Light Emitting Diodes

What is a LED ?

A light-emitting diode (LED) is a two-lead semiconductor light source. It resembles a basic pn-junction diode, which emits light when activated. When a fitting voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.

A LED

LEDs are so common, they come in dozens of different shapes and sizes. The LEDs you are most likely to use are the through hole LEDs with two legs. There are lots of LEDs that are small and hard to solder but these are easy to use with a breadboard because they have long wires we can stick in. The clear or clear-ish bulb is what protects the light emitter (thats where the magic happens). In fact, the first two letters of LED stand for Light Emitting.

A really nice thing about LEDs is that they are very simple. Unlike some chips that have dozens of pins with names and special uses, LEDs have only two wires. One wire is the anode (positive) and another is the cathode (negative). The two wires have different names because LEDs only work in one direction and we need to keep track of which pin is which. One goes to the positive voltage and the other goes to the negative voltage. Electronic parts that only work in 'one direction' like this are called Diodes, thats what the last letter of LED stands for.

• The longer lead goes to the more-positive voltage
• Current goes in one direction, from the anode (positive) to the cathode(negative)
• LEDs that are 'backwards' won't work - but they won't break either

It's all a little confusing - we often have to think about which is which. So to make it easy, there's only one thing you need to remember and that's the LED wont light up if you put it in backwards. If you're ever having LED problems where they are not lighting, just flip it around. Its very hard to damage an LED by putting it in backwards so don't be scared if you do.

What are LEDs used for?

LEDs are mostly used for two things: illumination and indication. These are technical words but are good to understand because if you want an LED for one thing and you buy the wrong thing you'll be pretty bummed.

Headlights should be bright!

Illumination means to "shine light onto something" - like a flashlight or headlights. You want your headlights to be bright as heck.

Brake lights should be bright enough to see, but don't need to light up the road!

Indication mean to "point something out" - like a turn signal or brake lights on a car. You don't want your car's turn signal to blind people!

If you get the wrong type you could end up with a DIY flashlight that is dim, or a control panel that burns people's eyes!

Diffused LEDs are really good at indication, they look soft and uniform and you can see them well from any angle.

Clear LEDs are really good at illumination, the light is direct and powerful - but you can't see them well from an angle because the light is only going forward.

How Can a Diode Produce Light?

Light is a form of energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called photons, are the most basic units of light.

Photons are released as a result of moving electrons. In an atom, electrons move in orbitals around the nucleus. Electrons in different orbitals have different amounts of energy. Generally speaking, electrons with greater energy move in orbitals farther away from the nucleus.

For an electron to jump from a lower orbital to a higher orbital, something has to boost its energy level. Conversely, an electron releases energy when it drops from a higher orbital to a lower one. This energy is released in the form of a photon. A greater energy drop releases a higher-energy photon, which is characterized by a higher frequency. 

As we saw in the last section, free electrons moving across a diode can fall into empty holes from the P-type layer. This involves a drop from the conduction band to a lower orbital, so the electrons release energy in the form of photons. This happens in any diode, but you can only see the photons when the diode is composed of certain material. The atoms in a standard silicon diode, for example, are arranged in such a way that the electron drops a relatively short distance. As a result, the photon's frequency is so low that it is invisible to the human eye -- it is in the infrared portion of the light spectrum. This isn't necessarily a bad thing, of course: Infrared LEDs are ideal for remote controls, among other things.

Visible light-emitting diodes (VLEDs), such as the ones that light up numbers in a digital clock, are made of materials characterized by a wider gap between the conduction band and the lower orbitals. The size of the gap determines the frequency of the photon -- in other words, it determines the color of the light. While LEDs are used in everything from remote controls to the digital displays on electronics, visible LEDs are growing in popularity and use thanks to their long lifetimes and miniature size. Depending on the materials used in LEDs, they can be built to shine in infrared, ultraviolet, and all the colors of the visible spectrum in between.