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Qualitative Understanding of MOSFET in Depletion

  1. Oct 20, 2012 #1
    1. The problem statement, all variables and given/known data

    No actual problem statement. I'm just having trouble understanding what is happening during the depletion and inversion stages.

    See the attached picture. During the depletion stage, what does it mean by "immobile acceptor ions?"

    2. Relevant equations

    None

    3. The attempt at a solution

    Accumulation stage
    There is a negative bias on the metal electrode that attracts positively charged hole carrier atoms to the oxide-substrate interface. Makes sense.

    Delpletion Stage
    Voltage on gate is increased to some positive value that is still less than the threshold. The text states that holes are repelled from the oxide-substrate interface. Hole density near that interface is reduced below the doping levels.

    Here's where I'm not sure about things.

    The positive charge on the gate needs to be balanced. Is this balanced by free electrons in the p-type substrate that are pulled to the oxide-substrate interface? Do these electrons fill in the holes near the oxide-substrate surface, resulting in the "immobile acceptor ions" labeled in (b) of the picture?

    Inversion Stage
    Gate voltage is increasingly positive resulting in more electrons being pulled to the oxide-substrate interface. An electron inversion layer is created.

    Why is the electron inversion layer created? Why don't the electrons that are pulled to the interface just ionize more holes, resulting in a larger depletion layer?
     

    Attached Files:

  2. jcsd
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