LED means light emitting diode. It is usually heavily doped. On forward biasing the diode majority carriers combine with minority carriers near the junction & emit energy (spontaneous radiation) in the form of visible light. Increase in the forward current results in decrease of light intensity. LEDs are biased such that the light emitting efficiency is maximum. The V-I characteristics of a LED is similar to that of a Si junction diode. But the threshold voltages are much higher and slightly different for each colour. The reverse breakdown voltages of LEDs are very low, typically around 5V. So care should be taken that high reverse voltages do not appear across them. LEDs must at least have a band gap of 1.8 eV (spectral range of visible light is from about 0.4 mm to 0.7 mm, i.e., from about 3 eV to 1.8 eV). The compound semiconductor Gallium Arsenide – Phosphide (GaAs1-xPx) is used for making LEDs of different colours. GaAs0.6 P0.4 (Eg ~ 1.9 eV) is used for red LED. Si (Eg ~ 1 eV), Ge (Eg ~ 0.9 eV), GaAs (Eg ~ 1.4 eV) are used for making infrared LED. LEDs are used in display of watches, calculators, telephones, measuring instruments etc., remotes of electronic devices, decorative items.