Beginner's question about transistors

Discussion in 'General Electronics Chat' started by gabrielw6, Jan 27, 2014.

  1. gabrielw6

    Thread Starter New Member

    Sep 21, 2013
    Hey people,
    I'm learning about FETs, and after remembering the classical image of a Bipolar transistor and of the FET, (image on I really like this site) I came up with a general question: in the bipolar, if both collector and emitter are with the same impurities, in the images symmetrical, why, or how, we differentiate the collector from the emitter? The same question on FETs, related to the drain and source. What am I misunderstanding? What is wrong or hidden?

    Thanks in advance.
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    You are first assuming symmetrical layouts (as you noted), which, when implemented, are not symmetrical.

    Were they symmetrical, you then have to worry about doping levels...e.g. the emitter is more heavily doped than the collector...

    From this page.



    Were they of the same doping levels and symmetrical, the drain and source could be swapped without regard, as is possible with some JFETs.
    Last edited: Jan 27, 2014
  3. crutschow


    Mar 14, 2008
    But note that many FETs have the substrate connected to the source so you can't, in general, interchange source and drain.
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    Yes, that is true.

    I meant this for discrete JFETs, but in updating my post with images, I messed up the arrangement and that caveat.

    Were everything else the same, most MOSFETs still cannot swap drain and source, per your reasoning.
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    Further to what has alredy been said, the collector (never the emitter I think) is often connected to the metal case or plate in power devices for heat dissipation purposes (it is assumed that power devices will be used in a configuration where the dissipated power is developed in the collector).
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
    newer designed rf power transistors have the emitter connected to the case for ease of heat sinking without having to insulate the case.
    back in the old days, some fet's were interchangeable between source and drain.I dont think they are anymore. has to do with the internal topography.
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    As far as good ole bipolar transistor go, yes, Virginia, you can swap collector and emitter and still get "transistor" action, albeit the current gain may be unity (or less).

    If this was impossible the entire TTL family of devices simply would not work.
  8. vk6zgo

    Active Member

    Jul 21, 2012
    Back in the day,Philips & Mullard made some special true "Bipolar" transistors which were completely reversible.
    I have no idea why!:D