LEDs

Although you may not realize it, you have probably already seen at least one light emitting diode (LED) device at some point in the day. LEDs, being found in a diverse array of consumer products, are ubiquitous in everyday life. For example, every morning you may look at a digital alarm clock; the lights that make up the numbers are very often a red or green LED. Modern automobiles are now often being produced with LED headlights, as opposed to the more traditional incandescent lights. Your television may have multiple LEDs built into it, with a red LED being used as a power indicator or white LEDs providing a backlight to illuminate the screen.

LEDs, at the most basic level, are small electronic semiconductors that are able to produce light when an electric current is passed through them. However, before we go more in-depth, a short primer on how semiconductors work is needed. Semiconducting materials contain a number of valence electrons which completely fill a low energy level referred to as a valence band. Semiconductors are often doped with impurities, such as aluminum or phosphorus, that contain differing numbers of valence electrons than the semiconductor material, leading to an increase (phosphorus) or decrease (aluminum) in valence electrons in the valence band. Semiconducting materials that contain more electrons are referred to as n-type materials. Since the valence band is already full of electrons, the excess are placed in the conduction band, which is higher in energy than the valence band. Materials that contain fewer electrons are referred to as p-type materials. In this material the valence band is not filled, and thus contains a number of “electron holes”. As a current is passed from the n-type to the p-type material, the excess electrons migrate towards the anode and the holes migrate towards the cathode. Eventually the meet at the interface and combine, resulting in the electrons dropping from a high energy state in the conduction band to a lower energy state in the valence band. The transition results in a release of energy which in an LED is often in the form of electromagnetic radiation, with a small amount of heat being generated.

The type of light produced by an LED is dependent upon the band gap (energy difference) between the valence band and the conduction band. In short, large band gaps produce shorter wavelength light while small band gaps produce longer wavelength light. There are two primary ways of adjusting the band gap. The first is to modify the amount of dopant added to the semiconductor. Increases in dopant lead to a smaller bandgap, while decreases lead to a larger band gap. However, it should be noted that doping will never increase the band gap beyond the band gap of a pure semiconductor. As a consequence of this, it is not possible to increase the band gap of a semiconductor through increased doping. Additionally, adding too much dopant may negatively impact the efficiency of the LED. Thus, while doping may be acceptable for fine tuning the band gap of a material, large changes are not possible with this method.

To dramatically alter the band gap, one must change the composition of the semiconductor itself. Several semiconductor compounds are commonly used in production of LEDs: Gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), aluminum indium gallium phosphide (AlInGaP), gallium arsenide (GaAs), and aluminum gallium arsenide (AlGaAs).[1] Both GaN and AlGaN semiconductors are used to produce light with wavelengths between 240nm-360nm, which results in UV light. These LEDs have a wide number of uses, including curing of chemicals or polymers, as an analytical tool in laboratories, and as a method of sterilizing water and medical equipment. InGaN is used to produce light in the 395nm-530nm range, resulting in near-UV, violet, blue, or green light. AlInGaP produces light with wavelengths between 565nm-645nm, which corresponds to colors from yellow-green to red. Both of these semiconductors are used to produce the wide array of colored LEDs used in everyday products. GaAs produces light in the 660nm-900nm range, which produces deep red to near infrared (IR) light. LEDs with this semiconductor are perhaps most well known for their use in television remote controls, which uses an IR LED to communicate with the television itself.

In regards to white light, there are several methods used to produce these types of LEDs, of which three will be described here.[3] The first is to use a red, green, and blue diode simultaneously, forming what is known as a RGB diode. By using this type of diode, one can carefully adjust the intensities of each individual color to produce a white light. However, a more common method is to coat an LED with a phosphor compound. To produce white light using this method, a blue LED is coated with yttrium aluminum garnet (YAG), which releases yellow light when exposed to the blue light of the LED. To the human eye, this combination of blue and yellow light will appear as white. Finally, recent advances in LED research have found that a new type of semiconductor wafer called gallium-nitride-on-silicon (GaN-on-Si) is able to produce a white light. Traditionally, GaN semiconductors were mounted on costly sapphire crystals. However, with this new method, GaN is mounted on much cheaper silicon, which should greatly reduce manufacturing prices and increase widespread use of LEDs.

References

 

  1. Dahl, R. (n.d.) Light-Emitting Diodes. http://www.photonics.com/EDU/Handbook.aspx?AID=36706

 

  1. Casidy, R. & Frey, R. (2002) Bonds, bands, and doping: How do LEDs work? http://www.chemistry.wustl.edu/~edudev/LabTutorials/PeriodicProperties/MetalBonding/MetalBonding.html
  2. Wikipedia. (2015) Light-emitting diode. http://en.wikipedia.org/wiki/Light-emitting_diode