Light-emitting diodes

A p-n junction made of a compound such as gallium arsenide (GaAs) gives out light near the junction when forward biased. Such a diode is a light-emitting diode (LED).

Figure to be added

Figure 11.18 — (a) the LED symbol; (b) a 5 mm LED, with the long leg the anode and the short leg, by the flat on the rim, the cathode; (c) the base seen from below.

Telling the legs apart

The long leg is the anode (+).
Looking at the base from below, the leg next to the flat is the cathode (−).

Red, yellow, green and blue LEDs are sold, and ones giving ultraviolet (UV) and infrared (IR). Early LEDs were mostly indicators; now they make large television screens too, and since white LEDs, home lighting, street lights and torches. They use very little energy and last about 50 000 hours.

Colours and voltages

Colour	Semiconductor	Minimum forward voltage
Red	GaAs	1.8 V
Orange	GaAsP	2 V
Yellow	AlInGaP	1.8 V
Green	GaP	2.2 V
Blue	GaN	5 V

A lit LED takes about 10–20 mA.
An LED gives light of a single colour; its casing is tinted only so you can tell its colour when it is off.
More current makes it brighter, but running it brighter shortens its life.