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LED colors question

8/14/1999 9:04 PM
Pat F LED colors question
This is one of those burning questions I've always meant to ask. Recently blue LEDs have become available. I've noticed that they are much more expensive than your run of the mill red, green and yellow LEDs. I guess I always thought that LEDs derived their color from the plastic they were encased in, until I bought some clear ones that were bright red when lit up. So, what is it that determines the color of LEDs and why are the blue ones so expensive? Is it the semiconductor material itself that determines color?
8/14/1999 11:36 PM

Hi, Pat. It is usually the material and how it is manufactured that determines the color i.e. wavelength. Other properties such as temperature can change this also. There may be more than this that determines color.  
One cool thing about blue LED's; they can be damaged by static discharge, like mosfets and some IC's. LED's that are static sensitive, hahahaha!  
8/15/1999 1:28 PM

What they told us in semiconductor physics class is that every semiconductor material has a unique energy band gap, a kind of energy hill that can be used to make semiconducting junctions.  
LED's are just diode junctions set up so that electrons crossing the gap can emit photons of the characteristic energy of the band gap.  
For silicon, the band gap is of a size that emits infra-red, so the light is not visible. The original LED's were what is called II-IV semiconductors, stuff made from mixtures of two materials instead of one like silicon. The originals were Gallium-Arsenide, which had a band gap that gave off a red light.  
In general, the bigger the band gap, the higher energy the photon. Light has energies that go from lower at the red end to higher in the blue and even higher at ultraviolet. The story of getting different color LED's has been one of diddling the semiconductor mixture to get the band gap to be higher and higher. They've use Gallium Aluminum Arsenide, Indium Phosphide, and a whole slew of others to get higher energies. The band gap is easy to see in diodes, as the forward voltage of a diode is about 1/2 the band gap.  
Silicon at 0.7V is in the infrared, and early LED's were 1.8V in the red. The voltages get higher for Green, at about 2.2-2.4V.  
Blue is the toughest, having a greater energy than the others. The first blue LEDs were actually silicon carbide diode, with a forward voltage of about 4V (!) to turn the junction on. These are incredibly difficult to make, and another mechanism was found to make blue light on ordinary silicon. These are the blue LED's we have today. The different mechanism makes blue light, but also makes for the static sensitivity.

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