What is an LED and how does an LED work?

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What is an LED and how does an LED work?
« on: 28/08/2009 12:30:03 »
Chad asked the Naked Scientists:
Hi there - I love the podcast!

I was wondering:
what exactly is a light emitting diode, or LED, and if they are so powerful and so energy efficient why don't we use them instead of regular light bulbs?


Chad Weirick
Norton, MA USA

What do you think?


Offline Vern

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What is an LED and how does an LED work?
« Reply #1 on: 28/08/2009 14:43:58 »
The light of the LED comes from electrons recombining with atoms in the junction of a diode. It is the nature of a semiconductor diode but special material selection and construction of the junction enhances the effect. I'm not sure they are so powerful. They are efficient because the light does not come from heat but from electron transitions.

I notice that there are now LED flash lights (Torches), but it was years before LED's were powerful enough to light up an area. They were mostly used for indicators in calculators and the like.



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What is an LED and how does an LED work?
« Reply #2 on: 28/08/2009 16:58:44 »
Have a look here. They make them operating at powers of more than a Watt, now. Bearing in mind the good efficiency, they are suitable for low level room lighting.


Offline techmind

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What is an LED and how does an LED work?
« Reply #3 on: 29/08/2009 00:29:29 »
Traditionally there's been several reasons why LEDs aren't used for general lighting:

1) Although they're relatively efficient, LEDs still dissipate heat. They need to be kept reasonably cool (under 80C-100C probably) to avoid damage. It's only within the past 5-7 years that we've developed construction/packaging techniques which can thermally conduct away the heat and has allowed high-power operation (a few Watts, as opposed to mW).

2) LEDs only natively emit a single colour. This makes them super-efficient compared to ordinary lights plus filter when you actually want coloured light (like for traffic lights). Through the 1980's and 1990's, LEDs were only available in red, orange, yellow and green colours. Conventionally you need to add blue to make white (spectral details aside). In the early 1990's, blue LEDs began to appear on the scene, but were expensive and not very bright. These got brighter and cheaper through the 1990s.

3) Blue LEDs open up a new way to make white from a single package: apply a yellow (red+green) emitting phosphor which is stimulated by an inherently blue LED. This works reasonably well for small torches, but the colour uniformity is pretty poor - it might be bluey-white in the centre of the beam, and yellowy around the edge. The "white" light colour tends to be rather "cold" (bluey) overall too. The spectrum of this light is also a bit peculiar, with a pronounced dip in the turquiose part of the spectrum, which might make things look a bit off-colour if used for lighting (rather like conventional fluorescent lamps, or worse).

4) High power LED's became widely available in the 2000's, and phosphor technologies are improving to give more welcoming "warm white" colours - albeit at lower efficiencies than harsh-white. The cost is still relatively high per unit (maybe 1 for a 1-3 watt device) and you still need half a dozen to get light output comparable to a 30p tungsten bulb ... and you still need to add a low-voltage power supply.

I think Philips now make an LED "bulb" which will fit a standard light fitting, but it's somewhat less bright than a regular 40watt bulb, has a slightly odd magenta-tinged white colour, and costs about 25 in the retail shops!

We're getting closer all the time to LED lighting, but it's not yet cost-effective for general lighting - although inroads are being made in specialist applications.
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"


Offline Madidus_Scientia

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What is an LED and how does an LED work?
« Reply #4 on: 29/08/2009 03:59:56 »
Vern's explanation is a bit hard to understand without prior knowledge, which I assume is absent and hence the question. So I offer my explanation;

A diode is a device which will only allow electric current to flow one way. They are made by joining two different types of semiconducter material together; N-type and P-type. The different types are named such because N-type has extra electrons which are negatively charged. P-type has positively charged atoms in it. Think of the positively charged atoms in P-type as holes that the electrons from N-type will try to fill. When the electron leaves its spot to go and fill a hole, it creates another hole where it just left, so the effect is that the holes themselves seem to move.

So when there is no current applied to the diode the electrons will fill all the holes near the junction of the two materials, called the depletion zone. So in the depletion zone because all the holes are filled and there is no free electrons, the material basically becomes an insulator and current doesn't flow.

If you connect this diode to a circuit, with the N-type connected to the negative, the electrons in the depletion zone will be forced out of their holes and current will flow. But if you connected the circuit the other way around, current cannot flow because the electrons will be attracted to the positive terminal, and the positive "holes" will be attracted to the negative terminal, so all that will basically happen is that the depletion zone will increase.

Anyway, each time an electron "falls" into a "gap" (A simplified explanation - let us know if you want the long one), it releases energy in the form of a photon. So all diodes emit light, the difference between standard diodes and what we call light emitting ones is that LEDs are are made with materials that are designed such that the frequency of the light emitted is in the visible spectrum, among other obvious design differences like a transparent case etc.

So the reason they are so efficient is because LEDs don't produce light by producing heat first, they just produce the light. (They do produce heat, but a miniscule amount compared to a light bulb)
« Last Edit: 29/08/2009 04:14:26 by Madidus_Scientia »