When does the NMOS form the channel? VGS > VTH or VGB > VTH?

Discussion in 'General Electronics Chat' started by vqtuyen, Oct 7, 2015.

  1. vqtuyen

    Thread Starter New Member

    Oct 1, 2015
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    When does the NMOS form the channel? VGS > VTH or VGB > VTH? I see that textbook talks about VGS > VTH, channel formed, but the capacitor is formed between the gate and the body, so i think it should be VGB >VTH?
    [​IMG]
     
  2. Bordodynov

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    May 20, 2015
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    When a source connected to the substrate question makes no sense. Otherwise, the on threshold (Vs> Vb, Vt> Vto) increases with Vsb. The threshold is measured for the Vgs. The threshold voltage is determined by the current, and not by the presence of an inversion layer.
     
    Last edited: Oct 7, 2015
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  3. vqtuyen

    Thread Starter New Member

    Oct 1, 2015
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    Can you explain more physically in the structure of the MOS?
     
  4. Jony130

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  5. dl324

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    Mar 30, 2015
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    In discrete (3 terminal) devices, bulk (or body) is internally connected to the source, so Vgs is essentially the same as Vgb.
     
  6. anhnha

    Active Member

    Apr 19, 2012
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    From this hanout, source and bulk are tied together so Vgs and Vgb are the same. How about the case source and bulk are not connected together?

    How about the state of mosfet, on or off? Is it determined by Vgs or Vgb?
     
  7. Bordodynov

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    May 20, 2015
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    Vgs_vs_Vgb.png See
     
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  8. anhnha

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    Apr 19, 2012
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    Thank you. Your simulation proves that you are right. However, I am still confused because it seems Vgb is the voltage creates inversion layer and so it determines mosfet threshold voltage.
     
  9. anhnha

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    Apr 19, 2012
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    Bordodynov, could you send me the model and simulation file?
     
  10. Bordodynov

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    May 20, 2015
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    anhnha,
    I am come file a little later (the File on my home computer). Much years back beside me to appeared such question. I came to output that channel there is, but he does not contact with source of the transistor (because of impoverished area of the spatial charge)
     
  11. Bordodynov

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    May 20, 2015
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    I am sending the file. I found it.
     
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  12. anhnha

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    Apr 19, 2012
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    Thank you. I tried that file and it is OK. However, I just test the circuit below. The source terminal is left open and bulk and source are tied together. Then I plot Vs when Vg sweeps from 0 to 5V.
    As you can see, Vs is equal to 5V for all values of Vg.
    Does this mean that Vgb is what helps form channel and the channel conducts now?

    Vgb - LTspice 2.PNG
     
  13. Bordodynov

    Active Member

    May 20, 2015
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    Yes, LTspice all calculated correctly. Even when the channel is not present, there is a leak p-n junction diode (i_leak_D-B). This tightens the substrate to leak Drain. Load the source resistor 1 Meg, and you will see the difference.
     
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