I'm looking at MOSFETs and I find a 2N7000 from Fairchild that has two rows in the table for gate threshold.
What nonsense is this? Why is the threshold higher lower for the larger drain current? Am I missing something here?
EDIT: Fixed dumb mistake.
This datasheet (from Digi-Key) is more specific ... the threshold is different due to MOSFET type (package).
The threshold is the point of switching off, and 'off' is defined by a residual current (much smaller than
the load current, typically 250uA or something like that). At the point of switch-off changes in gate voltage
will cause changes in the current flowing - nothing surprising that increasing the gate drive allows more
current to flow. The theoretical threshold voltage is when the drain current falls to zero, which it never
does owing to leakage (which is a LOT higher at 125C than 25C, BTW - the conditions for threshold voltage
are chosen to be insensitive to this leakage current change with temperature I suspect).
Remember the threshold is completely irrelevant for using a MOSFET(*) as a switch, you drive the gate voltage
to 0V anyway to switch it off, and switching fully on is a completely different gate voltage (typically 3
times the threshold voltage or more, as given by the Rds(on) condition)
(*) enhancement MOSFET, to be clear.
[ ooh yes, look, the leakage current is 1mA at 45V and 125C, that's why the extra threshold
value is specified at 1mA ]
Jiggy-Ninja:
Why is the threshold higher for the larger drain current? Am I missing something here?
Uh, yes.
If I say "the threshold voltage is X"
What does that mean? How do we define "threshold voltage"? It's when it starts to turn on, and current starts to flow. Well, how much current? We need to pick a value in order to define what "threshold voltage" means.
These manufacturers have been kind enough to provide two different figures, using different current for threshold - an naturally if you say "the threshold gate voltage is where 1mA flows" it's going to be higher than the threshold voltage if you define it at a quarter of a mA.
Yes, this means threshold voltage values are not directly comparable between datasheets oftentimes. The threshold voltage should not be confused with the gate voltage at which the transistor is fully on - I have seen people confuse the two.
dlloyd:
This datasheet (from Digi-Key) is more specific ... the threshold is different due to MOSFET type (package).
The datasheet I linked only has one package, TO-92. I don't think those are for the same MOSFET.
MarkT:
The threshold is the point of switching off, and 'off' is defined by a residual current (much smaller than
the load current, typically 250uA or something like that). At the point of switch-off changes in gate voltage
will cause changes in the current flowing - nothing surprising that increasing the gate drive allows more
current to flow. The theoretical threshold voltage is when the drain current falls to zero, which it never
does owing to leakage (which is a LOT higher at 125C than 25C, BTW - the conditions for threshold voltage
are chosen to be insensitive to this leakage current change with temperature I suspect).Remember the threshold is completely irrelevant for using a MOSFET(*) as a switch, you drive the gate voltage
to 0V anyway to switch it off, and switching fully on is a completely different gate voltage (typically 3
times the threshold voltage or more, as given by the Rds(on) condition)(*) enhancement MOSFET, to be clear.
[ ooh yes, look, the leakage current is 1mA at 45V and 125C, that's why the extra threshold
value is specified at 1mA ]
The gate threshold conditions do not specify different temperatures like the leakage current does.
DrAzzy:
Uh, yes.If I say "the threshold voltage is X"
What does that mean? How do we define "threshold voltage"? It's when it starts to turn on, and current starts to flow. Well, how much current? We need to pick a value in order to define what "threshold voltage" means.
These manufacturers have been kind enough to provide two different figures, using different current for threshold - an naturally if you say "the threshold gate voltage is where 1mA flows" it's going to be higher than the threshold voltage if you define it at a quarter of a mA.
Yes, this means threshold voltage values are not directly comparable between datasheets oftentimes. The threshold voltage should not be confused with the gate voltage at which the transistor is fully on - I have seen people confuse the two.
There is a rather unfortunately typo in my first post that I didn't notice until I read yours.
The threshold voltage in the datasheet is actually lower for the higher current. I know how enhancement MOSFETs work, so a higher threshold for a higher drain current wouldn't have confused me. This one seems backwards.
The gate threshold has a negative temperature coefficient, so the threshold decreases as the temperature rises. I guess the two rows for gate threshold could be for 25°C and 100°C respectively.
The gate threshold has a negative temperature coefficient, so the threshold decreases as the temperature rises. I guess the two rows for gate threshold could be for 25°C and 100°C respectively.
That makes sense, except the two lines do not specify different temperatures. It may be just an omission though.
