Semiconductor light emitting devices are grouped into two categories:

• Light emitting diodes (LEDs), which are basically general-purpose replacements for vacuum tube-based light emitters.

• Laser diodes that emit sharply focused monochromatic light beams.

We have already discussed laser diodes. Here we will look briefly at LEDs, which have come to dominate important segments of the lighting industry.

Light emitting diodes consist of p-n junctions in a special class of semiconductors that are capable of emitting light of many colors ranging from the infrared into the violet. Furthermore, of great practical importance is the fact that by combining LEDs of red, blue, and green, or using special phosphor coverings, white light can be produced, with conversion efficiencies from electrical input to useful light output that are very competitive with conventional vacuum-based devices.

When we discussed p-n junctions, we noted that one of their most valuable properties is their ability to inject carriers across the p-n interface. This is the principle behind LEDs. When forward-biased, they emit various colors of light, ranging from the blue into the infrared, depending on the semiconductor materials used.

Semiconductors useful as LEDs and lasers have special structures. As noted earlier when we discussed lasers, they are direct bandgap materials. Within them the recombination of an injected electron with a hole (in effect the capture of the free electron by an empty spot in the atomic outer orbit) results in the release of energy in the form of a photon. A photon is a unit of light energy.