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Discussion in 'Electronic Basics' started by abhishek, Apr 25, 2006.

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  1. abhishek

    abhishek Guest

    in CMOS when we are keeping source and substrate at ground potential
    then inversion layer is formed ,,,,,,how it is possible. because
    inversion layer forms only when there is revese bias.here it should be
    act as a simple p-n junction
     
  2. Hi,

    Let's take NMOSFET as an example first. Before looking into it, we
    further only consider the Gate and the Substrate. So, it is just a
    capacitor with 2 terminals (Gate and Substrate which are separated by
    SiO2). The substrate is usually p-type. So there are many holes (e.g.
    1E18) but very few electrons (e.g. 1E2) (n_i^2 = np). If we apply a
    negative bias to the gate, it will attract electrons towards the SiO2
    and repel the holes away from the SiO2. At the beginning, the
    population of holes decreases (towards 1E10) and then the region under
    the SiO2 will be depleted with holes. At that time, the electron
    concentration is not large yet (only 1E10 also). So the total number of
    carrier is just 2E10 which is much smaller than 1E18! This is the
    depletion layer.

    If the gate voltage continues to increase, eventually it will attract
    more than 1E18 electron towards the SiO2. Now, the channel is formed
    and very conductive (1E18 of electrons). But the carrier type is
    different! That's why it is called inversion!

    You may ask from where the huge amount electrons come, as the substrate
    is p-type. This is by electron-hole pairs generation, which is pretty
    slow!

    Now, let's add the source and drain to the structure. They are n+
    doped. They can supply the huge amount of electron needed to form the
    channel quickly! If you bias the source to the same potential as the
    substrate, would it affect how the inversion layer forms? No. Because
    they are at equilibrium anyway (same Fermi level). Of course, if you
    reverse or forward bias the Source to the Substrate, then the condition
    (or threshold voltage) for inversion layer formation will change.

    In summary, yes, there should be depletion layer formation before the
    inversion layer is formed. But that depletion layer is referring to
    that across the Gate and the Substrate. It has nothing to do with the
    p-n junction of the Source and the Substrate because they are
    physically at different locations. However, inversion layer forms at
    different gate voltage if the source and substrate are not biased at
    the same potential.

    Hope that helps!

    KodKodKod Learning Consulting -Here you get the answers
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