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n-type sample of silicon

Discussion in 'Electronic Basics' started by PZ, Jan 24, 2006.

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  1. PZ

    PZ Guest

    One question,

    "An n-type sample of silicon has a uniform density Nd= 10(power 16)
    atomes cm(-3) of arsenic. Find out the temperature at which half the
    impurity atoms are ionized. Assume that all mobile electrons and holes
    come from dopant impurities".


    don't understand the question at all. If assume all electrons come from
    dopant, that means I should I consider the intrinsic carrier? So, as
    temp goes up, some dopant get ionized, half which is 0.5 X 10(power 16)
    is the current. if intrinsic is not allowed to consider, can I still
    use p = Ni(power 2) / n to find p?


    thanks, I guess I don't get the meaning of this question. Any comments
    is appreicated.
     
  2. For materials in equilibrium, I think it holds.
    My general understanding is that at room temperature nearly all of the
    extrinsic dopant electrons are thermally ionized into the intrinsic's
    conduction band and that almost none of the intrinsic's electrons are
    ionized. This is seen on a plot of electron density versus inverse-
    Kelvins.

    But that isn't the question, really. The question is about at what
    temperature it would be the case that there would .5e16 be ionized.

    Perhaps this will help:
    http://ece-www.colorado.edu/~bart/book/book/chapter2/ch2_6.htm

    I'm thinking of figure 2.6.8 and the material both above and below it.

    Jon
     
  3. Rich Grise

    Rich Grise Guest

    Me neither. The way semiconductors were explained to me, a millennium or
    so ago, all of the dopant atoms are "ionized" all of the time, no matter
    what - that's what makes it an N-type semiconductor. They have no choice
    but to "ionize", because the valence electron won't fit into the crystal
    lattice.

    When a copper wire is carrying current, do the copper atoms get "ionized?"

    But, I'm confident that someone who knows their elbow from a hole in the
    ground will help clear this up for both of us. :)

    Thanks!
    Rich
     
  4. They can 'freeze out' if the temperature is low enough.

    You might think about this in terms of positive and negative
    attraction forces versus thermal energy. The extrinsic materials, if
    the temperature were at absolute zero or close to it, would keep their
    electrons very near their nucleus by + and - attractions. The
    probability of such an electron being buffeted away would be extremely
    low and near 0K temperatures. But since that net attraction isn't
    very strong in the case of extrinsic materials embedded in a silicon
    lattice, it doesn't take a lot of thermal jostling to bump them "up"
    into the conduction band of the silicon. At room temperature, none of
    them are able to settle for any length of time with an extrinsic
    nucleus. They keep getting kicked back up. So it appears that "all"
    of them are in the conduction band. But if you lower the temperature,
    there is less and less thermal energy and the probabilities shift. But
    we are talking "very cold" to reach 50%, even -- approximately the
    boiling point of liquid nitrogen at 1 atmosphere pressure, I think.

    Jon
     
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