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: Drift velocity of electrons in n-doped silicon

  1. Feb 5, 2009 #1
    1. The problem statement, all variables and given/known data
    "A cylinder of n-doped silicon is 10mm long, has a diameter of 5 mm and is known to have a mobility [tex]\mu[/tex]=0.15 m2V-1s-1. The resistance of the block measured along its length is 255 [tex]\Omega[/tex]

    a) What is the resistivity of the silicon block?
    b) Estimate the electron carrier density in the silicon
    A current of 1mA is passed along the length of the block
    c)What is the electric field inside the block?
    d)What is the drift velocity of the electrons?

    2. Relevant equations
    [1] Resistivity [tex]\rho=RA/l[/tex] ; R= resistance, A= cross sectional area, l= length
    [2] [tex]\frac{1}{\rho}=\mu ne[/tex] ; mu= mobility, n=electron carrier density, e=charge of electron
    [3] E=V/x ; E=electric field, V= voltage, x= length of cylinder
    [4] Vd=[tex]\mu[/tex]E ; Vd=drift velocity,

    3. The attempt at a solution

    Right so first I got resistivity= (255x(2.5x10-3)2pi)/10x10-3= 0.5

    Then using [2], I get n = 8.3x1019 (which to me seems too high?)

    By using [3] and saying that the voltage=0.255 and x= 10mm, I get E= 25.5

    Finally using [4] I get velocity= 3.825 ms-1 which seems pretty fast. I can't see where I could have gone wrong, thoughts? thanks for the help!
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted