LEDs and Laser Diodes

A light-emitting diode (LED) is a semiconductor diode that emits light when current passes through it in the forward direction. As with any diode it has a PN junction, but the materials are chosen so that, when electrons and holes recombine at the junction, the released energy appears as light rather than heat. LEDs are efficient, long-lived and rugged, and they have become the standard source for indicators, displays, and general lighting.

The colour of an LED depends on the semiconductor material, because the material sets the energy released in each recombination and therefore the wavelength of the emitted light. Different compound semiconductors give red, green, blue and other colours, and white LEDs are commonly made by using a blue LED to excite a phosphor coating. Because an LED is a diode, its current must be limited, usually with a series resistor or a current-regulating circuit, since it has a small forward voltage and would otherwise draw a damaging current.

A laser diode is a related device that produces coherent light through stimulated emission. Its structure forms an optical cavity that, above a threshold current, generates a narrow, intense and well-defined beam rather than the broad glow of an ordinary LED. Laser diodes are used in optical disc drives, fibre-optic communication, laser pointers and barcode scanners. Both LEDs and laser diodes are examples of optoelectronic components, which link electrical current and light.

Frequently asked questions

Why do LEDs come in different colours?

The colour depends on the semiconductor material, which sets the energy released when charge carriers recombine and therefore the wavelength of the emitted light.

Why does an LED need a series resistor?

An LED is a diode with a small forward voltage, so without a current-limiting resistor or regulator it would draw an excessive current that could destroy it.

How does a laser diode differ from an ordinary LED?

A laser diode forms an optical cavity and, above a threshold current, produces coherent light by stimulated emission, giving a narrow intense beam rather than the broad glow of an LED.