1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Fraction of ionized atoms in an intrinsic semiconductor

  1. Feb 19, 2014 #1
    1. The problem statement, all variables and given/known data
    Here is the problem in the book:

    Here is the solution:

    What I don't get is why the solution defines the fraction to be ni/(5*1022). Shouldn't it be 2*ni/(5*1022) since we have both donors and acceptors, so there are actually twice the number of atoms.

    Also, intrinsic means that the silicon is not doped right? If that is the case why are there even donors and acceptors, shouldn't the concentrations be zero?
  2. jcsd
  3. Feb 19, 2014 #2


    User Avatar
    Homework Helper

    There are no donors, no acceptors in an intrinsic semiconductor. Some silicon atoms get ionized by thermal energy. One of the electrons of a silicon atom gains enough energy to escape from its ion, which means leaving the valence band and becoming a "free" electron in the conduction band. The escaped electron leaves a positive ion behind, which means lack of an electron, a "hole" in the valence band.

    So excitation of a silicon atom produces an electron-donor pair and leaves one positive ion behind.

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

Have something to add?
Draft saved Draft deleted