Light-emitting Diode (LED):

When a conduction electron makes a transition to the valence band to fill up a hole in P-N                   junction, the extra energy is emitted as a photon. If the wavelength of this photon is in the visible range one can see the emitted light. Such a P-N junction is known as light-emitting diode (LED).

Example: Gallium arsenide, indium phosphate