I've been on this site now a couple of months and I notice various symbols used for MOSFETs. What is the preferred symbol for an N Channel MOSFET and why?
Answer
It is likely that you saw a Circuit Lab sysmbol and that this caused you to ask this question. The Circuit Lab N Channel MOSFET symbol is both unusual and illogical.
I'd avoid using them if at all possible.
Read on ...
Acceptable [tm] N Channel MOSFET symbol tends to have these characteristics.
Gate symbol on one side.
3 "contacts" on other side vertically.
Top of these is drain. Bottom of these 3 is source.
Middle has an arrow pointing INTO the FET and the outside end is connected to source.
This indicates that there is a connected body diode and that it is non conducting when the source is more negative than the drain (arrow is same as would be for a discrete diode).
Any symbol which obeys these guidelines should be "clear enough" and OK to use.
I have very occasionally seen people use a symbol which does not comply with these guidelines but which is still recognisable as an N Channel MOSFET.
SO. Any of these are OK, and you can see the differences for the unmarked P Channels.
But!!!
Jippie's example shows the rogue version.
[Note: See below - this is in fact intended to be a P Channel sysmbol].
Truly horrible. I'd have to wonder if this was a P Channel symbol or an N Channel one.
Even the discussion it is taken from has people expressing uncertainty re arrow direction. As shown IF that is an N Channel then it is implying body diode polarity and NOT current flow in source.
Thusly
________________'
Circuit Lab is apparently the (or a) culprit.
This is their symbol for an N Channel MOSFET.
A nasty piece of work, alas. Arrow shows usual drain-source conduction direction BUT as a MOSFET is a 2 quadrant device and will provide a true resistive on channel with \$V_{gs}\$ positive BUT \$V_{ds}\$ negative, the arrow is meaningless and, as it is in the opposite direction to most N Channel MOSFET sysmbols it is misleading to most. (Note the proper use of this symbol in table below).
USER23909 helpfully pointed out this page - Wikipedia - MOSFET . This page includes the following symbols. User xxx says these may be IPC standards, but Wikipedia is silent re their source.
Wikipedia MOSFET symbols
http://en.wikipedia.org/wiki/MOSFET#Circuit_symbols
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