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I know this has been asked a few times already, but if we disregard the body diode, or say we tie the body to a low enough voltage that it wouldn't forward bias those diodes, then are modern FETs actually symmetric?

I ask this because the FETs I've used will often have, out of 6 pins, 4 will be dedicated to the drain, 1 to the source, and 1 to the gate. Even if we could control the body and bias it to a voltage of our own choosing, it seems like the FET has an asymmetry built in. I've primarily seen this on higher power FETs.

EDIT: My question is mainly about integrated output power FETs or discrete component FETs.

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  • \$\begingroup\$ are you talking about 3rd quadrant operation? \$\endgroup\$ – JonRB Sep 21 '18 at 18:19
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"Modern FETs" is a pretty broad topic, but if you are talking about transistors built with a planar process, such as that used in digital integrated circuits, then they are symmetric. If you use a transistor as a transmission gate then the designations of source and drain can actually change as the input and output voltages change.

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It is true that a planar design is generally symmetric, but many MOSFETs have a power FET structure as described here. This results in a lower channel resistance and improve dissipation, but it cannot be made symmetrical due to the body diode being an inherent part of the structure. It also clearly favors gate-source voltage being significantly more significant than gate-drain, as opposed to a planar design.

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