MOSfet as diode: average knee voltages?

[Steben]

Is there a list or info about the average knee voltage of MOSfets used as diode?
Is there sort of a MOSfet that you can swap a red LED with?

(and in other words: can one speak of a "smallest knee voltage MOSfet"?)

EDIT: is it what they call "gate treshold voltage"?

[R.G.]

Is there a list or info about the average knee voltage of MOSfets used as diode?
Is there sort of a MOSfet that you can swap a red LED with?
(and in other words: can one speak of a "smallest knee voltage MOSfet"?)
EDIT: is it what they call "gate treshold voltage"?

Interesting question, but very difficult answer.

The technical answer is that no, there is no average, typical, or smallest knee voltage MOSFET.  MOSFETs can and have been made with gate thresholds from several volts positive to zero, to negative - that is, depletion mode instead of enhancement mode.

That being said, most discrete MOSFETs will have thresholds between 2V and 5V, and the more common to-92s will have thresholds about 3-4V. And manufacturers don't specify gate thresholds too tightly, and the diode-connection voltage not at all.

The knee voltage is a function of the gate threshold voltage, so you're on the right track. But it's going to be hard to filter that info out of a data sheet in a useful manner.

I think you may have to fall back on buying some MOSFETs, testing and characterizing them for your use, then worrying about the next batch being the same, if there is a next batch.

[Steben]

Is there a list or info about the average knee voltage of MOSfets used as diode?
Is there sort of a MOSfet that you can swap a red LED with?
(and in other words: can one speak of a "smallest knee voltage MOSfet"?)
EDIT: is it what they call "gate treshold voltage"?

Interesting question, but very difficult answer.

The technical answer is that no, there is no average, typical, or smallest knee voltage MOSFET.  MOSFETs can and have been made with gate thresholds from several volts positive to zero, to negative - that is, depletion mode instead of enhancement mode.

That being said, most discrete MOSFETs will have thresholds between 2V and 5V, and the more common to-92s will have thresholds about 3-4V. And manufacturers don't specify gate thresholds too tightly, and the diode-connection voltage not at all.

The knee voltage is a function of the gate threshold voltage, so you're on the right track. But it's going to be hard to filter that info out of a data sheet in a useful manner.

I think you may have to fall back on buying some MOSFETs, testing and characterizing them for your use, then worrying about the next batch being the same, if there is a next batch.

thanks;
all in all it seems using a MOSfet + germanium in a 9V circuit will get you very close to opamp clipping... (if there is an opamp of course)
Most opamps will clip at best at 3.5 V - 4 V.

[WGTP]

It's time to whoop my favorite Mosfet clipping thread out again. 

http://www.diystompboxes.com/smfforum/index.php?topic=38581.0

[robmdall]

It's time to whoop my favorite Mosfet clipping thread out again. 

http://www.diystompboxes.com/smfforum/index.php?topic=38581.0

Nice thread.... Thanks for whooping it!!