Haitz’s Law states that the brightness of a LED package doubles every 18 to 24 months. There are signs that this rate is actually increasing as the era of LED- based displays and solid state lighting advances rapidly. Today, we learned of another very interesting LED device development, this time from NIST (National Institute of Standards and Technology) where researchers have developed an LED that is seven times brighter than conventional LEDs.

These LED die include a series of 10 concentric rings around the emitting area of the diode. The rings take the light that is normally trapped inside the high-index LED material by total internal reflection, extract it from the die and direct it perpendicular to the LED surface. This leads to light perpendicular to the LED being about 41% of the total generated, compared to about 2% if nothing special is done to the die.

Luminus Devices claims an extraction efficiency of about 85%, although their devices have a near-lambertian distribution where much of the extracted light is far from perpendicular to the die. The narrower cone angle produced by the NIST technique may have significant advantages in étendue-limited applications such as LED projection systems and some biomedical applications. On the other hand, the Luminus Devices system is currently a product, while NIST is reporting on a research project in Physical Review Letters*.

The NIST team fabricated their own infrared LEDs consisting of gallium arsenide packed with "quantum dots" of assorted sizes made of indium gallium arsenide. Quantum dots are nanoscale semiconductor particles that efficiently emit light at a color determined by the exact size of the particle. The LEDs were backed with an alumina mirror to reflect the light emitted upwards.

The researchers experimented with different numbers and dimensions of grooves. The brightest output was attained with 10 grooves, each about 240 nanometers (nm) wide and 150 nm deep, and spaced 40 nm apart. The team spent several years developing the design principles and perfecting the manufacturing technique. Fortunately, lead author Mark Su says the design principles are transferable to other LED materials and emission wavelengths, as well as other processing techniques, such as commercial photolithography.

Insight Media has always believed that the importance of the LED to multiple applications would generate research funding from large corporations, VCs and government. This broad community of R&D would then ensure that the forecasts of Haitz’s law would be met or exceeded. If you search the scientific literature, you will find hundreds, even thousands of papers detailing large and small improvements to LEDs. While it will be years, if ever, before you see a LED-based RPTV or LCD-TV based on this NIST technology, this not-so-small addition to the knowledge base ensures the continued improvements of LEDs for displays and multiple other applications –MB

* M. Y. Su and R. P. Mirin, Enhanced light extraction from circular Bragg grating coupled microcavities, Applied Physics Letters. July 17, 2006.