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A sure-fire way to increase the value of any piece of electronic equipment is to add some superfluous flashing red, yellow and green LEDs to it. (Light Emitting Diode.) They serve no use, but their presence is somehow comforting (especially in sci-fi films) and gives the impression that the equipment is busy doing something useful.

There was a time however when this task wasn’t so easy. In the early days of LEDs, you could have any colour you wanted, so long as it was red. We’ve been looking at LEDs (more specifically, diodes in general) in one of my classes recently. The colour is determined by the band gap of the material from which the diode is made. What’s a band gap?  Well, in a semiconductor, just like any other substance, electrons can’t have any-old-energy. They have to sit in very specific energy levels. In a solid, these levels form bands of permitted energies. A semiconductor has two bands that are close in energy, but are distinct; the lower band, called the valence band, is mostly occupied by electrons, and the upper, called the conduction band, is mostly unoccupied. The material can conduct electricity because electrons can move in the conduction band, and also ‘holes’ (where there are states not occupied by electrons) can also move in the valence band.

In an LED, light is emitted when an electron drops from the conduction band into a hole in the valence band. The wavelength of light is very specific – depending on the size of the bandgap. Large bandgap materials give higher frequencies (frequency is proportional to the energy drop through E=hf where h is Planck’s constant) and so have shorter wavelengths. (9 June 2010 – Whoops, I said longer wavelength in my original post – now it’s correct)

That was the problem with developing blue LEDs.  Blue is short-wavelength visible light, and so a blue LED would need a material with a large bandgap. Developing such a material that was also cheap, robust, easily workable etc wasn’t an easy undertaking. By contrast, the long wavelength red LED was much easier.

I had a suggestion from a student that you could just turn a red LED to a blue one by putting it in a blue piece of plastic. I’m afraid this won’t work. Blue plastic looks blue because it absorbs the red and green light from a white light source. Since a red LED produces red light, if you put it in blue plastic we would just find that the red would be absorbed, and nothing would escape. You wouldn’t get any light out at all.

However, there is a way that you can shift the colour of an LED, namely changing the temperature, as this movie demonstrates.

http://www.youtube.com/watch?v=yNaYYhogGTk&feature=related