Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Bandgaps - how are they tuned?

  1. Aug 28, 2010 #1
    I know the width of possible band gaps for something like an LED depends on the P and N material present, and that they can change them by altering the density (doping) of the charge carriers in the two.

    Do they also have ways to implant or strip carriers from the layers other than doping?

    I've heard of things like ion implantation. How do they do that? Are there others as well?

    All the best shipmaties!
  2. jcsd
  3. Aug 28, 2010 #2
    The band gap is the energy difference between the valence band and conduction band. There is an empty region between the two bands where there are no available states for electrons to occupy. It's fairly constant for a given type of semiconductor.

    What you really change with doping (aka ion implantation) is the Fermi level. You can think of the Fermi level as the midpoint of the probability distribution that defines the likelihood of an electron occupying some state. If you dope Silicon with an N-type dopant (donor ion), the Fermi level moves closer to the conduction band which means the free roaming electrons become more likely. The positve charge of the ions that give up their electrons balance the negative charge but they're stuck in place so they can't carry current.

    In LED's or photodetectors, the goal is to implant some atom that will have an energy state somewhere in the bandgap. That way, when electrons jump into or out the state there will be an associated photon of a desired wavelength (color). A professor once told me that they don't really have it down to an exact science. The quantum models are not perfect so what many companies actually did is to have grad students cook up lots of batches of LED's with different dopants to get the colors right.
  4. Aug 28, 2010 #3
    Interesting, interesting...

    I know that commercially the error in the wavelength gets 'fixed' by simply sorting them into different bins, like ball bearings are 'fixed' for size by sorting them by size after production.

    In my possibly over simplified take on things, I thought the light emitted was due purely to the band gap being direct and the size of the gap. ---------->Wiki explanation; Implications for radiative recombination

    The method you described sounds like the dislocation one described there.

    I have no idea what they use commercially as their primary choice. I'd have thought the simple band gap route would be cheaper and easier. Perhaps the dislocation methods makes tuning the emission wavelength easier.
    Last edited: Aug 28, 2010
  5. Aug 28, 2010 #4
    No, it's not just the bandgap. Generally, you have to add dopants to the semiconductor that create electron steps inside the bandgap. Also, don't forget that these are diodes! Diodes are either made by doping one side of a semiconductor or by joining different materials together.
  6. Aug 28, 2010 #5
    I understand diodes and depletion zones between two different materials, like Schottkys, and that dopants are added to silicon to make it P or N and increase the current carrying capacity.

    Adding things that offer steps within it would seem to be drifting from the idea of creating a direct gap, where the large separation means the energy has to come out as EM.

    Here's Shuji talking about his work on the first blue LEDs, seems like he's a lot more concerned about the intrinsic band gap than doping the materials.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook