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[Transistor] Vpinch-off & Vthreshold

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Lucyfer

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Ok i was reading and the came the phrase: " Vgs-V(x)< Vt. At that point, the induced charge is zero, and the conducing channel disapears os is Pinched off.
Now the question is, whats the difference betwen Vth and V pinch-off since they are both close to each other... V th is when the channel appears and the current flows from Gate to source... and V pinch-off is the value where the channel no longer exists and there is no current flowing.... now
whats the value differenre betwen each other ? thanks!
best regards
correcting.. there is still current in pinch off mode but its constant...
 
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crutschow

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Most Helpful Member
V pinch-off is not normally specified but, as you note, it is very close to V threshold. The difference between them is probably a few tens of millivolts. V pinch-off is not normally of concern when designing circuits since you typically turn off a FET by applying a voltage well below the threshold.
 

BrownOut

Banned
Your idea of Vpinchoff is totally incorrect. The channel does not disappear at pinchoff. That voltage is at the boundary between a MOSFET's "linerar" or "triode" region, and the "constant current" or "saturation" region ( NOT TO BE CONFUSED WITH BJT TRANSISTOR SATURATION ) If VDS - VGS goes above the pinchoff voltage, then the FET operates in the constant current mode. If VDS - VGS goes below the pinchoff, the FET operates in the linerar or triode mode. Pinchoff voltage references VDS. Threshold voltage references VGS.
 
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crutschow

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Most Helpful Member
I was incorrect in my first explanation.

V threshold is normally used with an enhancement-mode FET and refers to the point at which the transistor just starts to conduct.

V pinchoff is normally used to refer to the point at which a depletion-mode transistor is just cut-off.

It has nothing to do with the boundary between the linear and saturation regions of a FET.
 

BrownOut

Banned
It has everything to do with the boundary between linear and saturation regions of a FET.
 

Lucyfer

New Member
Ok ... i know these values are specified, but im trying to take some measures of a transistor we build in our own...
like gm, µ, Vth, Vpinch-of.... and what else we can measure!
I was seaching 1st for that answer u guys gaved me... now im thinking if i may find the ζ so i can calculate µ= Vd/ζ ...
i was thinking if the Vg makes any kind of interference in the Vds... i would try to find it by V(x)=-dE/dx .......is it right ?
 

crutschow

Well-Known Member
Most Helpful Member
It has everything to do with the boundary between linear and saturation regions of a FET.
Can you post a reference for that?
 

BrownOut

Banned
Try this

Look at paragraph 4.4 on page 125. That's the only defn of pinch off voltage I know.

Hope you can get to it.
 
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crutschow

Well-Known Member
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Try this

Look at paragraph 4.4 on page 125. That's the only defn of pinch off voltage I know.
.
Thanks. That's a good description of the pinch-off operating region of a transistor, which apparently is the formal definition of that term.

There is also a common definition of the pinch-off voltage (as opposed to the pinch-off operating region) which is the cutoff voltage at which a depletion-mode FET just turns off. See the last paragraph of .
 

BrownOut

Banned
I think we're gonna just disagree on this one. I have some problems with the information in the links you've provided. I don't think pinchoff should ever describe the "turn on/turn off" voltage of a MOSFET in any configuration. I think that usage of the term bled over from JFETS, where it was an unfortuante useage too.
 
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crutschow

Well-Known Member
Most Helpful Member
I think we're gonna just disagree on this one. I have some problems with the information in the links you've provided. I don't think pinchoff should ever describe the "turn on/turn off" voltage of a MOSFET in any configuration. I think that usage of the term bled over from JFETS, where it was an unfortuante useage too.
That may be true. The term pinch-off is normally used with JFETS and I've seldom seen it used with MOSFETS. But common useage sometimes supersedes correct useage, unfortunately.
 
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