Cgs, Cgd, Cgb should be in parallel to form the total gate capacitance equation. However source drain and body are at different potentials so how are they in parallel?
When Vgs is less than zero, the charge carriers at the Si-SiO2 interface are holes, the positive carriers. Since the substrate is P-type, it has more than enough holes to accumulate at the interface, making it such that the electric field, that is observed within a capacitor doesn't penetrate into the substrate, thus the capacitance, which is C= (epsilon)A/L has no changing terms. But when Vgs is negative, the charges to accumulate at the interface are to be negative. But since they are less in number, what happens is that there will be a deficit of negative charges. To compensate, the holes of nearby dopant atoms are pushed away, creating a depletion region of immobile negative acceptor ions. This region isn't very close to the surface, causing the field to penetrate into the substrate, effectively varying the "L" in C=(epsilon)A/L. Further, as depletion continues, we get to the point where the negative charge carriers, the electrons can be easily accumulated at the interface than the immobile carriers. This is where inversion starts and they act like holes in accumulation case, terminating field before it penetrates. Thus, it reaches the value before depletion. Although there is a slight difference as some field still penetrate into substrate due to presence of immobile negative carriers, it is highly negligible in region where we operate on.
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Cgs, Cgd, Cgb should be in parallel to form the total gate capacitance equation. However source drain and body are at different potentials so how are they in parallel?
nice lecture
Why does it have a constant capacitance when Vgs is below zero ?
because of accumulation
When Vgs is less than zero, the charge carriers at the Si-SiO2 interface are holes, the positive carriers. Since the substrate is P-type, it has more than enough holes to accumulate at the interface, making it such that the electric field, that is observed within a capacitor doesn't penetrate into the substrate, thus the capacitance, which is C= (epsilon)A/L has no changing terms.
But when Vgs is negative, the charges to accumulate at the interface are to be negative. But since they are less in number, what happens is that there will be a deficit of negative charges. To compensate, the holes of nearby dopant atoms are pushed away, creating a depletion region of immobile negative acceptor ions. This region isn't very close to the surface, causing the field to penetrate into the substrate, effectively varying the "L" in C=(epsilon)A/L.
Further, as depletion continues, we get to the point where the negative charge carriers, the electrons can be easily accumulated at the interface than the immobile carriers. This is where inversion starts and they act like holes in accumulation case, terminating field before it penetrates. Thus, it reaches the value before depletion. Although there is a slight difference as some field still penetrate into substrate due to presence of immobile negative carriers, it is highly negligible in region where we operate on.
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