Cree pushes white LEDs to limit

Cree continues to make improvements to its blue LEDs. Speaking at the Intertech LEDs 2003 conference, Bernd Keller described blue (460 nm) LEDs that have an output of 19 mW when driven at 20 mA, with 35% quantum efficiency and 29% wall-plug efficiency. These chips enable very-high-efficiency white LEDs that provide 4.7 lm when driven at 20 mA, with an efficacy of 74 lm/W.

Kilolumen lamp Keller reported that Cree is working to increase the efficiency, operating power density and yield of its LEDs, and to move to larger wafer diameters (LEDs are currently produced by Cree on 2 inch SiC wafers).

"The industry is now capable of producing about 300 times the number of lumens per wafer compared to the first commercial blue LEDs," he said. "By 2010 this factor is anticipated to increase by about 15,000 times, i.e. 50 times from today." The actual lumen/wafer value is dependent on packaging and other factors.

Cree has also been developing high-flux LEDs that measure 900 x 900 µm and have an output of 250 mW when driven at 350 mA. These blue chips have been packaged with a commercially available phosphor to make white LEDs with an output of up to 48 lm at a color temperature of 6000 K, and an efficacy of 40 lm/W.

Further increases in the flux per chip can be made by increasing the operating power, which in turn increases the need for innovative packaging solutions that minimize thermal resistance and remove heat from the LED junction.

Keller also described an R&D lamp module that has an output exceeding 1000 lm. The multichip lamp produced 1200 lm when driven with an electrical power of 37.5 W, giving an efficacy of 32 lm/W; this is about twice the efficiency of incandescent lamps with a similar lumen output.