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A bit confused about BJT charge flow

  1. Sep 20, 2007 #1
    In this picture, as well as countless others:

    http://en.wikipedia.org/wiki/Image:NPN_BJT_Basic_Operation_(Active).svg

    we have electrons flowing from emitter to base, and 'holes' from base to emitter.

    Why the distinction?

    When an electron flows from emitter to base, is it not the same thing as a hole flowing from base to emitter? Holes are just a 'technique' to quantify electron flow, correct?

    Also, my book states this verbatim:

    "If the concentration of electrons in the n-type emitter is much larger than the concentration of holes in the p-type base, then the number of electrons injected into the base will be much larger than the number of holes injected into the emitter."

    Not understanding how they are distinct components.

    Thanks for any help guys.
     
  2. jcsd
  3. Sep 20, 2007 #2

    mgb_phys

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    Correct

    It's just another way of saying that the base will become negatively charged
     
  4. Sep 20, 2007 #3

    Gokul43201

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    Because in an n-type material, the mobile electrons live in the conduction band, while in a p-type material, the mobile electrons live in (near) the valence band.

    Now, if I take a material with N (~1023) atoms/cc and dope it with n (~1015 to 1019) n-type dopants/cc, then I get no more than n mobile electrons/cc, but if I dope it with p (~1015 to 1019) p-type dopants/cc, then I can have as many as N-p mobile electrons/cc (alternatively, p mobile holes/cc). Rather than talk about nearly all of electrons moving, it is simply easier to talk about the vacancies (holes) moving in the opposite direction.
     
    Last edited: Sep 20, 2007
  5. Sep 20, 2007 #4

    rbj

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    also, in the p-type material, when electrons move (say from right to left) into the neighboring vacancy, the "hole", besides that it can be visualized as the hole moving from left to right, this motion of electrons behaves like negative charges (as electrons are) but with negative effective masses. particles of negative effective mass will move in the opposite direction than they would if positive mass and the same force. but the force is also reversed if the effective charge is reversed in polarity.

    so negative charges of negative mass behave, in a given electric field, the same positive charges of positive mass. so the holes can be treated as positive massed particles of positive charge and we can do hole-electron ballistics more easily with that model.

    then, at the junction or just past it, the holes that move in the p-type material (from right to left) combine with electrons that are moving from left to right in the n-type material and, essentially anihilate each other. (at the junction, the free electon in the n-type simply plug the holes in the p-type.)
     
  6. Sep 26, 2007 #5
    Thanks for the very prompt response guys, but I hate to say that I'm still a bit confused. I guess now it's more with the semiconductor physics, but the BJT is a great example for my confusion.

    I understand that, but I don't understand how that is cause for a distinction in the model.

    I must have some flaw in my thinking, so please pick it out:

    The n-type emitter clearly has most of its electrons in the conduction band. Now the electron injection into the base is straightforward--it just moves into the conduction band of the base and then quickly falls into the valence band because of it being p-type and its Fermi Level.

    In essence now a hole has been moved into the n-type.

    I don't understand how any other route could exist for charge movement, be it hole or electron, so I don't understand how holes could move from base to emitter. I also 'believe' that electrons in the emitter valence band cannot transfer to the base valence band, and holes in the valence band can't do anything. I basically view the valence band as the energy band of the 'valence shells' of the atoms, and thus they are bound to them.

    Sorry if I'm being thick here, but I must be making some mistake.

    The emitter will then have an equal positive charge right?

    What I don't understand though is why the say that the number of carriers are different. In my mind the number should be the SAME, since one stems from the other. Maybe the book meant to say the concentration, or percentage?

    Thanks for all help in advance guys. It's great to have friendly physics geniuses around.

    My professors for the most part shrug off all of these questions...this is where I turn to!
     
  7. Oct 4, 2007 #6
    noesis,
    consider an npn transistor,working in active mode,i.e,emitter junction forward biased and collector junction reverse biased.

    the conduction electrons in n diffuse to base.these electrons belong to the dopant atoms.
    (at the same time,very few holes from base ,diffuse to emitter).but we choose to ignore the hole current because it is small in magnitude.

    a small percent of electrons entering base recombine with the holes in base.the rest can move over to the collector.

    but the base lost some holes,which must be replenished.hence a covalent bond in base is broken,creating an electron-hole pair.the electron goes out of the base lead and that hole remains in the base.

    and the emitter will not have equal number of positive charge, because when CONDUCTION electrons leave the emitter,an equal number of electrons enter the emitter through the lead connected through it...

    i hope that helped...
     
  8. Oct 7, 2007 #7
    Charge flow mechanism in semi conductors

    I understand that noesis is primarily confused abt the distinction between charge transfers through holes and electrons.
    Unlike metallic conductors, charge transfer in semi-conductors doesn't take place exclusively via "conduction band electrons". Electrons, which don't have sufficient energy to come out of the nuclear influence and hence remaining in valence band, can also participate in the process of charge transfer by moving into the "holes" - (vacant spaces caused by displacement of electrons from valence band to conduction band)- of neighbouring atoms.
    Thus a hole which existed in a neighbouring atom, has now in effect moved into the atom from which a valence electron jumped. Thus valence band electrons also contribute to charge transfer in semi conductors.To have a convenient distinction between charge transfer in conduction band and charge transfer in valence band people say that electrons move in conduction band while holes move in valence band.
    Hope this helps... don't hesitate in asking for further clarification..
     
    Last edited: Oct 7, 2007
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