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What is source of base current in transistor?

  1. Jun 23, 2015 #1

    goodphy

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    Gold Member

    Hello.

    Let me think about the NPN transistor (Collector-Base-Emitter order). Here base-emitter voltage is forward-biased (The base is higher than the emitter in voltage.) thus current flow of majority carriers (Here they are electrons in N and holes in P) occurs from emitter to base.

    In next, the collector and base is now reverse-biased. (Collector is higher than base in voltage.) This reverse bias basically prevents majority carrier flow in NP junction but allows only minority carrier flow. But here in base there is presence of large number of electrons which actually came from the emitter by forward-bias between PN junction and electron in base (P-type extrinsic semiconductor) is minority! Thus they can flow to collector. In conclusion, current flow from emitter and collector ICE is made.

    Here is my question: in this logic, where is base current? what is the source of base current? I've suspected it is minority current in reverse-biased collector-base but they can be going to emitter since they see forward-bias between base to emitter!

    Could you please clarity this?
     
    Last edited: Jun 23, 2015
  2. jcsd
  3. Jun 23, 2015 #2

    NascentOxygen

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    Staff: Mentor

    The base is only lightly doped, so that a majority of the electrons that come from the emitter will be swept through into the collector to give the device a high current gain. But it is inevitable that some of the electrons will combine with holes in the base P material, and it's this flow of holes to the active base region that accounts for base current.

    Read more: http://www.radartutorial.eu/21.semiconductors/hl19.en.html
     
  4. Jun 23, 2015 #3

    goodphy

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    Thanks. Thus..some of electrons are combining holes in base thus concentration of holes in that area becomes lower, leading diffusion of holes from base conductor to base to re-fill the holes. Is it right summary?
     
  5. Jun 23, 2015 #4
    Actually, the base current due to recombination is only a small fraction of the total base current. When an external source forward biases the b-e junction, electrons from the emitter, and holes from the base, are acted upon by the E field so that base holes move towards emitter and emitter electrons move towards the base. Because the base is very lightly doped in comparison to the emitter, the density of holes towards emitter is much lighter than that of emitter to base.

    The component of base current due to holes moving from base to emitter is called the "injection component" of base current. When a few electrons from the emitter fail to reach the collector and recombine in the base, electrons eventually exit the base lead to preserve charge neutrality. This tiny part of the base current is called the "transport component" of base current.

    Also, there is yet another component of base current called "displacement" or "charging" component. The b-e junction has a diffusion capacitance. As frequency increases, more ac current is needed to charge and discharge this capacitance. At a high enough frequency, it takes as much base current to charge/discharge Cdiffusion as there is collector current. In other words, for 1.0 mA of Ic, a full 1.0 mA of Ib is needed. At this frequency, the current gain beta equals 1.0. This is called the transition frequency "ft".

    Aren't you glad you asked?

    Claude
     
  6. Jun 23, 2015 #5

    goodphy

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    Yes, very detailed. Thanks for giving such a explanation which I ever didn't find in google.
     
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