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Homework Help: Current flow in NPN transistor

  1. Jun 10, 2013 #1
    Here is an image show how currents flow in NPN transistor from Microelectronic Circuits by Sedra Smith:
    Can you explain more about injected holes (iB1)? I can't imagine how holes can flow into emitter (N type)
    Is it possible to for holes to flow from base to emitter?

    Attached Files:

  2. jcsd
  3. Jun 10, 2013 #2
    I think my main question is how holes diffuse from P type to N type semiconductor.
    Can anyone help me explain it (how holes diffuse) in terms of electrons?
    I can't imagine how holes can exist in N type by diffusion.
  4. Jun 11, 2013 #3


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    It's been a few years so I'm a bit rusty but..

    The base is +ve with respect to the emitter so you would expect holes to flow from base to emitter and electrons the other way. The number of holes injected into the emitter is a lot less than the number of electrons injected the other way due to the doping.

    Due to the number of free electrons in the n-type emitter a hole injected there is not likely to go far before it meets an electron and is destroyed.

    Since the base is short most of the electrons make it from the emitter to the collector and since only a few holes make it from base to emitter Ic>>Ib and the transitor has current gain.
  5. Jun 11, 2013 #4
    I am confused about this:
    When we say that holes are injected from base to emitter it means that electrons in valence band (not electrons in conduction band) move from emitter to base.
    Is this right?

    And this is another question:
    When an electron in conduction band from emitter falls into hole in base, it become electron in valence band. I wonder where the electron will go.
    - the electron now is in valence band and it moves in valence band to positive lead of battery VBE
    - the the electron now is in valence band and it moves in valence band to depletion region in CB junction and here it gets enery from VCB exits valence band and go into depletion region.

    The first possibility is what I understand from the book but I wonder if the second is possible.
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