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21 November 2006


Shimei to produce GaN-on-silicon LEDs for blue, then green and red emission; developing blue lasers

In September 2006, Shimei Semiconductor Co Ltd of Kyoto Research Park, Japan has developed prototype 0.3mm x 0.3mm blue LEDs on silicon substrates. The maximum output power (emitting at 450nm wavelength) is 10mW (on a par with existing products) and luminous intensity is 1.5-2.0cd when driven at 20mA.

Shimei was founded in May 2005 by chief technology officer Hirofumi Yamamoto with $5.5m from Shin-Etsu Chemical Co Ltd and venture-capital companies in Japan. Shimei says it is developing technology that focuses on not only boosting LED emission power but on cutting cost, through the use of silicon substrates (which are much cheaper than sapphire substrates).

Shimei says it applies proprietary calculation theory to simulate MOCVD configuration and surface and vapor phase reactions, helping to cut development time and cost, and claims that this enables epitaxial growth of high-quality III-nitride compounds on any substrate.

Because silicon absorbs more light than sapphire, Shimei forms a reflective structure in the blue LED's buffer layers between the substrate and the active emitting layer, improving the efficiency of light emission through
the top surface. Also, because silicon is electrically conductive (unlike sapphire, which is an insulator), the n-type electrode is made with a direct contact to the bottom of the chip rather than indirectly via the top.

Shimei expects to improve performance through prototyping. The company is readying production lines for a monthly capacity of 3 million units and aims to ship samples in April 2007, in bare chip or wafer form.

Shimei is also looking at the untapped potential for III-nitride LEDs with longer wavelengths, including green and even red, the company's Isao Kohda told Semiconductor Today. This would enable them to produce all three
primary colors (for full-color LEDs), unlike III-V compounds for existing red and infrared LEDs (which also contain toxic arsenic and phosphorus). Shimei has also started blue laser development, added Kohda.

Also, apart from lowering LED cost, simplifying structure, and extending lifetime, compared to sapphire substrates "it is easy to make large-area LEDs", says Yamamoto. "Using silicon wafers could enable the integration of optical devices with CMOS circuits drivers and other circuits on the same substrate," he adds.