The ABCs of LEDs – Illumination & Junction

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The ABCs of LEDs – Illumination & Junction

  
  
  
  

I – Illumination

LEDs are bright, efficient, and quick to react. They have become a substitute for light bulbs in many applications because they use less power, have longer lifetimes, produce little heat, and emit colored light.LED christmas lights

Using LEDs for general lighting is becoming more practical as they become increasingly more efficient, generating more lumens per watt. For example, a fluorescent tube equivalent of 3,000 lumens would have required over 1,300 LEDs using 30 lumens/watt at 2 to 3 lumens per LED in 2003. However, by 2005, there was a 20X reduction in the number of LEDs required for the same florescent tube, to around 50, using 50 lumens/watt or higher at 60 lumens per LED.

We are seeing more and more innovation in form factor and functionality in solid-state lighting products as lighting manufacturers better understand how to leverage LED sources. Cree, for example, combined LEDs in an indirect fashion with a complex optical design to deliver a new parking-garage fixture that delivers uniform light. Peerless has announced a 4-inch cylindrical fixture that includes an optic that provides continuous smooth lighting and can produce direct or indirect illumination or both. GE Lighting, meanwhile, added a lower-cost planar model to its Lumination family that is lit from the rear.

J – Junction

One of the crucial keys to solid state electronics is the nature of the P-N junction. When p-type and n-type materials are placed in contact with each other, the junction behaves very differently than either type of material alone. Specifically, current will flow readily in one direction but not in the other, creating the basic diode. Also, large pits can short the p-n junction, causing device failure.

The majority of LED failures are temperature related; an LED's performance and projected lifetime correlate tightly to thermal management and the resulting junction temperature of the LED semiconductor chip. Elevated junction temperatures cause a reduction in light output and accelerated LED lifetime degradation. Proper thermal management of an LED luminaire and mechanical construction are vital for performance. To assess the mechanical construction, an evaluation should include techniques such as x-ray photographs of LED soldering. Measuring actual thermal performance to validate thermal design assumptions is necessary to ensure quality and reliability of SSL products.

However, thermal measurements of SSL can be challenging. Positioning the thermal couple at the wrong point or having large amounts of photonic energy illuminate the thermal couple will result in an incorrect temperature measurement. Such errors could result in design issues that may compromise LED lifetime for the finished product.  

Excess heat leads to a whole range of performance and reliability issues for high-power semiconductor applications such as high-brightness LEDs (HB LEDs). The need for maximum heat extraction from the LED junctions has led to the use of aluminum nitride (AlN) ceramic packages in most high-power LEDs, whereas mid-power LEDs can utilize lower-cost alumina or even plastic packages. A cost-reduced approach to AlN, however, can deliver the thermal performance needed in HB LEDs while also delivering package costs more in line with mid-power devices. A reduction in HB LED package cost could help spur broader deployment of LEDs into solid-state lighting (SSL) applications.

The thermal management challenges in HB LEDs are simple to describe and hard to overcome. HB LED light output is sensitive to heat. As an LED gets too hot, more input energy is converted to heat instead of light, which further increases the LED temperature, leading to potential catastrophic failure. The figure below shows the degradation of light output as a function of HB LED junction temperature. L70 refers to the point where the current light output of the LED is 70% of the original light output; at L50 the light output falls to 50%. This example illustrates clearly the critical importance of pulling heat away from the HB LED in order to ensure a low junction temperature and stable light output.

LED, LED junction temperatures

Conducting heat away from the semiconductor junction involves heat transfer in the z axis, as well as in the x and y axes. The z-direction thermal conduction happens directly from the diode junction heat source toward the heat sink. The x,y-direction conduction, or heat spreading, moves the heat horizontally away from the junction. The closer to the semiconductor junction, the more critical a role a material plays in thermal management.

Sources:
HB Matrix LED Assembly

LEDs Magazine, “LED-based luminaires: New designs from Cree, Acuity, and GE Lighting”, http://ledsmagazine.com/news/10/8/5.

LEDs Magazine, “LED system evaluation yields quality analysis” http://ledsmagazine.com/features/10/9/8.

LEDs Magazine, “Cost-reduced AlN delivers thermals needed in HB LED packages”, http://ledsmagazine.com/features/10/9/7.

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html

Solid State Technology Journal: “The gleam of well-polished sapphire”, http://electroiq.com/blog/2013/01/the-gleam-of-well-polished-sapphire/.

Photo credits:
LEDs Magazine, “Cost-reduced AlN delivers thermals needed in HB LED packages”, http://ledsmagazine.com/features/10/9/7.

LEDs Magazine, "LEDs transform a White Christmas into a Green Christmas", http://ledsmagazine.com/features/5/1/1/OwenHolidays10.

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Janine Hueners
Marketing Specialist
Palomar Technologies, Inc.

 

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