Can D and S be swapped? (Assuming the body is not internally connected to S.) The structure of a MOSFET is totally symmetric. Even the LTspice simulation shows that it can be swapped.
Does this behavior also exist in reality?
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2\$\begingroup\$ Assuming body is not connected to S - but it is. That's why MOSFETs always conduct in one direction even if you turn them off, and this property is sometimes used in charging circuits. If you want to block the current both ways, you can use double MOSFETs back-to-back (D-S-S-D or S-D-D-S) with gates tied together. \$\endgroup\$– IlyaCommented Mar 2, 2022 at 11:47
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2\$\begingroup\$ @Ilya: No. the Body-source connection is a deliberate manufacturing decision, for discrete MOSFETs \$\endgroup\$– tobaltCommented Mar 2, 2022 at 11:48
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\$\begingroup\$ I didn't argue with that. Except that they ARE manufactured like that. So it IS connected. I think I saw some FETs with body separately maybe on mouser, but I'm not sure. \$\endgroup\$– IlyaCommented Mar 2, 2022 at 11:50
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2\$\begingroup\$ @Ilya what I was trying to suggest is, that this connection is not mandatory and in ICs this connection can be easily omitted. \$\endgroup\$– tobaltCommented Mar 2, 2022 at 11:54
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1\$\begingroup\$ @Ilya Well, no, it is not true that all MOSFETs are manufactured with the body connected to the source. It's just not true. \$\endgroup\$– Elliot AldersonCommented Mar 2, 2022 at 12:50
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1 Answer
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It depends on the type of MOSFET. Simple planar FETs are symmetrical if the gate is placed in the middle of the channel. The same applies to more modern logic MOSFET such as FinFETs and GAA-FETs.
For vertical power MOSFET, like Trench FETs the drain is constructionally different. The drain is separated via a longer "drift" region.
The important part in the figure is the difference between the drain (bottom) and source (top) regions. The different doping profiles (P and N region) are not decisive for this distinction.
However, it is easy to see why it is often chosen to connect the source to both the n+ terminals and the P- well. Not doing that would complicate the top layout.
For logic MOSFETs in ICs which don't need large area metal contacts, it is more feasible to establish thin vias to the drain and source region and leave the channel "unconnected". The body is then part of the substrate, which is conventionally tied to the most negative available potential (VSS) for n-MOSFETs.
