Posts tagged LED

(See also LEDs on Wikipedia).

Light emitting diodes (LEDs) are 2-terminal semiconductor devices whose operation is based on a PN junction. Their terminals, as with all diodes, are called anode (A) and cathode (K).

Note: sometimes they are redundantly called LED diodes, although the acronym already has the word "diode" in it.

Depending on their specific construction, they emit "light" with different spectral characteristics. Some LEDs emit in the IR range, some in the visible range (with different colors) and other in the UV range.

The semiconductor materials used to fabricate LEDs are quite exotic compared to the ubiquitous silicon used for most other semiconductor devices. For example Gallium Phosphide (GaP), Gallium Arsenide-Phosphide (GaAsP) and Aluminium Gallium Nitride (AlGaN).

There are three main application of LEDs:

• As indicator lights on panels and instruments to convey information to human operators. LEDs intended for this application are also called indicator LEDs.

• As light sources. These are often called lighting LEDs and are nowadays widespread in lighting fixtures and LED lamps that use white LEDs.

• As optical communication transducers, i.e. as devices to convert electrical signals into optical signals. An example of this application are the ubiquitous TV remote controls, that use an IR LED to send commands to the TV-set.

LEDs turn on when their forward voltage (voltage on the anode relative to cathode) reaches a threshold voltage and current starts flowing from anode to cathode.

The threshold voltage depends on the specific semiconductor material employed in the construction and roughly increase with the frequency of the emitted radiation (decreases with the wavelength).

So, for example, IR LEDs have lower threshold than red LEDs, which in turn have lower threshold than blue LEDs. UV LEDs have the highest thresholds. The following are ballpark values for the thresholds for various kind of LEDs:

White LEDs are actually blue LEDs whose emitting area is covered with a special phosphor material that converts part of the emitted blue light into longer wavelengths (toward yellow light). These latter mix with the residual blue light to give the sensation of white light to the human eye.