Why can't LEDs be made to operate at any voltage?

lightningbug

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With all the technology progress being made these days, you'd think someone would mass produce LEDs to run on 1.5V or 1V or 1000V or whatever. What are the limitations to producing LEDs that will run on different voltages just like all other electronic devices? Isn't it possible to design an LED that would run safely on 1-2V, be very bright, and still be effecient?
 

tetranitrate

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Well, I think you'd have pretty much the same problems as getting any IC to run on any voltage. The problems are mainly physical. I think it has to do with the size between the PN junction (or other junction types) along with the actual electrical pathway size found in the semiconductor. The smaller the junction, the less voltage it can safely handle, otherwise you get arcing. If you notice the size of power transistors, capacitors, etc seem to increase with the amount of voltage they are rated for. There are probably other factors that I don't understand that contribute to the problem (stray capacitance?).
 

SemiMan

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Yes but with all the advances in technology, they still have not been able to change the speed of light... :)

The forward voltages are a factor of the junction potentials of the materials that are used to make LEDs.... the combination of which somewhat defines the wavelength of light they emit. Sorry, that is about as simple as I can make it. The materials have been picked to emit the wavelength of interest, and also to be as efficient as possible. This would be somewhat akin to why are NiMh and NiCad batteries about 1.2V and Lithium between 3 and 4, though we would really like a one cell 12V battery, in order to do this, we must stack a bunch of lower voltage ones.

Semiman
 

matrixshaman

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I believe that is a limitation of the materials currently available. I think you could say it's limited by the chemistry or physics of the semiconductive materials. But I'm mostly guessing here....
 

Biker Bear

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It's simply the nature of the beast; Light Emitting Diodes are semiconductor junction devices. That means you have a piece of the semiconductor with additives that provide extra electrons mated with a piece with additives that leave it deficient in electrons. ("N" and "P" types.) Right at the junction, these characteristics cancel out, and it requires a certain amount of voltage - dependent on the material and the additives, or "doping" - to push electrons through that "barrier".

This is why certain LED colors require different voltages - in order to get the right energy release for that color of light, the semiconductors have to be formulated with that in mind. It's just not possible to make the low-voltage materials used for red or yellow LEDs produce blue light - if it could, we'd have had blue LEDs a long time before we actually did.
 

tetranitrate

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Edit: same content as above

The problem with running semiconductors at low voltages is that there is a lower threshold beyond which electrons cannot "jump" the PN junction. It's really more complicated than that, but I never could understand it. There are certain materials that have better lower operating voltage characteristics (CMOS vs. TTL), but I think the problem with LEDs is that you can't easily switch materials since the color of the light outputted depends heavily on the materials used, and the materials used defines the forward voltage.
 
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abvidledUK

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What you need is some sort of feedback loop, I or V, similar to multi voltage PR2 LEDs, built into LED, over low dc range, 1-50v
 

elgarak

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None of the answers get the most important feature of semiconductor LEDs: It's their energy bandgap. In metals, the electrons fill up energies up to a certain level, the Fermi level. In order to travel through the material, the electrons have to get into an empty state above that. In metals, the empty states start directly above the Fermi level (actually, that's the definition of a metal).

In semiconductors, there's a forbidden zone above the filled levels. The width of this gap is specific to the material used and defines both the color of the LED and its forward voltage. You have to 'lift' the electrons above the band gap to transport them into the LED and get light out. So your applied voltage has to be higher than the band gap. The band gap is given in eV, and if you just add the "e" to the forward voltage, you'll get the band gap of the used semiconductor in eV. If you calculate 1242/E (band gap in eV), you'll get the wavelength of the emitted light in nm.

The band gap has nothing to do with the doping, or the characteristics of the pn junction (though both are important to make the LED work). If you want emission of a certain wavelength of light, that defines the set of materials you have to use to make the LED, which has this specific band gap. (I'm a physicist, and for me the wavelength and energy (and voltage) are one and the same.) That's why it took so long to make blue and with this white LEDs: There simply was no usable material with the right band gap.

The price you have to pay for a high energy/low wavelength LED (blue/UV) is the high band gap/forward voltage.

"Nifty is the Lord" - A. Einstein
 
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jtr1962

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It is possible to make LEDs operate at higher voltages by simply incorporating a resistor and/or multiple dies in series in the same package. In time we may even integrate driver circuits so as to allow them to efficiently operate off 12VDC or 120 VAC. In theory it is possible to make step-up driver circuits to allow LEDs to operate off lower voltages as well. The problem with both these approaches are cost and added complexity as well as loss in flexibility. A driver circuit might drive the LED at 20 mA regardless of input voltage. You might want to drive it at higher or lower currents instead. In the end I think we're better off with external driver circuits instead.

As stated in previous posts it isn't possible to make an LED operate directly at different voltages than they already do. The lower limit for blue (and hence white) LEDs seems to be around 2.8V. The upper limit is around 3.6V. These variations are due to process differences, material differences, and die size relative to drive current (larger dies have a lower Vf at any given current).
 

el_vato

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SemiMan said:
Yes but with all the advances in technology, they still have not been able to change the speed of light... :)

The forward voltages are a factor of the junction potentials of the materials that are used to make LEDs.... the combination of which somewhat defines the wavelength of light they emit. Sorry, that is about as simple as I can make it. The materials have been picked to emit the wavelength of interest, and also to be as efficient as possible. This would be somewhat akin to why are NiMh and NiCad batteries about 1.2V and Lithium between 3 and 4, though we would really like a one cell 12V battery, in order to do this, we must stack a bunch of lower voltage ones.

Semiman


umm RE: speed of light...

http://www.answersingenesis.org/docs2001/0201light_stop.asp
 

srvctec

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elgarak

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One comment: A lot of people working in the field do not distinguish anymore between semiconductors, semi-metals, non-metals and insulators. If it doesn't have a band gap, it's metal. If it does, it's not a metal, and can be used like a semiconductor.

The reason being that even very high band gap materials (typical insulators) like diamond are nowadays being used for semiconducting devices.
 
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