Why do blue LEDs have a higher forward voltage than expected?

[madmike159]

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 anyone 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 anyone knows a reason for this your help would be appreciated.

 

[madmike159]

Sorry I ment to put this in General Physics.

 

[Manchot]

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.

 

[cmos]

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.

 

[madmike159]

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]

Blue LEDs are not -as far as I know- simple "bandgap devices" in the same way as red LEDs. Instead, blue and UV LEDs are quite complicated devices built up from several layers of III-V semiconductors with different energy gaps. The transitions that emitt the light are due to these layers forming a quantum well. I.e. there is no direct correlation between the bandgap of the material and the frequency of the emitted light.

Building quantum wells is fairly easy in theory (it basically involves "designing" the right potential using the Schroedinger equation, it is a very direct application of basic quantum mechanics) but in real devices you also need buffer layers etc. This is why reliable, cheap blue LEDs haven't been around for more than a decade or so.
Blue laser diods are even more complicated, but the basic principle is the same as for LEDs.