The ABCs of LEDs – Gallium Nitride & High-Brightness LEDs
G – Gallium Nitride (GaN)
Last week we talked about epitaxy and film, which ties into our discussion this week about gallium nitride. Gallium nitride is used as an epitaxial layer that is grown on the polished upper surface of the sapphire substrate. Most advanced LED devices are built upon sapphire (Al2O3) substrates. Since GaN and sapphire have a huge lattice mismatch, the GaN “epi layer” is a highly stressed film.
To make an LED, crystal layers of GaN are grown on a substrate material. The substrate material must have certain qualities, and the most commonly used for LEDs today are sapphire and silicon carbide. Due to differences in material properties between GaN and these materials, the GaN crystal grows imperfectly on such foreign substrates, and produces a high incidence of imperfections which reduce the light generation efficiency of the LED.
The ability to grow GaN crystals on its native GaN substrate (“GaN on GaN”) is significant. The crystal grows much more perfectly, can accommodate much higher power densities, and allows the LED emit 5-10 times more light from the same crystal area. This is called “light density”- it’s the reason that GaN on GaN devices exhibit far more point-source-like qualities. The picture to the left below shows GaN grown on a foreign substrate, while the one on the right shows GaN grown on GaN.
As an added benefit, the GaN on GaN technology is more heat tolerant than other substrate types and allows higher energy conversion in small form factors. The recent commercialization of GaN on GaN technology is a significant breakthrough in energy density, allowing 5-10 times more lumens to be generated per unit volume.
H – High-Brightness LEDs
Since the original LED in 1962, the technology has grown rapidly. With seven-segment LED displays seeing popularity in the 1970s and 1980s, but falling off due to liquid crystal displays being more flexible and lower cost at the time, for a while it seemed like LEDs would not be a popular technology. Due to defects in crystal structure and poor substrate creation, the light output of LEDs was not anywhere near its potential. However, in the 1990s high-brightness gallium nitride (GaN) LEDs were discovered, and it was a short step to the 2000s, when bright white LEDs became a lighting revolution.
One of the fastest growing technologies in the world is High-Bright/High-Power LEDs. Industrial and commercial lighting contribute a large part in the HB LED market growth. Other end uses are found in aircraft lighting and powerful heaters in water purification systems.
For high-brightness LED applications, such as outdoor lighting or rear projection screen lighting, a matrix configuration of LEDs is used. In this configuration, the LEDs are placed in tightly packed rows and columns to generate the most light.
These LEDs are very efficient by a factor of ~60%. They are durable, long lasting, and over time, cost reducing. HB LEDs can drive very high currents into the hundreds of mA; some HB LEDs can emit lumens in the thousands. High-brightness LED applications require maximum thermal transfer to achieve performance requirements.
HP/HB LEDs consume only a fraction of power in comparison to incandescent bulbs, but LED applications tend to generate a lot of heat, and in a very small localized area. Temperature per unit or area is high with HP and HB LEDs, requiring an inherent cooling process if they are to last a long time. In response, some pretty interesting heat sinks and heat spreaders are available. But it all starts inside the LED, and that heat has to be removed and controlled, or the LED will self-destruct.
Stay tuned next week for more on LEDs!
LED Journal: white paper "A Critical Advance in MR16 LED Lamps" http://www.ledjournal.com/main/category/white/. http://www.soraa.com/public/docs/A_Critical_Advance_In_MR16_Lamps-Benya.pdf.
LEDs Magazine: “SSL industry celebrates 50th birthday of visible LED light”, http://ledsmagazine.com/news/9/10/7.
Solid State Technology Journal: “The gleam of well-polished sapphire”, http://electroiq.com/blog/2013/01/the-gleam-of-well-polished-sapphire/.
Photo credit: http://www.edn.com/electronics-blogs/powersource/4374006/GaN-on-GaN-breakthrough-LED-boosts-MR16-performance.
Palomar Technologies, Inc.