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Bipolar Junction Transistor - Saturation

  1. Jan 16, 2010 #1
    Hello All,

    I have very basic questions with respect to what we mean when we say a Transistor is in "Saturation". As I have read almost everywhere, we need to Forward bias both the emitter-base junction and Collector-Base Junctions for transistor to go to Saturation.

    Before I post my questions, I would like to put a small paragraph so that I can reference it in my questions.

    Let us consider an npn transistor in the common emitter configuration. We know that the emitter is very heavily doped, the base is very thin and lightly doped and the collector is moderately doped. Now, to forward bias the emitter-base junction, we connect the negative terminal of the battery to the emitter(which is n-type), and positive terminal to the base. When we say forward bias, the electrons from the battery travel through the wire and in turn provide energy to the electrons in the emitter which flow further causing a base current. An in increase in the bias voltage is like increasing the energy supplied to the emitter causing more electrons from the emitter to be pushed into the base and a higher base current. Now, let us bring in the collector base junction. In a common emitter mode, we would connect the battery between the collector and the emitter. Given the fact that we should forward bias the collector base junction for "saturation", we should ensure that the collector is "less positive" compared to the base. If this is maintained, the collector-base junction is very similar to the emitter-base junction and the movement of electrons should be similar to what happens in the emitter-base junction. That is, electrons should be pushed towards the base side resulting in the flow of an "outward CONVENTIONAL current" from the collector. However, we always see that the direction of "CONVENTIONALcollector current" is always "inward".

    Here are my questions:
    1) Could you please provide a basic understanding of the directions involved in transistor operation?
    2) What exactly is happening "physically" in the transistor in the "saturation region" of the transistor output characterestics?
    3) How does an increase in base current bring about an increase in the collector current?
    4) What is exactly the physical quantity that is "saturated" and why is it called "saturated"?

    Note to the Admin: I am sorry for having posted this thread at two places. Since I am a newbie, I was not aware of how to post new threads. Henceforth, I shall take care of posting threads correctly. Thanks
  2. jcsd
  3. Jan 16, 2010 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF. I think you will find a very helpful place.

    On your questions, it's called saturation because the collector voltage cannot go any lower. You've done all you can do with your base current (in Common Emitter configuration) to lower the collector voltage and support the collector current.

    Here is a previous EE thread about saturation. There are several others that you can find with the search feature at the top of the page:


  4. Nov 29, 2010 #3
  5. Nov 29, 2010 #4
  6. Nov 30, 2010 #5
    Hello berkeman,chr0mium and cabraham,

    Thank You very much for the replies and directions for the explanations. It has been very helpful to understand the fundamental concepts...
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