MOSFETS and pn junctions in substrate / source and drain

Discussion in 'Homework Help' started by mofet, Oct 5, 2010.

  1. mofet

    Thread Starter New Member

    Oct 5, 2010
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    Hi


    I attached a sheet that shows my doubts about MOSFETs.
    It is used to say that to body doesn't matter that much in MOSFET analysis. I do not know why is this.
    Later, in some books they refer the effect body (I didn't read yet about it, but must be for very accurate models. It seems (?) that body effects must be minor.. only being important in critical applications).

    But my doubts are:
    1 - If we apply a positive voltage in DRAIN (Vds) that has n+ (plenty of electrons ) is this turns out the n+ region in one p-region ?

    2 - Why connecting a substrate(=body) terminal to the source terminal put the pn junctions reverse-biased!?! (see the sheet attached, please). And in this way they told that the body doesn't matter much in operation device??

    3 - If really Vgs is turning out more positive and positive... it will pull down all holes to the base of the substrate and it has no anymore carriers - (no free electrons neither holes) causing a depletion layer.... the positive charges (from Vgs positive) will attract the negative charges to the other plater under gate... these negative charges comes from n+ drain and n+ source regions... forming a n-channel. But if this is so... how electrons then can travel in an one negative channel if they would be repealed again to the source (electrons will come from source to the drain in the current - provided that Vds is positive)? Please help me in these questions. If anything is not clear, ask me , I'll try to expose better my doubts.


    ps English is not my native language, so sorry for any unclear sentence, or typos.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    I believe you are trying to describe a FET, not a MOSFET, at least judging from your drawing. There are many similar components, names for them are important.

    MOSFETs

    Don't worry about your English, we get a lot of that here. Everyone is welcome.
     
  3. mofet

    Thread Starter New Member

    Oct 5, 2010
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    thanks for input. I'm trying to represent what is known as IGFET (insulated gate FET).

    I really would like to clear out these doubts. I'm confusing about the pn junction and the reason why the body terminal is not , usually, take in account for analysis in FET.
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Becaues if you connect (short) "body" with source and if Vd is positive.
    Then it's clear that this "build in diode" is reverse-biased.
    But reality is much more complicate.
    http://www.irf.com/technical-info/appnotes/mosfet.pdf
     
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  5. mofet

    Thread Starter New Member

    Oct 5, 2010
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    yes. That was my suspection as I sketched in the diagram. Thanks!

    Only it lacks to know/clarify about the 1st and 3rd questions (this third question seems I have to take a look at SC Physics) . :)
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    As for first question i think that you are wrong.

    As for 3rd question I think that when you apply positive voltage to Vgs. The electric field produce by Vgs repels holes in P-substrate to uncover bound of acceptors atom.
    Electric field attracts mobile electrons from N+ regions and any minority electrons form substrate.
    So N channel is establish (induced channel) between source and drain.
    And we call this inversion layer.
    http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html
    http://books.google.pl/books?id=n0r...AEwAA#v=onepage&q=induced channel mos&f=false
     
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