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Transistors & Conventional Current

  1. Feb 19, 2014 #1
    Hi, please could someone help me with NPN transistors. I found this veritasium video on YouTube () and do not think it is correct. He shows a diagram towards the end of the video with a source and drain which I looked up and source is collector and drain is emitter. He clearly shows electrons flowing from source to drain (collector to emitter) but I don't think that's right. In a conventional circuit current does indeed flow from collector to emitter for an NPN transistor but this assumes conventional current flow (i.e. positive charges flowing). Therefore, surely you must state that holes flow from source to drain to be correct here? If your talking about electrons I would have thought you would have to talk about non-conventional current flow i.e. electrons go from negative terminal across the transistor from emitter to collector to the positive terminal? Or to put in better words - electrons move from emitter to collector or holes move from collector to emitter. Or have I got it all wrong?
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Feb 19, 2014 #2


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    The smart guy should have mentioned the type of transistor he was explaining.
    The video diagram is for a certain type of FET transistor.

    One basic type of transistor is the BJT ( bipolar junction transistor ) which is formed from the npn and pnp arrangement that you mention with the collector, base and emitter.

    A second is the FET ( field effect transistor ), which has the source, drain and gate. ( JFET and MOSFET).

    A BJT and an FET are not compatable with each other and work by different principles.
    A BJT is a current device and has holes/electrons moving within it. Simply put regulation is done by the amount of current flowing in/out of the base.
    An FET is a voltage device and is a single carrier - either holes or electrons, but not both, Regulation is by a voltage applied to the gate.


  4. Feb 19, 2014 #3
    Thanks for your answer 256bits! I think that makes sense now. I have attached a diagram from my textbook of a relay circuit which uses an NPN transistor and would be very grateful if you could clarify a few things. So, with this transistor I assume it would be a BJT, is that right? When the base has a positive voltage it says that the current flows from +5V through the relay and then through the transistor from collector to the emitter. Inside the transistor would you therefore have to say that 'holes' are flowing from collector to base? Similarly if you were talking about electrons would you have to say that when a positive voltage is applied to the base, electrons flow from ground (0V) through the transistor (this time from emitter to collector) then through the relay to +5V. Is that correct?

    Attached Files:

  5. Feb 19, 2014 #4


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    Your circuit uses the symbol for an NPN BJT transistor, so there's no reason to think it is something different.

    If you want to understand what the transistor does as part of the complete circuit, don't tie yourself in knots thinking about electrons and holes. Just think about conventional currents, that always flow from positive to negative voltage.

    If you want to understand the physics of how the transistor actually works, then of course you do need to think about electrons and holes. But the best way to deal with complicated situations is break them down into simple pieces. The "physics" of how the transistor works is one thing. The "electrical engineering" of how to use it in a circuit is another thing. You don't often need to think about both things at once.
  6. Feb 19, 2014 #5
    Thanks AlephZero. So what would be the correct way of interpreting the BJT NPN transistor in the diagram in terms of electrons/holes? If we stick with conventional flow like you said which means the collector is positive and the emitter is negative. Would you therefore have to talk about holes moving within the transistor from collector to emitter? Or does it now start to get too complicated.
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