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I am a student and for my next exam, as part of the tasks, I need to identify which terminals (pins) of P and N type mosfet are gate, source and drain. I have an example photo here, but if possible, I would appreciate a more general answer besides solution to this exercise. I've tried searching on Google, but I maybe didn't understand well enough.

Please disregard the pencil markings on the photo for G S D as they are probably wrong.

Thank you for your help.

enter image description here

mkdavor
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  • Do you mean the symbol terminals on the schematic or the terminals on an actual device? – Kevin White Jan 29 '22 at 18:40
  • @KevinWhite on the schematic. – mkdavor Jan 29 '22 at 18:44
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    Your pencil marks are correct, at least for the two transistors you've marked. The far-left one seems to have some erased marks that I can't make out, though. – Hearth Jan 29 '22 at 18:52
  • @Hearth how would it be for the far-left? And more importantly, why? Thanks – mkdavor Jan 29 '22 at 18:53
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    Well, how did you get the ones for the other two? Clearly you're doing something right, you just need the confidence to know it's right. – Hearth Jan 29 '22 at 18:54

2 Answers2

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In this kind of diagram, using discrete FETs, source and body (the middle connection, opposite the gate) are almost always connected together (which also implies a body diode.) This leaves gate, which is the 'capacitor', then drain which has no body connection. (Note: there is such a thing as a 4-terminal FET with separate body connection, and these can be symmetrical.)

For additional emphasis, sometimes the gate has the bend of the 'L' facing source, as is done here. This isn't always the case however, sometimes gate connection is shown connecting to the center of the gate capacitor.

Finally, one other detail: the source, body, and drain have separate 'plates' to the gate. This indicates an enhancement-mode FET. Depletion mode shows these 'plates' as one unbroken line.

More here: MOSFET symbol - what is the correct symbol

hacktastical
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The terminal connected with the middle line is always Source 'S' (which also helps determines its channel type. Inward arrow: N- channel, Outward arrow: P-Channel). While the remaining terminal is Drain 'D'. The individual terminal on opposite side is Gate 'G'.

AG47
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  • What do you mean by "middle line"? You should note that the source and body of a MOSFET are not always connected together, so looking for such a connection is not a reliable way to identify the source. – Elliot Alderson Jan 29 '22 at 19:17
  • Please see MOSFET symbol for reference. @mkdavor needs a general answer for identifying terminals. – AG47 Jan 29 '22 at 19:20
  • If the OP needs a general reference then your answer does not provide it. You talk about just one way of indicating the terminals and completely ignore the international standards for drawing MOSFETs. – Elliot Alderson Jan 29 '22 at 19:23
  • Let Op decide his/her understanding with the answer with respect to the schematic shown.You can write (or I would rather say copy) your own detailed answer for Op. – AG47 Jan 29 '22 at 19:26
  • There's no need for me to write an answer. This question has been answered over and over again on this site, as I suggested in my close-as-duplicate vote. My comments were for the benefit of other future readers of your answer, not just the OP. – Elliot Alderson Jan 29 '22 at 19:30
  • Better benefit the future readers by posting your "international standard" answer here rather than extending this thread. – AG47 Jan 29 '22 at 19:34
  • I provided a link to it in the comments to the question, as did hacktastical. – Elliot Alderson Jan 29 '22 at 19:41
  • Thats great. Well done – AG47 Jan 29 '22 at 19:44