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I noticed a question regarding mosfet polarity asked here and wondered whether a four terminal mosfet would behave the same way under reversed polarity. Since the four terminal mosfet has no internal diode (I think), I do not see how there could be a distinction between source and drain. Is this true?

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It would be possible to construct a four-terminal MOSFET that was symmetrical, with source and drain indeed being interchangeable. On the other hand, even with a four-terminal device, a non-symmetrical shape may allow performance in one direction to be improved beyond what would be possible with a symmetrical device. A MOSFET integrated onto a chip will often have a gate which is confined to a single plane above the active channel, but power MOSFETs have much more complicated three-dimensional structures which generally favor non-symmetrical designs.

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  • \$\begingroup\$ So with integrated mosfets, this may be the case, but in power mosfets, this is most likely not the case. Is there any case type of mosfets are actually symmetrical or will I just have to make due with jfets? \$\endgroup\$ – Alex Eftimiades Apr 14 '12 at 1:15
  • \$\begingroup\$ Do not "make do" with JFETS. The world largely uses MOSFETs quite happily. Learn how to use their strengths and design around their limitations - as we need to do with ANYTHING we do. That's a major part of what engineering is about. If you really must have a symmetric device connect two MOSFETS in series. Join source to source, gate to gate. Inputs/outputs are the two drains. For N channel apply +ve to gates relative to source to turn on. Magic. Works well. The gate-source "junction" is floating but it just takes engineering to deal with that. I have used this arrangement with great success. \$\endgroup\$ – Russell McMahon Apr 14 '12 at 3:16

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