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Why p type semiconductor is electrically neutral

  1. Dec 22, 2012 #1
    why p type semiconductor is electrically neutral and what is the concept of drift current?
  2. jcsd
  3. Dec 22, 2012 #2
    Semiconductors are made of atoms.

    All atoms are electrically neutral.

    Do you understand why?
  4. Dec 22, 2012 #3
    But they have holes greater in number then electrons so it will not affect its neutrality and will make it positive in nature
  5. Dec 22, 2012 #4

    Philip Wood

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    Gold Member

    Here's a simplistic answer...

    You mustn't forget the atomic nuclei. With p-type doping, the doping atoms will have 3 electrons with which to bond, instead of the 4 that silicon has, but the doping atom will also have one fewer proton in its nucleus (assuming simplest case) than silicon.

    I expect to be shot down in flames...
  6. Dec 22, 2012 #5
    Think about my question, which you didn't answer.

    Atoms are electrically neutral because they have the same number of electrons as protons.

    Atoms can combine together to form molecules, which are also electrically neutral.
    Atoms combine by sharing electrons in some way within the molecule.
    This is called chemical bonding.
    Protons are not shared although they belong to the molecule.

    Usually only small percentage of the total electrons are shared. Can you think of any exceptions?

    A typical example would be two oxygen atoms both with 8 electrons and 8 protons combining to form a neutral molecule with 16 electrons and 16 protons.
    Of these 16 electrons 4 are shared.

    With oxygen only two atoms are needed to make a complete molecule. Some elements can combine or chemically bond together in much larger numbers to form large lumps of solid. We don't call these lumps molecules because the number of atoms can vary enormously, unlike oxygen which is fixed at two.

    They are however chemically bonded aggregates with electron sharing in the same manner.

    And they are electrically neutral.

    An element such as gold, has 79 electrons of which 8 can be shared in the solid to form the metal lump we see.

    Other elements such as carbon can share 4 electrons to form the solid diamond.

    The difference between solid is that diamond is an electrical insulator, whereas gold is an electrical conductor.
    Good electrical conduction is one of the properties of metallic elements. They conduct because some of the shared bonding electrons can break free of a particular atom and move freely around the solid.

    This movement is called electron drift and answers the second part of your question.

    Remember that the solid is still electrically neutral overall.

    These free electrons normally move in no particular direction on average. However when a voltage is applied (which pushes them in one particular direction) they carry the electric current in metals.

    Diamond has no such free electrons, which makes it an insulator.

    Yet other elements such as germanium and silicon have some free electrons, but not as many as metals.
    These are semiconductors and their ability to conduct electricity is markedly less than for metals.

    So far I have only mentioned single elements but different elements also combine together to form molecules, metallic aggregates and semiconductor aggregates.

    These combination too are electrically neutral because when they combine there are the same number of electrons and protons.

    Examples would be:-
    molecules : carbon dioxide;
    metallic : aggregates brass : (zinc - copper alloy);
    insulator : silicon dioxide (glass);
    semiconductor : gallium arsenide.

    Once again they are electrically neutral.

    These would all be combinations in definite proportions so for example every silicon atom is joined to the same number of oxygen atoms.

    So far I have talked of the total number of electrons but all the action takes place via the shared electrons alone.
    So what follows will be about shared electrons only.
    So long as we remember that each of these shared electrons is electrically balanced by a proton to maintain neutrality whether it is free as in a metal our not as in an insulator we will be OK.

    All the semiconductors so far are intrinsic semiconductors and N type.

    If, however, we introduce only a small proportion a second element we can obtain different properties.

    If we replace a few of the atoms of the semiconductor (say germanium or silicon with 4 shared electrons) by an atom having one less shared electron (boron or indium with 3) we obtain the P type semiconductor you asked about.

    The P refers to the fact that there is one fewer shared electron since the impurity has one less to offer.
    However the impurity also has added one fewer proton to the aggregate so maintaining electrical neutrality.

    Yes we call the missing electron a ‘hole’ and calculate as though it were a real positive charge because that makes the maths easy, but the overall aggregate is still electrically neutral.

    As well as decreasing the supply of electrons we can also increase them by a similar process, but with an atom that has more electrons (and more protons) eg arsenic with 5.

    This enhances the N type effect, but as additional protons have also been added the material remains electrically neutral as before.
  7. Dec 23, 2012 #6
    P-type semiconductor means that the charges 'free' to move munder the influence of an electric field are 'holes'...+charges.
    It does not mean that there are more +charges than - charges.
    In the same way an n-type semiconductor means that the charges 'free' to move are electrons. It does not mean that there are more electrons than + charges.
  8. Dec 23, 2012 #7
    The number of holes is greater than the number of conduction electrons. Not all electrons in the crystal are conduction electrons.

    The condition of electrical neutrality is the number of actual electrons in the crystal have to equal the number of protons in the crystal. Some of the electrons reside in the core electronic states, some in the valence electronic states, and some in the conduction electrons states.

    Both valence holes and conduction electrons are quasiparticles. The number of valence holes and the number of conduction electrons has nothing to do with the total number of actual electrons in the crystal.

    I have a heuristic way of looking at it. The protons in the lattice are balancing the charge of electrons and holes.

    The crystal lattice is comprised of nuclei at every lattice point. Each nucleus has a number of protons. The crystal has electrons and holes moving around between the lattice points.

    For every hole that is in the valence band, there is one proton mission from the lattice. The proton is missing at the lattice point holding an acceptor atom. For every electron in the conduction band, there is an extra proton. The extra proton is at the lattice point corresponding to a donor atom.

    The charge of the protons in an acceptor point cancel out the charge of a conduction electron. The absence of proton charge at a donor point cancels out the charge of a valence hole.
  9. Dec 24, 2012 #8
    Thanks to all i got it now,its just that mere presence of an electron or a hole doesn't make it charged as in p type or in n type semiconductor ,electrons and protons are equal in number ,i was confused about the term impurity ,was not sure that they were atoms .......
  10. Dec 24, 2012 #9
    Careful, it is not electrically charged.

    P type means that the majority carriers are holes

    N type menas that the majority carriers are electrons.
  11. Dec 27, 2012 #10
    Suppose an impurity particle/atom with 5 valence electrons is induced into a bulk material whose atoms have only 4 valence electrons. The "excessive" electron of the impurity particle can be promoted to a higher energy state such that we can say the impurity particle/atom is "ionized". In this case, do we still call this particle atom?
  12. Dec 27, 2012 #11

    Philip Wood

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    Gold Member

    I'd prefer to call it an ion. Indeed, I call the nuclei and their bound electrons in an ordinary pure metal 'ions', because each atom has contributed at least one electron each to the 'sea' of 'free' electrons.
  13. Dec 27, 2012 #12
    You can call it an ion if you like, but that is only a transitory state.

    What do you call it before the electron is promoted or after it gets one back?

    And what about the normal covalently bonded lattice centres? What happens when one of their electrons is promoted. You have a 'regular?' lattice that is a mixture of ions and atoms.

    Once you embark on this model you soon need to consider delocalisation, with a regular lattice of nuclei.

    But for the purposes of answering the original question I think the least confusing is to observe that you start with neutral atoms (equal numbers of electrons and protons) and reconfigure to a giant 'molecule', still neutral because it still has an equal number of electrons and protons.
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