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FET: drain&source can be swapped?

Discussion in 'Electronic Components' started by Tony, Oct 18, 2003.

  1. Tony

    Tony Guest

    Hello,
    in a bipolar transistor collector and emitter aren't freely
    interchangable, because they have a polarity.
    But, can I freely swap drain and source of a FET? Or also
    FETs are polarized (drain-source wise, not the obvious gate)?

    I ask this because as far as I know a FET behaves as a simple
    switch, where even alternate current can flow.

    Thanks!
    Tony
     
  2. Tony,

    You can't. The gate voltage has its polarity (normaly related to the source)
    and the FET would not be happy to feel it changed so radically. You may
    destroy it.

    pieter
     
  3. Actually, in general your statement is as wrong as it is right. Alot
    of small signal FETS are equivalent when switching the drain and source.
    Alot of special purpose or power FETS have structures that certainly would
    disable or distroy the component if the 'drain' and 'source' were
    interchanged. (Sometimes, it isn't the internal FET itself but it
    is the protection circuitry that keeps inverse use from working.)

    Not all fets work the same in 'inverse' mode, but again, most likely
    such fets that do work are likely to be small signal.

    John
     
  4. Mosfets only block voltage in one direction. They have an
    inherent body diode which allows current to flow in the opposite
    direction subject to the voltage drop characteristic of the body
    diode. Also, as indicated in another post, gate voltage is specified
    in relation to the source.
    When using mosfets to control alternating current, it is standard
    practice to use 2 of them in series (opposite polarities) so that
    current can be controlled in both directions.
     
  5. If the FET is made symmetrical, i.e. the drain and source are the same
    relative to the gate, then the performance with the source and drain
    swapped should be the same. Some data sheets say you can swap drain
    and source.

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  6. BTW, I should have said JFET. Like the other followups point out,
    some FETS will conduct like a forward biased diode if the polarity is
    reversed. JFETs are simple and can be symmetrical.

    Also, some power FETs are used in the third quadrant, for zero voltage
    drop rectifiers.


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  7. Dave Platt

    Dave Platt Guest

    But, can I freely swap drain and source of a FET? Or also
    Actually, if I recall correctly, there are some FETs (depletion-mode
    JFETs, and perhaps some small-signal MOSFETs) which have a fully
    symmetrical internal construction - the identification of "drain" and
    "source" on these is largely arbitrary. An N-channel FET of this
    type can be hooked up either way, as long as the gate is driven in the
    correct direction with respect to whichever pin is acting as the
    source.

    This is _not_ true of most power MOSFETs. Due to their construction,
    there is an "intrinsic" or substrate diode between source and drain.
    If you try to reverse the leads (or polarity) on these, the intrinsic
    diode will go into conduction.
     
  8. Hi
    Actually transistor are symmetrical as far as collector
    emitter polarity is concerned - it is just that they work very
    badly upside down. Having said that, years ago I seem to remember
    there was a transistor designed to be used either way round which
    found use in the odd bilateral circuit.

    As for the FET, I believe that the early JFETs, such as the
    MPF102, were quite happy standing on their heads.


    Cheers - Joe
     
  9. Well, your comment about a BJT working poorly when used in
    'inverse' mode is true for some applications. However, the
    saturation voltage (the effective Vce drop) is much lower
    in inverse mode, even though the Beta is effectively much
    lower. There are some useful circuits when using some BJTs
    in inverse mode.

    John
     
  10. Uwe Bonnes

    Uwe Bonnes Guest

    : Hello,
    : in a bipolar transistor collector and emitter aren't freely
    : interchangable, because they have a polarity.
    : But, can I freely swap drain and source of a FET? Or also
    : FETs are polarized (drain-source wise, not the obvious gate)?

    : I ask this because as far as I know a FET behaves as a simple
    : switch, where even alternate current can flow.

    Look at the datasheet, if you can do. E.g. the bipolar JFET BF862 from
    Philips explicitly states it

    Bye
     
  11. John,

    You're right. I made it too simple.

    pieter
     
  12. justa

    justa Guest

    I think it's a matter of capacitance...

    And no, they are not necessarily symmetrical.
    The layering/fabrication of the transistor's
    structure is optimized for the particular
    application. This is one reason why some
    trans' are spec'd for RF purposes. Attempts
    to reverse their polarity drastically effects
    their performance. Again...capacitance...
     
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