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Transistor Action

  1. Mar 2, 2009 #1
    "Transistor Action"

    Dear Forum,

    I have a question about what is often referred to as "transistor action".

    I understand how a transistor operates within a circuit, and the characteristics of such a circuit, but I would like to learn physically how the bipolar transistor is able to control a collector current through base current or base voltage?

    Lets assume the transistor is properly biased and the base emitter junction is passing current i.e. Vbe>0.6V. When a base voltage is applied and hence a base current is flowing there then appears a collector current of hfe*Ib. OK. This collector current is supplied by the source (normally written as Vcc).

    Can you please answer the following questions.

    1.) Does a change in base voltage (or current) cause the collector voltage (voltage at the bottom of a collector resistor) to change, thereby causing a change in current where the current is given by (Vcc-Vc)/Rc.?
    2.) If this is the case, how does the change in Vb get "translated" into a change in Vc?
    3.) Does a change in base current (or voltage) cause a change collector current to be drawn which then passes through the resistor and drops a voltage thereby changing the collector voltage i.e. Vc=Vcc-IcR?
    4.) I guess this question is does the collector voltage change first, leading to a change in collector current, OR does the collector current change first leading to a change in collector voltage?

    5.) Now for the interesting question. Whether 1 or 3 is correct (or both, or neither) how does the change in Vb (or Ib) pass through the transistor and lead to a change in Vc (or Ic). What physical processes are happening in the transistor materials. Mathematical or qualitative explanations welcomed.

    Thank you very much.

    Kind Regards

  2. jcsd
  3. Mar 6, 2009 #2
    Re: "Transistor Action"

    Dear Forum, It has been several days with no reply. Can you tell me if this is a difficult question? Is it a poorly understood area of physics? Or is it in the wrong area of the forum?


  4. Mar 7, 2009 #3


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    Science Advisor
    Gold Member

    Re: "Transistor Action"

    The web is full of sites that explain transistor operation. Start with wikipedia, then search on semiconductor and bipolar transistor operation, transistor tutorials, etc. Here are a few representative sites:

    http://www.spectrum.ieee.org/may04/3992/s101" [Broken]

    http://www.uoguelph.ca/~antoon/tutorial/xtor/xtor.html" [Broken]

    http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html" [Broken]

    You'll see that transistors are current-driven devices rather than voltage-driven. The bias voltage permits current to flow across the B-E junction, and charge carriers diffuse out of the base towards the collector. If you still have questions, please come back and post them.
    Last edited by a moderator: May 4, 2017
  5. Mar 7, 2009 #4
    Re: "Transistor Action"


    The first link there was very helpful.


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