# Blue LEDs and band gap energy

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## Main Question or Discussion Point

My physics/electronics teacher told me some thing really interesting and confusing a wile ago and I have been trying to get my head round it. The forward voltage in a LED (voltage needed to drive the current) is proportional to the band gap energy.
However Blue LEDs have a higher forward voltage than the band gap energy suggests. I haven’t been able to figure out why. Does any one know?

This is a link to a page with some band gap energies for different semi-conductors. http://en.wikipedia.org/wiki/Bandgap

This is a link to a page on LED’s and what they are made out of http://en.wikipedia.org/wiki/LED

I up loaded a .txt of forward voltages and band gap energies so you don't have to look through lots of internet pages. The blue LED forward voltage defiantly looks too high. If any one knows a reason for this your help would be appreciated.

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Sorry I ment to put this in General Physics.

There are really a lot of other factors that go into the forward voltage of an LED, such as doping, the quality of the interfaces, the intrinsic resistive losses of the material, etc. Even if you account for all of these, there isn't really even a well-defined "forward voltage," because the current / light output varies continuously with voltage.

So, you're right that the forward voltage isn't completely proportional to band gap. As a rule of thumb, though, it's close enough.

Hey madmike, do you think you can point out a specific example that this pertains to? Be as quantitative as possible. If by "forward voltage" you mean the voltage necessary to turn the device on, then this shouldn't be substantially near the bandgap.

On a sidenote (may be of use), I know InGaN is common for blue LEDs. Additionally, there several designs which use LEDs of longer wavelength and then send that light through some sort of upconverter to get blue light out of the system.

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Hey madmike, do you think you can point out a specific example that this pertains to? Be as quantitative as possible. If by "forward voltage" you mean the voltage necessary to turn the device on, then this shouldn't be substantially near the bandgap.
I didn't say near to I said proportional.

Thanks i though there would of been something I would of missed.

f95toli