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I'm trying to get a better understanding of how the NMOS works when a negative voltage is applied to the gate.

So I currently understand that the $P^-$ substrate is lightly doped with such a material from the group III elements. This allows for an excess of holes in the material.

What I don't understand is why these holes get attracted to the SiO2 insulator, creating a $P^+$ Accumulated channel under the material, when a negative voltage is applied.

Why is that negative voltage pulling the holes towards the SiO2 insulator? I would like to be able to visualize what exactly is happening.

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The NMOS is a metal-oxide-semiconductor structure with a p-type semiconductor substrate. The oxide (SiO2) is an insulator. It is mainly used for n-MOSFETs which have an electron (n) inversion layer at the surface formed upon application of a positive voltage to the metal gate with respect to the substrate. The modulation of this electron inversion channel with applied gate voltage is the working principle of the metal-oxide-semiconductor field effect transistor. If a negative gate voltage is applied to the n-MOS structure, the positively charged holes in the p-type substrate are attracted to the SiO2/semiconductor interface by the applied negative electric field and produce an accumulation layer there. NMOS devices are, however, mostly used with positive applied gate voltages.

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  • $\begingroup$ I understand that. But why exactly does the negative potential attract holes? $\endgroup$
    – user367640
    Commented Sep 11, 2016 at 1:29
  • $\begingroup$ The negative potential on the gate creates an electric field in the insulator (SiO2) and in the first instant upon application also in the p-type substrate directed towards the metal gate. Holes are particles of positive charge which experience a force in the direction of the electric field so that they will move in this direction until they are blocked by the interface to the SiO2. Therefore they will accumulate there in a very thin accumulation zone. In the final state the accumulation layer shields the penetration of the electric field into the substrate stopping the hole movement. $\endgroup$
    – freecharly
    Commented Sep 11, 2016 at 2:06

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