Help understanding current flow of transistor in saturation mode

Discussion in 'Homework Help' started by anhnha, Aug 26, 2012.

  1. anhnha

    Thread Starter Active Member

    Apr 19, 2012
    773
    45
    Would anyone please help me this problem?
    When NPN bipolar transistors operate in saturation mode, both junctions forward-biased with Ve < Vb > Vc. With Vbe >0 then electrons will flow from emiter to base but I am confused that how electrons can flow from base to collecror when Vbc > 0, it mean that electrons flow in the direction of electric field in base-collector junction?
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The base-collector of a transistor is a diode and current flows if the forward voltage is met or exceeded.
     
  3. anhnha

    Thread Starter Active Member

    Apr 19, 2012
    773
    45
    When transistor operate in saturation, bot junctions forward-biased. With Vbe >0 then electrons will flow from emiter to base but how can these electrons go to collector to create Ic current when Vbc> 0?
    Please see picture attached.
     
  4. bretm

    Member

    Feb 6, 2012
    152
    24
    If a transistor were just two diodes then you're right - current wouldn't flow. But it's not. A more reasonable model includes dependent current sources in parallel with each diode to represent the emitter-collector action. This is the basis of the Ebers Moll model.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Of course current flows in both diodes (junctions) because they are both forward biased when the transistor is saturated.
    In an NPN transistor the base is P which is the anode of both diodes.
    The collector and the emitter are N which is the cathode of each diode.
    So when the base is +0.75V, the emitter is 0V and the collector is about +0.05V then the base-emitter conducts and the base-collector also conducts.
     
  6. bretm

    Member

    Feb 6, 2012
    152
    24
    Sorry, i misread his post. I thought he was asking about collector emitter current.
     
  7. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Shouldn't your first question be asking about the nonsaturated region where the base-collector junction is reverse biased. How is it that currect flows when the diode is reverse biased?

    When you can answer that, you will be able to answer your original question.
     
  8. anhnha

    Thread Starter Active Member

    Apr 19, 2012
    773
    45
    Sorry but I still cann't understand this.
    This is my understanding about the operation of transistor in saturation mode.
    Please correct me when I am wrong.
    When transistors operate in saturation mode, both junctions forward-biased. It mean that with NPN transistor Vbe> 0 and Vbc > 0. Because Vbe > 0, electrons which are emitted from emiter will flow in opposite direction
    of electric field created by Vbe and finally go to base. In order to create Ic these electron that have just gone to base have to flow to collector. But how these electrons can flow when the electric field created by Vbc not support for them? These electrons are pushed back and cann't reach to collector, therefore Ic = 0. But in fact, when transistor in saturation Ic ≈ Ie and Vce ≈ 0.2 V.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Forget about electrons and which way they flow.
    When the base-emitter is forward-biased then base-emitter current flows. This causes collector-emitter current to flow according to the amount of current gain that the transistor has.
     
Loading...