Everything Science Grade 12

14.4 CHAPTER 14. ELECTRONICS

The Light-Emitting Diode (LED) ESCGH

A light-emitting diode (LED) is a diode devicethat emits light when charge flows in

the correct direction through it. If you apply avoltage to force currentto flow in the

direction the LED allows, it will light up.

Figure 14.11: Symbol for a light-emitting diode with anode and cathode labelled.

Extension: Circuit Symbols

This notation of having two small arrows pointing away from the device is com-

mon to the schematic symbols of all light-emitting semiconductor devices. Con-

versely, if a device is light-activated (meaning that incoming light stimulates it),

then the symbol will have two small arrows pointing toward it. It is interesting

to note, though, that LEDs are capable of actingas light-sensing devices:they

will generate a small voltage when exposed to light, much like a solar cell on a

small scale. This property can be gainfully applied in a variety of light-sensing

circuits.

The colour depends onthe semiconducting material used to constructthe LED, and

can be in the near-ultraviolet, visible or infraredpart of the electromagnetic spectrum.

FACT

Nick Holonyak Jr.
(1928) of the Uni-
versity of Illinois at
Urbana-Champaign
developed the first prac-
tical visible-spectrum
LED in 1962. Light emission

The wavelength of the light emitted, and therefore its colour, depends onthe materials

forming the p-n junction. A normal diode, typically made of silicon or germanium,

emits invisible far-infrared light (so it can’t be seen), but the materials used for an LED

can emit light corresponding to near-infrared, visible or near-ultraviolet frequencies.

LED applications

LEDs have many uses. Some of these are given here.

• thin, lightweight message displays, e.g. in public information signs (at airports
  and railway stations, among other places)