about the arrow direction of bulk of pmos and nmos

Hi All

I would like to confirm with you all if the arrow convetion of the bulk
for pmos and nmos is the flow direction of electron?
Kindly comment
Thank you so much


best regards
Jason

 

"The arrow points North"

 

: Hi All

: I would like to confirm with you all if the arrow convetion of the bulk
: for pmos and nmos is the flow direction of electron?
: Kindly comment
: Thank you so much

No.

In an NMOS, the arrow points toward the gate. In a PMOS, the
arrow points away from the gate.

This has nothing to do with electron flow. In addition there
generally isn't much current (or electrons, by your convention) flowing
through the bulk, at least not more than very locally, as the
bulk-to-source and bulk-to-drain junctions are generally reverse-biased.
There is not generally any gate-to-bulk (or vice-versa) electron flow,
either, except, perhaps in the weak-inversion region of operation. In
strong inversion, there is an inversion channel between the gate and the
bulk, so the current would flow from gate-to-channel instead of
gate-to-bulk.

That probably more-than-answers your question...

Joe

 

Hi Joe

Thank you so much for giving the opinion and sharing the wonderful
idea.
That was my initial thought. But I went thinking that there could be
more before one adopts any convention.

Nmos if has body effect, meaning the Vsource is higher than Vbulk,
therefore there could be current flowing to the bulk. Since the arrow
is pointing to the gate for Nmos, then it might be saying the electron
is flowing from bulk to source.

That was a guess with some logic isn't it?

Thank you Joe for sharing the knowledge.

best regards
Jason

 

The conventional power fet symbol is a mess. We draw fets that look
like regular NPN or PNP transistors, but we add the insulated gates.
It's easier to resolve and far more intuitive.



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John

 

: Hi Joe

: Thank you so much for giving the opinion and sharing the wonderful
: idea.
: That was my initial thought. But I went thinking that there could be
: more before one adopts any convention.

: Nmos if has body effect, meaning the Vsource is higher than Vbulk,
: therefore there could be current flowing to the bulk. Since the arrow
: is pointing to the gate for Nmos, then it might be saying the electron
: is flowing from bulk to source.

: That was a guess with some logic isn't it?

Hey Jason,

Actually, in an NMOS with body effect, there is still not really
any chance of current flowing from source to bulk. I had to draw this out
to re-confirm, but an NMOS has a bulk made out of p-type material, and
source/drain made out of n-type material. The source end of the
source-to-bulk junction (diode) is the CATHODE of the diode, while the
bulk end is the ANODE. If the source (cathode) of that diode is at
a higher voltage than the bulk (anode) the source-to-bulk junction is
reverse biased, and no significant current will from from source-to-bulk
or vice versa.

Forward-biasing either the source-to-bulk or drain-to-bulk
junctions is REALLY BAD, and can lead to latchup. Therefore, that is why
the bulk voltage is picked to be the smallest possible voltage (for the
bulk of NMOS devices, or p-type bulk) and why it is picked to be the
largest possible voltage for the bulk of PMOS devices, which have n-type
bulk.

Hope that helped explain that.....

Joe

 

Hi Larkin

Thank you for explaining and giving the confirmation after Joe. It is
very kind of you to explain and took the effort to draw the nice
looking Nmos.

Joe,
You are very right about the reverse biased diode which I almnost for
got haha
You explained in a very detailed sentences.
So in the end , I may think that the arrow is just showing :
1) direction of initial electron flow to the gate to form electron
sheet charge (NMOS). Therefore pointing into the gate
2) direction of initial electron deplete the area near to the gate for
PMOS (leaving the gate surface)

What do you think?
That is just some guess to help one to rememebr

Thank you all for your comments
Thank you Joe and John Larkin

best regards
Jason

 

John Larkin wrote...

It's a mess allright. That symbol conflicts awkwardly with the
appointed symbol, with it's substrate diode going the other way.

.. |
.. |__|
.. | |
.. -----| |<-,
.. | | |
.. |--+ N type
.. |
.. |

And then we have these symbols, for N and P type.

.. N | P |
.. |--' |--'
.. --| --o|
.. |--, |--,
.. | |

And we have these symbols, where it's understood what polarity
you're to assume, N or P type.

.. N | P |
.. __|--' __|--'
.. |--, |--,
.. | |

Some solve this by using gate position to indicate the source,

.. N | __ P |
.. |--' ||--'
.. __||--, |--,
.. | |

There's more...

 

*I* appoint the symbols. Mine looks like, and behaves like, an NPN
transistor with an insulated gate. I don't have to show the substrate
diode any more than an NPN has to show the b-e zener.

But the substrate diode is source-to-drain, not halfway up, and it's
unaffected by the gate. And there are just too many bits and pieces
for this version to look right on a well-proportioned schematic.

John

 

John Larkin wrote...

That's so, it is complicated, and if the (redundant) drain-source
substrate diode is explicitly shown, as some like to do, especially
for certain relevant circuits, it becomes a very complex symbol.

Your simple MOSFET symbol has sufficient use that most anyone seeing
it on your schematics will know what's meant. Go in peace my son.

 

Blessings on you, and all your schematics, as well.

Another homemade symbol I like is the one we use for Schottky
diodes...


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where the little dot in the middle is the hot carrier.


John

 

Hi Winfield and Larkin

Thank you so much for the posting.
Winfield , you have been drawing so many circuits. I was wondering if
those are true ones.
Then was told by somebody you are a author of a 5-star electronics
book. I am sure you must be giving some value-added circuit.
I will digest it. So when will you publish the book?

Thank you all.

best regards

 

Win co-authored "The Art of Electronics", which everyone should have.
The second edition is still in print, and he has been promising a
third for, um, a while now.

His circuits are, to my knowledge, always correct, if a tad complex
and fussy at times. But that happens when people treat electronics as
art and sport.

John

 

Hi John

Thank you for the info given.
Look forward to Win's new book then.

Thank you all for writing to this post


best regards
Jason