Question about the source of FET

Discussion in 'General Electronics Chat' started by Nano001, Jan 12, 2010.

  1. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Hi everyone. This is the first of my general electrical engineering questions that I have. I am a newbie and I would like to get to know the field better. I am stuyding FETs, and I was wondering if it is possible for the source (of an enhcncment mode nmos FET) to ever "run out" of electrons to be supplied to the drain. It would seem that there are a fixed number of impurities to contribute to the drain current. Am I understanding this correctly?
     
  2. Paulo540

    Member

    Nov 23, 2009
    188
    0
    Electrons are 'replaced' by the power source.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    They keep getting replenished by the current source. One comes in as one goes out.

    If the current source stops supplying electrons, it's like stopping the music during musical chairs - except everyone always has a chair.
     
  4. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Thank you. I am still confused however. The source is grounded, what current source are you talking about? The drain-source voltage?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes.

    Conventional flow is positive to negative. However, electron flow is from negative to positive. So, for an N-ch enhanced MOSFET, the electrons come in through the source, and flow out through the drain.
     
  6. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Ok thanks. So essentially the electrons flow from the positive end of the "VDS source" to ground then into the source of the MOSFET since the MOSFET source is grounded?

    Also, then it seems here that the drain current is with the flow of electrons. I have seen many places where the current is denoted as opposite to the flow of electrons. Where and when are these two conventions used?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You're getting electron flow and conventional flow confused. That is very easy to do, and will get you very confused. Thank Benjamin Franklin for that (of course, he had no way of knowing at the time...)
    Read through this, it will help to explain:
    http://web.engr.oregonstate.edu/~traylor/ece112/lectures/elect_flow_vs_conv_I.pdf

    You can use either, but you need to be consistent. Much of the time, I use conventional flow - but I'm old school like that. ;) However, you need to be prepared to switch to electron flow when the situation requires.
     
  8. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    So if I understand this correctly, electrons from the source flow to the positively charged VDS being applied to the drain when a Vg is applied, and hence the conventional current flows from drain to source. Electrons now need to be replaced in the MOSFET source, but if electron flow is from negative to positive, how can VDS supply electrons if the ground of VDS is AT the source?

    I see how electrons can get replaced at the drain, but not the source.
     
Loading...