charge flow in semiconductor

Discussion in 'Physics' started by whale, Jan 11, 2009.

  1. whale

    Thread Starter Active Member

    Dec 21, 2008
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    electrons density in semiconductor is less compared to conductors like copper.
    but the charge flow ie,current in materials depend upon the number of free electrons.
    in semiconductor(doped) the electron flow is free ,and there is no traffic jam between the free electrons.
    but in conductors,although they have many number of free electrons there is traffic jam between electrons.so,this effect will affect current flow.
    thus in doped semiconductor ,free electron density is less and the charge flow is free.and in conductor free electron density is high and ther is traffic jam in charge flow.

    so,my question is
    for a standard voltage the current flow in standard doped semiconductor slice will be greater than same standard copper?

    in other words ,
    is that the resistance of standard doped semiconductor is less than same standard copper?
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well you have some misconceptions to overcome first.

    The electron desity in terms of electrons per cubic metre is not not so very different in all (solid) materials. The electrons are fixed by the number of electrons per atom, which is fixed by the atomic number of the atom. So for example the atomic number of copper is 29 and germanium 32. thus copper has 29 electrons and germanium 32 i.e.more per atom than copper.

    The real difference is that in both materials, in the solid, the energy levels permitted to the electrons form two distinct "bands" These are called the valence band and the conduction band. The electrons in the valence band stablise the crystal structure and play no part in conduction. It is only electrons in the conduction band that take part in conduction.

    The valence band has lower energy and is occupied first in preference in both substances.

    The energy difference (or gap) between the valence band and the conduction band is much much greater in semiconductors, like germanium, than in metals, like copper.

    However if we dope a semiconductor with an atom that has more electrons then the excess ones have to go into the conduction band.

    Also if a proportion of the electrons are energetic enough to jump from the valence band to the conduction band they will find this easier from a metal atom than from a semiconductor atom.

    So there are fewer electrons in the semiconductor conduction band than a metal one, even with doping (which only affects a very small percentage of atoms)

    So the semiconductor always has lower conduction than metal.

    You also need to see how holes affect this equation as they form an equal conduction path (to electrons) in semiconductors.

    Look here at the AAC textbook. It is quite good.

    http://www.allaboutcircuits.com/vol_3/chpt_2/4.html
     
  3. whale

    Thread Starter Active Member

    Dec 21, 2008
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    then ,what is the purpose of collector region with relatively large area in bjt,and how current amplification takes place in bjt?
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Did you read the AAC textbook?

    Was there anything you need clarification on?

    I am asking because feedback helps improve things.
     
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