Light-emitting Diode: Structure and Function

How does a Light-emitting Diode Work?

Light-emitting diodes (LEDs) consist of semiconductor crystal layers that, depending on the material used, emit light of certain wavelengths when a current flows through them in the forward direction (electroluminescence). The individual diodes shine either red, green or blue; white light can be obtained from them in various ways. In the past, light-emitting diodes mainly served as colourful but dim signal lamps. After decades of research and development, these digitally controllable, energy-saving light sources are now revolutionising lighting by replacing the outdated, inefficient incandescent lamps.

Light-emitting Diode: Structure and Function

Semiconductors have the property of conducting electricity well in only one of two possible flow directions. The LED chip is coupled to a cathode (excess of electrons, negative charge pole) via a crystalline silicon layer. On top of the silicon layer is a second, thinner layer of semiconductor material that has an electron deficiency. In the transition area, the so-called barrier layer, the excess electrons hit the 'holes' where electrons are missing and close them, releasing energy as light. The connection to the anode (positive charge pole) is made via a very thin gold wire, which only minimally affects the light emission. A protective plastic layer surrounds the LED chips and improves the light emission.

A typical light-emitting diode is about one millimetre in size. The lighting element emits its light in a very targeted manner instead of radiating in all directions like a light bulb. However, the individual LED chips can be spatially combined in any way to direct the light output in the desired directions. Another aid to directing light are bezels that serve as reflectors.

Coloured and White LED Light

RGB LEDs each have a red, a green and a blue light-emitting diode and can control the amount of light they emit, which then results in the desired colour. The most complex method is to produce white LED light. Essentially, there are two different methods for producing the desired white from different light colours.

• One variant combines a blue LED with a phosphor layer, the yellow light of which gives the overall white colour.
• Additive colour mixing can also create the white LED light from red, blue and green light of certain wavelengths.