When a conduction electron makes a transition to the valence band to fill up a hole in a p-n junction, the extra energy may be emitted as a photon. If the wavelength of this photon is in the visible range (380 nm-780 nm), one can see the emitted light. Such a p-n junction is known as light-emitting diode abbreviated as LED. For silicon or germanium, the wavelength falls in the infrared region. LEDs may be made from semiconducting compounds like gallium such as, arsenide or indium phosphide. LEDs are very commonly used in electronic gadgets as indicator lights.
