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FET: drain&source can be swapped?

T

Tony

Jan 1, 1970
0
Hello,
in a bipolar transistor collector and emitter aren't freely
interchangable, because they have a polarity.
But, can I freely swap drain and source of a FET? Or also
FETs are polarized (drain-source wise, not the obvious gate)?

I ask this because as far as I know a FET behaves as a simple
switch, where even alternate current can flow.

Thanks!
Tony
 
P

petrus bitbyter

Jan 1, 1970
0
Tony said:
Hello,
in a bipolar transistor collector and emitter aren't freely
interchangable, because they have a polarity.
But, can I freely swap drain and source of a FET? Or also
FETs are polarized (drain-source wise, not the obvious gate)?

I ask this because as far as I know a FET behaves as a simple
switch, where even alternate current can flow.

Thanks!
Tony

Tony,

You can't. The gate voltage has its polarity (normaly related to the source)
and the FET would not be happy to feel it changed so radically. You may
destroy it.

pieter
 
J

John S. Dyson

Jan 1, 1970
0
Tony,

You can't. The gate voltage has its polarity (normaly related to the source)
and the FET would not be happy to feel it changed so radically. You may
destroy it.
Actually, in general your statement is as wrong as it is right. Alot
of small signal FETS are equivalent when switching the drain and source.
Alot of special purpose or power FETS have structures that certainly would
disable or distroy the component if the 'drain' and 'source' were
interchanged. (Sometimes, it isn't the internal FET itself but it
is the protection circuitry that keeps inverse use from working.)

Not all fets work the same in 'inverse' mode, but again, most likely
such fets that do work are likely to be small signal.

John
 
G

Gary Reichlinger

Jan 1, 1970
0
But, can I freely swap drain and source of a FET? Or also
FETs are polarized (drain-source wise, not the obvious gate)?

Mosfets only block voltage in one direction. They have an
inherent body diode which allows current to flow in the opposite
direction subject to the voltage drop characteristic of the body
diode. Also, as indicated in another post, gate voltage is specified
in relation to the source.
When using mosfets to control alternating current, it is standard
practice to use 2 of them in series (opposite polarities) so that
current can be controlled in both directions.
 
W

Watson A.Name - Watt Sun

Jan 1, 1970
0
Hello,
in a bipolar transistor collector and emitter aren't freely
interchangable, because they have a polarity.
But, can I freely swap drain and source of a FET? Or also
FETs are polarized (drain-source wise, not the obvious gate)?

I ask this because as far as I know a FET behaves as a simple
switch, where even alternate current can flow.

If the FET is made symmetrical, i.e. the drain and source are the same
relative to the gate, then the performance with the source and drain
swapped should be the same. Some data sheets say you can swap drain
and source.
Thanks!
Tony


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W

Watson A.Name - Watt Sun

Jan 1, 1970
0
Watson said:
If the FET is made symmetrical, i.e. the drain and source are the same
relative to the gate, then the performance with the source and drain
swapped should be the same. Some data sheets say you can swap drain
and source.

BTW, I should have said JFET. Like the other followups point out,
some FETS will conduct like a forward biased diode if the polarity is
reversed. JFETs are simple and can be symmetrical.

Also, some power FETs are used in the third quadrant, for zero voltage
drop rectifiers.


--
@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
 
D

Dave Platt

Jan 1, 1970
0
But, can I freely swap drain and source of a FET? Or also
Tony,

You can't. The gate voltage has its polarity (normaly related to the source)
and the FET would not be happy to feel it changed so radically. You may
destroy it.

Actually, if I recall correctly, there are some FETs (depletion-mode
JFETs, and perhaps some small-signal MOSFETs) which have a fully
symmetrical internal construction - the identification of "drain" and
"source" on these is largely arbitrary. An N-channel FET of this
type can be hooked up either way, as long as the gate is driven in the
correct direction with respect to whichever pin is acting as the
source.

This is _not_ true of most power MOSFETs. Due to their construction,
there is an "intrinsic" or substrate diode between source and drain.
If you try to reverse the leads (or polarity) on these, the intrinsic
diode will go into conduction.
 
J

Joe McElvenney

Jan 1, 1970
0
Hi
in a bipolar transistor collector and emitter aren't freely
interchangable, because they have a polarity.

Actually transistor are symmetrical as far as collector
emitter polarity is concerned - it is just that they work very
badly upside down. Having said that, years ago I seem to remember
there was a transistor designed to be used either way round which
found use in the odd bilateral circuit.

As for the FET, I believe that the early JFETs, such as the
MPF102, were quite happy standing on their heads.


Cheers - Joe
 
J

John S. Dyson

Jan 1, 1970
0
Hi


Actually transistor are symmetrical as far as collector
emitter polarity is concerned - it is just that they work very
badly upside down. Having said that, years ago I seem to remember
there was a transistor designed to be used either way round which
found use in the odd bilateral circuit.
Well, your comment about a BJT working poorly when used in
'inverse' mode is true for some applications. However, the
saturation voltage (the effective Vce drop) is much lower
in inverse mode, even though the Beta is effectively much
lower. There are some useful circuits when using some BJTs
in inverse mode.

John
 
U

Uwe Bonnes

Jan 1, 1970
0
: Hello,
: in a bipolar transistor collector and emitter aren't freely
: interchangable, because they have a polarity.
: But, can I freely swap drain and source of a FET? Or also
: FETs are polarized (drain-source wise, not the obvious gate)?

: I ask this because as far as I know a FET behaves as a simple
: switch, where even alternate current can flow.

Look at the datasheet, if you can do. E.g. the bipolar JFET BF862 from
Philips explicitly states it

Bye
 
P

petrus bitbyter

Jan 1, 1970
0
John S. Dyson said:
Actually, in general your statement is as wrong as it is right. Alot
of small signal FETS are equivalent when switching the drain and source.
Alot of special purpose or power FETS have structures that certainly would
disable or distroy the component if the 'drain' and 'source' were
interchanged. (Sometimes, it isn't the internal FET itself but it
is the protection circuitry that keeps inverse use from working.)

Not all fets work the same in 'inverse' mode, but again, most likely
such fets that do work are likely to be small signal.

John

John,

You're right. I made it too simple.

pieter
 
J

justa

Jan 1, 1970
0
I think it's a matter of capacitance...

And no, they are not necessarily symmetrical.
The layering/fabrication of the transistor's
structure is optimized for the particular
application. This is one reason why some
trans' are spec'd for RF purposes. Attempts
to reverse their polarity drastically effects
their performance. Again...capacitance...
 
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