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pnp/npn transistors

Discussion in 'Electronic Repair' started by wonderer, May 15, 2006.

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  1. wonderer

    wonderer Guest

    A high school teacher tells the class that pnp stands for positive,
    negative, positive and that npn stands for negative, positive,negative.

    Is he right?
     
  2. Guest

    Yes, he is. The letters mean the order of the types of semiconductor
    materials in the structure of the component. N-type (negative)
    semiconductor has extra electrons which it wants to get rid of, P-type
    (positive) lacks some electrons and it wants to get ones.
     
  3. Pooh Bear

    Pooh Bear Guest

    Please explain how any material has extra ( or lacks ) any electrons !

    Graham
     
  4. Pooh Bear

    Pooh Bear Guest

    N actually stands for 'n material', p for 'p material'. The silicon ( typically
    ) is 'doped' with other elements to make the n and p material which have
    different electrical properties. The difference is that in one conduction
    through the material takes place using electrons ( n-type ) and in the other by
    so-called 'holes' ( p-type ).

    http://en.wikipedia.org/wiki/Semiconductors#N-type_doping

    Graham
     
  5. mc

    mc Guest

    Please explain how any material has extra ( or lacks ) any electrons !

    Well, for starters, it is *not* electrically charged. The electrons *do*
    match the protons, making the whole thing neutral, just as with most of the
    other matter in the world.

    What differs is whether there's *room* for the electrons in the crystal
    structure.

    Around every silicon atom, there's room for 4 electrons in the crystal
    structure of silicon. These are the 4 valence electrons (outermost shell
    electrons) of silicon.

    Put in an atom with only 3 electrons in its outer shell, and there's *still*
    room for 4. There is no net electrical charge, but there is a "hole" which
    an electron could move into. (Put in a lot of these and you get P-type
    silicon.)

    Put in an atom with 5 electrons in its outer shell, and there's an electron
    left over. In terms of electrical charge, it needs to stay there (to
    balance the charge of the protons in the nucleus), but the crystal structure
    doesn't accommodate it. It hangs around without fitting tightly into a
    specific position. That is, it becomes a conduction electron. (Put in a
    lot of these and you get N-type silicon.)

    Conduction electrons make it easy for matter to conduct electricity because
    they're free to move. Holes also make it easy for matter to conduct
    electricity because a hole can move; that is, an electron from the next atom
    can fall into it, leaving a hole at the next atom instead of where it
    started out.

    Now then. In a diode, you apply a voltage to pull the holes (in the P-type
    material) toward the conduction electrons (in the N-type material). In the
    middle of the diode, they meet and join, and conduction occurs. If you
    apply the voltage the opposite way,
     
  6. mc

    mc Guest

    Reposting in order to finish the message...

    you apply a voltage to pull the holes (in the P-type
    material) toward the conduction electrons (in the N-type material). In the
    middle of the diode, they meet and join, and conduction occurs. If you
    apply the voltage the opposite way, the holes and conduction electrons
    never meet, and no current flows.

    Transistors are more complicated. One key concept is that if you can
    somehow bring a lot of electrons into a piece of P-type material, it starts
    to act like N-type. That is, it has conduction electrons because you
    brought them in by conduction, not because they came in with the atoms.

    Well... Consider a three-layer stack, N, P, N. Use the first two (N, P) as
    a diode and make it conduct. That brings plenty of electrons into the P
    layer.

    While this is going on, also apply a positive voltage to the third electrode
    (N). That ought not to make it conduct, because the second P N junction
    would have voltage on it in the reverse direction. But what actually
    happens is that when you make the first (N P) junction conduct, it's like
    opening a floodgate. Once those electrons get into the central P region,
    they go right on through. (The second P N junction doesn't stop them
    because the P layer is full of electrons and no longer acting like P
    material, no longer relying on holes to do its conducting.) You get many
    more electrons that you ask for; that is, the small current through the
    first junction is amplified. That's how an NPN transistor works.

    I predict that someone will jump on me and say, "No, it's not that way at
    all, it can only be described mathematically." To which I reply: "Just
    because *you* can only quote formulas doesn't mean the physical phenomenon
    isn't real."
     
  7. Guest

    Pooh Bear kirjoitti:
    I should use "quotation" marks :) or something like that. It's true
    that in this case the material hasn't extra electrons to get rid of (or
    lack, for getting them), but there are "extra" electrons to transport
    the charge in the material, and in the p-type, there is "lack of them",
    meaning those "holes" to transport charge (electron gets to a hole).

    But, there is actually such thing as material having extra, or lack of,
    electrons (meaning that the atom is out of octet, and it "wants" to get
    electrons, or get rid of them, to get the octet), but then it would be
    a case of chemistry (off-topic).
    Yes, but the P and N, in the material types, stand for the positive and
    negative
     
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