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My question is related to this post (the comments below).

Basically, how can I identity the source and drain of a transistor ? I thought that drain and source were totally symmetric inside of NMOS or PMOS from the P/N structure behind as also explained here.

But apparently, in the comment below this post, a PMOS source should be the highest voltage of a circuit and the NMOS source is the ground.

I would like to understand why. Also I am not sure to understand this explanation because what happens if actually a NMOS transistor is not plugged to the ground on any of its port (but at some "intermediate" voltage).

StarBucK
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    3-terminal PMOS and NMOS FETs (i.e. virtually all of them) have a parasitic diode between source and drain, called the "body diode". One way round, this conducts... –  Jan 09 '21 at 16:23
  • There is absolutely no need to start a new question when your (now) current question is still dealing with this subject. – Andy aka Jan 09 '21 at 16:28
  • @Andyaka this question is related but different. In the other post I am asking how to understand the inverter. But the real physics is about a transistor alone. For me the question are different. The other topic is juste more confusing than this focused one – StarBucK Jan 09 '21 at 16:29
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    *I would like to understand why* Then start reading about MOSFETs, there are plenty of books on the subject. Do realize that this is getting close to "hardcore" electronics, I mean, you don't need to understand all that to be able to **use** a MOSFET properly. If you just follow what is done in most schematics, you would be fine without knowing **why**. – Bimpelrekkie Jan 09 '21 at 16:44
  • Some links: https://components101.com/articles/mosfet-symbol-working-operation-types-and-applications#:~:text=In%20general%2C%20the%20MOSFET%20works,the%20source%20and%20drain%20terminal. These are more "hardcore": https://www.infineon.com/dgdl/mosfet.pdf?fileId=5546d462533600a4015357444e913f4f and http://homepages.rpi.edu/~sawyes/Models_review.pdf – Bimpelrekkie Jan 09 '21 at 16:44

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