Editing MOSFET (section)

=== Metal–oxide–semiconductor structure ===
The traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a layer of [[silicon dioxide]] ({{chem|Si|O|2}}) on top of a silicon substrate, commonly by [[thermal oxidation]] and depositing a layer of metal or [[polycrystalline silicon]] (the latter is commonly used). As silicon dioxide is a [[dielectric]] material, its structure is equivalent to a planar [[capacitor]], with one of the electrodes replaced by a semiconductor.

When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a p-type semiconductor (with ''N''<sub>A</sub> the density of [[acceptor (semiconductors)|acceptors]], ''p'' the density of holes; ''p = N''<sub>A</sub> in neutral bulk), a positive voltage, ''V''<sub>G</sub>, from gate to body (see figure) creates a [[depletion layer]] by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see [[doping (semiconductor)|doping]]). If ''V''<sub>G</sub> is high enough, a high concentration of negative charge carriers forms in an ''inversion layer'' located in a thin layer next to the interface between the semiconductor and the insulator.

Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the [[threshold voltage]]. When the voltage between transistor gate and source (''V''<sub>G</sub>) exceeds the threshold voltage (''V''<sub>th</sub>), the difference is known as [[overdrive voltage]].

This structure with p-type body is the basis of the n-type MOSFET, which requires the addition of n-type source and drain regions.