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Name: Alan
Status: Other
Grade: Other
Location: N/A
Country: United States
Date: March 2006


Question:
First off I'm not sure if this should have been a question asked in the computer section but here it goes. I know that light bulbs produce light via heat. Tungsten filament, argon gas environment so it does not ignite. Seems simple enough.



Replies:
LEDs (light emitting diodes) give off light via an aluminum gallium arsenide semi-conductor (I think). However, how exactly is electrical current turned into light in LEDs? According to me physics and chemistry teachers there is little to no heat given off, and technically the light is not being "converted" to light. I am not sure what they mean exactly. So how does an LED produce light? And why do they never burn out? (unless of course you exceed the 50 mA limit).

Bob Erck


What your teachers mean by "no heat given off" is that LED's do not *radiate* infrared light, as light bulbs do. LED's do give off heat. In fact, most of the energy that goes into an led comes out as heat. Less than 20% is converted to light.

LED's give off light when electrons drop from a high energy state to a low energy state. The material in which this occurs has a junction between two kinds of semiconductors, each of which permits electrons to have only specific amounts of potential energy, and the energies permitted in the two materials are different. When electrons move from one material to the other, they are no longer permitted to have the energy they had in the first material, so they drop down to a lower energy state that is permitted, and the energy difference is emitted as a photon.

LED's do burn out, though not in the same way light bulbs do. The heat produced gradually changes the properties of the semiconducting material. This reduces the light output. Many commonly available white-light LED's go dim in just a few months of continuous use. Low-power LED's, and some high-power LED's with carefully engineered paths by which heat can be extracted from the semiconductor material, last much longer.

Tim Mooney


A detailed explanation of "How do LED's work? is too long for a forum like NEWTON. The most basic principle is that electrons are driven from a state of high energy to a state of low energy in a semiconductor diode. The energy gap between the two states falls in the visible region of the electromagnetic spectrum and the energy is released as a photon of light. However, I do not think this is really very helpful, so I recommend (and did) a search on the terms: "How LED's work?" and "light emitting diodes". Of many I got the following two "hits":

http://electronics.howstuffworks.com/led.htm

http://en.wikipedia.org/wiki/Light-emitting_diode

Although the latter is not 'peer reviewed' -- anyone can contribute -- it is kind of a "living" encyclopedia, but it usually is self correcting. These sites provide sufficient detail so that you can understand LED's at any depth you choose to pursue the question.

Vince Calder



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