FET drain and source distinction

[Walter Harley]

A fellow asking a question on sci.electronics.components reminded me of
something that I find confusing; maybe someone can help.

Some FETs are symmetric, in that there is no distinction between the source
and drain. Others are asymmetric, but the distinction may still be minor
(e.g., slightly different capacitance from the gate to one or the other...).

My question is this: the amount of current that flows from drain to source
(or vice versa) is related to the voltage between the gate and source (Vgs);
but there is a voltage differential between the source and drain, so Vgd !=
Vgs. How does the FET "know" which one to pay attention to? Or does it use
the average, or whichever one is lower or higher, or what?

I'm sure the actual explanation is very simple, but I would appreciate it if
someone could help me get it into my head

Thanks,
-walter

 

[Ken Smith]

My question is this: the amount of current that flows from drain to source
(or vice versa) is related to the voltage between the gate and source (Vgs);
but there is a voltage differential between the source and drain, so Vgd !=
Vgs. How does the FET "know" which one to pay attention to? Or does it use
the average, or whichever one is lower or higher, or what?

Any fet can be though of as infinitely many infinitely small fets in
series. Each of these tiny fets has no voltage difference between its
source and drain. The source to drain resistance is controlled by the
voltage from the gate to the source/drain of each tiny fet.

 

[John Larkin]

My question is this: the amount of current that flows from drain to source
(or vice versa) is related to the voltage between the gate and source (Vgs);
but there is a voltage differential between the source and drain, so Vgd !=
Vgs. How does the FET "know" which one to pay attention to? Or does it use
the average, or whichever one is lower or higher, or what?

I'm sure the actual explanation is very simple, but I would appreciate it if
someone could help me get it into my head

Thanks,
-walter

Yup, simple. Consider a symmetrical n-channel enhancement mosfet. If
the ends are at the same voltage, there's no source or drain, but then
there's no current flow anyhow. If one end is more positive than the
other, call the positive side "drain" and the other end "source". So
the gate voltage to enhance the channel is relative to the
most-negative terminal, which acquires the name "source". Symmetric
jfets (more common in real life) work like this too: negative Vg
relative to the most-negative end will pinch it off.

In the case you cite above, what's different about the two voltages is
that they are opposite in *polarity.* The fet "pays attention" to the
one that makes the gate "positive."

Interesting: Rds(on) is lowest when the s-d voltage is very low, which
is when both "ends" of the fet are essentially sources, so twice as
much enhancement is available!

John

 

[Lizard Blizzard]

A fellow asking a question on sci.electronics.components reminded me of
something that I find confusing; maybe someone can help.

Some FETs are symmetric, in that there is no distinction between the source
and drain. Others are asymmetric, but the distinction may still be minor
(e.g., slightly different capacitance from the gate to one or the other...).

My question is this: the amount of current that flows from drain to source
(or vice versa) is related to the voltage between the gate and source (Vgs);
but there is a voltage differential between the source and drain, so Vgd !=
Vgs. How does the FET "know" which one to pay attention to? Or does it use
the average, or whichever one is lower or higher, or what?

From what I understand, there is more pinch effect towards the drain,
which has a greater diff in voltage, and less pinch effect towards the
source, which has less diff in voltage. But since the device is
symmetrical, this effect is reversed when the electrodes are swapped.

I'm sure the actual explanation is very simple, but I would appreciate it if
someone could help me get it into my head

Thanks,
-walter

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