to improve the yield. Ziming Semiconductor plans to begin sampling supply in April 2007 and is currently preparing for a monthly production of 3 million chips. In addition to being sold as a bare chip (Bare Chip), Ziming Semiconductor also considers wafer supply.
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