Posted on 27 March 2011 at 19:19h

We were doing about light sources the other day as part of the HL EM option and one of the students asked how an LED worked and why they are so efficient, first some background about semiconductors. Semiconductors are solids so their electrons exist in energy bands rather than levels. The highest band is fairly empty so there are few free electrons to enable conduction, semi conductors are therefore poor conductors, however adding an impurity (doping) can increase their conductivity either by adding free electrons (N type) or an impurity with missing electrons (P type). It sounds like a material with missing electrons shouldn’t increase the conductivity but it does since it allows other electrons to move into the holes. If a piece of semiconductor is doped with N at one end and P at the other a junction is formed between the two halves, this is called a diode. When current flows through the diode, electrons conducting though the N type combine with holes in the P. When this happens the electrons move from high to low energy level resulting in the emission of light.

So why should they be so efficient? If we compare to an incandescent light bulb then to produce light the electrons first have to be excited into the higher energy level, this happens when the lattice atoms gain energy from the conduction electrons. In a diode the electrons are already in a higher energy level so don’t need to be excited there by the material getting hot. Well that’s my theory and I’m sticking to it.