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

A Derivation of DOS

  1. Apr 8, 2016 #1
    Hi guys,

    I have a very particular question on the derivation of DOS.

    For a particle in an infinite box k=π/L. However, when deriving the density of states, all text books use k=2π/L

    Now you could argue that they account for spin degeneracy, but its not that! Because in the text books that happens (again) in a later step

    So where is this factor 2 coming from?


  2. jcsd
  3. Apr 8, 2016 #2
    the solution of Schrodinger’s equation for particle in a 3D box
    where Each solution can be uniquely associated with
    a k- space vector ; k^2 = k(x) ^2 +k(y)^2 + k(z)^2 the boundary condition gives k =n.pi/L
    The k has three axes k(x) ,k(y), k(z) the conditions at the walls are k(x) =n(x).pi/L .... and similarly for k (y) and K*z) and so on where n(x), n(y) and n(z) can take values as +/- 1, +/-2,. +/-3..........
    some of the texts might have used n(x) taking values 1.2,3,..... so for proper counting the states they might have used 2.(pi)/L
    see for details-http://web.eng.fiu.edu/npala/EEE6397ex/EEE_6397_Ch2_Energy%20Levels%20and%20Charge%20Carriers%20in%20Semiconductors_PART3.pdf [Broken]
    Last edited by a moderator: May 7, 2017
  4. Apr 8, 2016 #3
    Thanks for the link, I derive it the same way.

    You're right. If they use 2π/L then they must take the entire k-space into consideration, because they didn't differ between +/-. Where as if you take the π/L if did already differ between positiv and negative solutions and you only take the respective segment in k-space?!

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

Have something to add?
Draft saved Draft deleted