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Jan 1, 1970
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 it should be
act as a simple p-n junction

KodKodKod Learning Consulting

Jan 1, 1970

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

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!

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