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
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!
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