Uniroyal Optoelectronics of Sarasota, Fla., has significantly boosted the light output of its blue LED chips. Some recently shipped test chips have achieved brightness levels ranging from 3.5 to 4 mW, equalling the current industry-best performance, the company says. When packaged, these LEDs will produce 8 to 10 mW of light, promoting the development of new LED-based applications with blue light sources.
These blue LEDs are noteworthy, not so much for their current level of performance as for their promise of even brighter output levels in the near future. Their development also signals the growing competition among LED chip makers to produce high-performance blues. These trends may drive down the price of blue LEDs while enabling their application in a host of new products that now rely on other light sources. As their light output rises, blue LEDs will find new uses in outdoor signs, traffic signals, and medical equipment.
According to Jeff Nelson, Uniroyal's chief technical officer, two other vendors currently offer packaged blue LEDs with 6 mW of output. That value, he says, corresponds to about 3 to 4 mW for the unpackaged LED die—equivalent to what Uniroyal is now achieving. All three vendors use indium gallium nitride (InGaN) on a sapphire substrate. Blue chips also are being produced on silicon-carbide substrates.
However, Uniroyal is new to the LED arena. Its blue-chip development began about a year ago. Recently, it stepped up its efforts by forming an R&D team in Tampa, Fla. Uniroyal credits the new team of researchers with raising the output of the chips from the 2 mW of their current production chips to the 3 to 4 mW of the test chips. The team did so primarily by increasing the chips' ability to generate light rather than by tweaking device construction to maximize the percentage of light extracted.
"We have really focused most of our improvements at the epitaxial level," Nelson notes. These improvements involved optimizing the quantum well structure and doping the LEDs' n- and p-type structures. Nelson says these modifications account for about 80% of the LEDs' increased output. The other 20% is attributed to better device construction. The team boosted the efficiency with which light is extracted from the chip by improving the transparency of the conducting p-type contact.
Because most of the development was done at the epitaxial layer, there's still much room for improvement in performance at the device-fabrication level. Nelson believes that with chip shaping and other efforts in this area, the light output of the blue LEDs should double. As for other opportunities for increasing light output, the blue LED test chips aren't particularly big. These 13-mil chips have an emitting area that measures 250 by 250 µm. Enlarging the die may increase light output still more.
Contact Jeff Nelson at (813) 630-9100 or visit www.uniroyalopto.com.