You have probably seen some kind of LED display or an LED light source recently. You may even be looking at one now if you're reading this on a particular type of computer monitor. LED, which stands for "light emitting diode," has become a ubiquitous component behind everything from lamps to digital watches, traffic lights, auto headlights, text displays and in a plethora of electronic devices including cell phones and TVs, just to name a few. But behind this seemingly simple device, there lies a complex yet fascinating technology involving the fields of electronics and even advanced physics.
Energy Bands
In order to learn exactly how LEDs work, it is important to first understand the fundamentals. Electricity consists of electrons that can be either particles or waves. Now think of any solid material and imagine it possesses two energy bands: a valence band and a conduction band. For a solid to conduct electricity, electrons must move from the valence band to the conduction band. The difference between a metal that conducts electricity and an insulator, which does not, is that the insulator has a boundary called a "band gap" between the valence and conduction bands. This band gap makes it more difficult for electrons to move to the conduction band, so an insulator generally does not conduct electricity.
Semiconductors
Some insulators, however, can be forced to allow the flow of electrons by applying energy in the form of heat or light. The added energy excites the electrons in the valence band enough for them to jump over the band gap and into the conduction band. These types of materials, called semiconductors, make up most electronic components. One type semiconductor is the diode
Diodes
Diodes are a key component in electronic circuits.
Electricity tends to flow in all directions at once. However, electronic circuits require that electrons flow in a certain direction for devices to work. Diodes make this possible by allowing the current to move only in a single direction. A diode acts like a one-way lane; on one direction, the diode serves as a conductor; on the other direction it acts as an insulator and stops the flow of electrons.
Where LEDs Come In
The only thing separating a "normal" diode and the light-emitting variety is the material it consists of. A standard diode has silicon-based semiconductors, while an LED utilizes a combination of elements, among them gallium, indium, aluminum, arsenic, phosphorus and nitrogen. In every diode, every time you apply current in the "proper" direction, electrons move from the valence band to the conduction band. This leaves empty spaces in the valence band, and after a time, electrons will shift back to fill these empty spaces. This shifting process releases energy. Silicon doesn't lend itself to emitting light; as a result, silicon diodes only give off heat. But with materials such the ones used in LEDs, this shifting of electrons causes the LED to give off light.
Colors
Each time an electron jumps back across the band gap from the conduction band to the valence band in an LED, it emits energy in the form of light. The width of the band gap determines the color of the light the LED gives off. The first LEDs were red, but with advances in technology and with adjustments to the band gap, LEDs today come in a wide variety of colors.
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