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I have learn that the FET looks like this:

enter image description here

However industrial FETs are not built like that.The source and the substrate have common potential.However if the source and substrate have common potential that means the conductive channel formed is not wide enough to connect the source with the drain there is a gap between the n type channel formed and the source.So how does it really work?

Clone Wars
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  • Can you tell us **why** you think the channel won't reach the drain? Did you read this somewhere, and if so please give us a link. – Elliot Alderson Jan 14 '21 at 13:41
  • *However industrial FETs are not built like that.* The MOSFETs in the ICs I design **for Industrial purposes** are exactly constructed like this. There Source and Substrate do not have to be connected. Maybe you actually want to ask about **Power MOSFETs** which have a different construction, see: https://ecee.colorado.edu/~bart/book/book/chapter7/ch7_8.htm – Bimpelrekkie Jan 14 '21 at 13:46
  • I don't just imagine the black line named source has to touch the p type substrate there fore the yellow lines must be a little bit more right and therefore , the n type conducting channel appears just below the yellow line so the source won't 'touch' the conducting channel – Clone Wars Jan 14 '21 at 13:46
  • Bimpelrekkie I have bought FETs myself and it had 3 pins not 4. – Clone Wars Jan 14 '21 at 13:47
  • *However if the source and substrate have common potential that means the conductive channel formed is not wide enough to connect the source with the drain* Source and Substrate being connected **has nothing to do with the size of the channel**. The channel is formed when enough positive voltage develops between Source and Gate: \$V_{GS} > V_{Threshold}\$. May I suggest that you read a book or tutorial on how MOSFETs work because I sense that you lack some knowledge there. – Bimpelrekkie Jan 14 '21 at 13:48
  • Yes OK but the channel is formed just below the gate correct?It has the same width with the gate. – Clone Wars Jan 14 '21 at 13:49
  • Does this answer your question? https://electronics.stackexchange.com/a/72603/17608 – Phil Frost Jan 14 '21 at 13:49
  • @PhilFrost not really. – Clone Wars Jan 14 '21 at 13:54

1 Answers1

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The source and the substrate have common potential. However if the source and substrate have common potential that means the conductive channel formed is not wide enough to connect the source with the drain there is a gap between the n type channel formed and the source. So how does it really work?

It works precisely how you say it does not. The conductive channel formed is wide enough to connect the source with the drain. The channel might be a little asymmetrical, like this:

enter image description here
(image from electrical4u.com)

Provided the gate voltage is high enough there is no gap and the conductive channel is, well, conductive.

Phil Frost
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