FETs inserted the wrong way around
 

One thing puzzles me, why does it matter if a FET is put into
a circuit the wrong way around, with source and drain interchanged,
because a FET (JUGFET, MOSFET) is symmetrical in its construction?

FETs inserted the wrong way around
 

On 18/09/2018 20:12, Gareth's Downstairs Computer wrote:
One thing puzzles me, why does it matter if a FET is put into
a circuit the wrong way around, with source and drain interchanged,
because a FET (JUGFET, MOSFET) is symmetrical in its construction?


It depends.

Some FETs are symmetric and should, in theory, work if 'reversed'. In
JFETs it depends on the doping process. From memory, if a double
diffusing process is used you're out of luck- but that is just from memory.

Strictly speaking, symmetric JFETs are the only ones where the arrow
should join the bar at mid-point in the symbol. The others it should be
offset. Having said that, I've rarely seen that applied.



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FETs inserted the wrong way around
 

One thing puzzles me, why does it matter if a FET is put into
a circuit the wrong way around, with source and drain interchanged,
because a FET (JUGFET, MOSFET) is symmetrical in its construction?

Your "because" is too general a statement.

Some types of FETs are, in fact, physically symmetrical in
construction, and the source and drain are interchangeable. Most
JFETs are like this, as are "lateral" MOSFETs (the usual sort in IC
manufacturing, and some older audio power MOSFETs.)

However, the commonly-used "vertical" MOSFET is different... it's very
asymmetric in its construction.

https://electronics.stackexchange.co...ted-with-arrow
has a good set of explanations... including the detail that in a
vertical MOSFET, the substrate is normally shorted to the source right
at the semiconductor die, in order to suppress the "parasitic" BJT
which lurks inside the silicon structure.

FETs inserted the wrong way around
 

On 18/09/2018 20:17, Dave Platt wrote:
One thing puzzles me, why does it matter if a FET is put into
a circuit the wrong way around, with source and drain interchanged,
because a FET (JUGFET, MOSFET) is symmetrical in its construction?

Your "because" is too general a statement.

Some types of FETs are, in fact, physically symmetrical in
construction, and the source and drain are interchangeable. Most
JFETs are like this, as are "lateral" MOSFETs (the usual sort in IC
manufacturing, and some older audio power MOSFETs.)

Which is where I came in and what prompted my question.



However, the commonly-used "vertical" MOSFET is different... it's very
asymmetric in its construction.
https://electronics.stackexchange.co...ted-with-arrow
has a good set of explanations... including the detail that in a
vertical MOSFET, the substrate is normally shorted to the source right
at the semiconductor die, in order to suppress the "parasitic" BJT
which lurks inside the silicon structure.

Brilliant, Read, marked, learnt and inwardly digested, and recommended
in turn to other enquirers.

Thank-you very much.