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[Transistor] Vpinch-off & Vthreshold

Discussion in 'General Electronics Chat' started by Lucyfer, Oct 19, 2009.

  1. Lucyfer

    Lucyfer New Member

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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...
     
    Last edited: Oct 19, 2009
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    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.
     
  3. BrownOut

    BrownOut Banned

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    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.
     
    Last edited: Oct 19, 2009
  4. dave

    Dave New Member

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  5. crutschow

    crutschow Well-Known Member Most Helpful Member

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    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.
     
  6. BrownOut

    BrownOut Banned

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

    Lucyfer New Member

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    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 ?
     
  8. BrownOut

    BrownOut Banned

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    I don't know if that's right, becuase I don't undersand what you wrote.
     
  9. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Can you post a reference for that?
     
  10. BrownOut

    BrownOut Banned

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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.

    Hope you can get to it.
     
    Last edited: Oct 19, 2009
  11. crutschow

    crutschow Well-Known Member Most Helpful Member

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    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 The transistor as a switch : JUNCTION FIELD-EFFECT TRANSISTORS or the third paragraph in http://www.zen118213.zen.co.uk/RFIC_Theory_Files/MOS_Transistor.pdf.
     
  12. BrownOut

    BrownOut Banned

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    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.
     
    Last edited: Oct 20, 2009
  13. crutschow

    crutschow Well-Known Member Most Helpful Member

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    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.
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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  15. Lucyfer

    Lucyfer New Member

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