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!

Mean Free Time Silicon (PURCELL 4.12)

  1. Oct 3, 2012 #1
    1. The problem statement, all variables and given/known data
    This is Purcell 4.12... Given that conductivity in silicon at 500K is 0.3 sec-1, deduce the mean free time between collisions of an electron.

    2. Relevant equations
    Electron mass is 9.1x10-28 grams (mass of a hole is exactly the same), the charge on an electron is 4.8x10-10, the density of electrons and holes is each 1015.

    3. The attempt at a solution
    I solved Purcell's equation for conductivity to find mean free time instead:

    T=(Mass*σ)/(2Ne2)... and I get something like 5.92x10-25 seconds. Isn't this way too small? Or did I do something wrong?
  2. jcsd
  3. Oct 6, 2012 #2
    Are you sure that the holes count as mobile? What is their mass? Would counting them effectively halve the time you're looking for?
  4. Oct 6, 2012 #3


    User Avatar
    Homework Helper
    Gold Member

    According to page 147 of Purcell, the conductivity of pure silicon at 500 K is 0.3 (ohm-cm)-1 rather than 0.3 sec-1.
  5. Oct 7, 2012 #4
    Ah! So the OP's problem was conversion.

    What about that factor of 2?
  6. Oct 7, 2012 #5


    User Avatar
    Homework Helper
    Gold Member

    I believe the factor of 2 is there because an electron could collide with either another electron or a hole. So, the number density of particles that the electron could collide with would be twice that of just the number density of electrons alone.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Mean Free Time Silicon (PURCELL 4.12)
  1. Phonon mean free path (Replies: 4)

  2. Griffiths 4.12 (Replies: 2)

  3. Mean free path help (Replies: 0)