Advances in Chip Technology, Packaging Enable White LED Breakthroughs
Significant advances in chip technology have enabled Cree, Inc.'s Santa Barbara Technology Center to demonstrate white LEDs with record efficacies as high as 74 lumens per watt - on par with fluorescent lighting systems and more than four times as efficient as incandescent sources.
This achievement was part of a three-year project focused on demonstrating that existing white LED technology could be successfully scaled up (in terms of electrical input/optical output power) to levels suitable for general illumination applications, and with superior energy efficiency. This goal required significant improvements in such diverse areas as chip efficiency, optical design, and thermal management. To meet the challenges, Cree's expertise in LED design and fabrication was combined with Lawrence Berkeley National Laboratory's expertise in optical design and low thermal resistance material bonding.
Chip Development Yields Record Efficiencies
The primary light source in the white lamps is an InGaN-based blue LED (whose blue emission is subsequently converted to other colors using phosphors to create a white light source). Efficiency and output power of the blue chip are therefore critical parameters. Several generations of novel chip designs were investigated over the course of the project. This work led to a number of results believed to be world records, including a 0.9 x 0.9 mm2 chip operating at 350 mA with quantum and wall plug efficiencies of 33% and 27%, respectively, after packaging. In addition, some of the technology has been incorporated into Cree's recent chip products.
By combining the high performance chip designs with commercial phosphors in developmental LED packages, Cree was able to demonstrate white LEDs with record efficacies as high as 74 lumens per watt for a 0.3 x 0.3 mm2 chip operating at 20 mA, and 57 lumens per watt for a 0.9 x 0.9 mm2 chip operating at 350 mA.
Packaging Development Contributes to 1000 Lumens Target
Cree and Lawrence Berkeley also focused on demonstrating a compact LED with output in the 1000+ lumens range (1000 lumens is the approximate light output of a 60 watt incandescent bulb). To reach the 1000 lumens target, high radiance arrays were developed using multiple chips and custom LED packaging. The optical size of the arrays was less than 50 mm in diameter to make them comparable in size to existing incandescent lamps. Special consideration was given to the optical design to minimize optical losses.
Thermal management was another key area of research. Electrical energy not converted to light is converted to heat, the effective dissipation of which is challenging. An effective maximum temperature of only ~125 °C (due to available LED encapsulation materials) severely limited the allowable thermal resistance of the system. At the same time, the small size of the LED chips tended to create a "thermal bottleneck". Using high performance materials and bonding techniques to insure low thermal resistances in the critical heat flow path, a compact 1000 lumens LED lamp capable of dissipating 20 watts of heat was successfully demonstrated.