How Everything Works
How Everything Works How Everything Works

Question 518

How do light emitting diodes work and what is responsible for their different colors?
Light emitting diodes are diodes that have been specially designed to emit light rather than heat during their operations. Whenever current is flowing through a diode, electrons are moving from the n-type semiconductor on one side of the diode's p-n junction to the p-type semiconductor on the other side of the junction. Once an electron (which is negatively charged) arrives in the p-type semiconductor, it's attracted toward an electron hole (which is positively charged) and the two move together. The electron soon fills the hole and it releases a small amount of energy when it does. In a normal diode, electrons lose energy at a rate of 0.6 joules of energy per coulomb of charge as they recombine with the electron holes. That means that the current flowing through the normal diode loses 0.6 volts as it flows through the diode. The missing energy becomes thermal energy or heat.

But in a light emitting diode (an LED), each electron that arrives in the p-type semiconductor after crossing the p-n junction recombines with an electron hole in a remarkable way. It gives up its extra energy as light! Each time an electron and an electron hole recombine, they emit one particle of light, a photon, and the frequency, wavelength, and color of that light depends on the amount of energy given up by the electron as it falls into the electron hole. The semiconductor material from which an LED is made has a characteristic called its band gap. This band gap measures the energy needed to pull an electron away from an electron hole in the material. If this band gap is small, the LED will emit infrared light. If this band gap is larger, the LED will emit red, orange, yellow, green, or even blue light (the farther to the right in that list, the more energy is required). Because each electron loses more energy in recombining with an electron hole in an LED than it would in a normal diode, the current flowing through an LED loses more voltage (typically 2 volts for red LEDs and as much as 4 volts for blue LEDs) than does the current flowing through a regular diode (typically 0.6 volts).

Physicists, chemists, materials scientists, and engineers have been working for years to perfect the materials used in LEDs, making them more and more efficient at turning the electrons' energies into light. Until recently, there were no suitable materials from which to build blue LEDs, but recent developments of large band gap semiconductors have made blue LEDs possible. In fact, even blue laser diodes are now being made. A laser diode is a specially designed LED in which all of the photons are copies of one another rather than being emitted independently by the individual electrons as they drop into their respective electron holes.

One final note: it's now possible to obtain a "white" LED! This device is actually a blue LED, combined with a fluorescent phosphor that converts the blue light into white light.


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