MOSFET operation
Example application of the N Channel MOSFET. At the time when the switch is pushed the LED lights up.
Metal-oxide-semiconductor structure on P type silicon
Metal-oxide-semiconductor structure
A metal oxide semiconductor (MOS) structure is obtained by placing a layer of silicon dioxide (SiO2) on top of the silicon substrate and depositing a layer of metal or polycrystalline silicon (the latter is broadly used). As the silicon dioxide is a dielectric material, its structure is equivalent to a capacitor, with one of the electrodes replaced by the semiconductor.
At the time when the voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a P-type semiconductor (with NA the density of acceptors, p S density of holes; p = NA in neutral bulk), a positive voltage, VGB, from gate to body (see figure) creates a depletion layer by forcing positively charged holes away from gate insulator interface, leaving exposed a carrier free region of immobile, negatively charged acceptor ions (doping (semiconductor)). If VGB is high, a high concentration of negative charge carriers forms in the inversion layer located in a thin layer next to interface in between the semiconductor and the insulator. Unlike MOSFET, where the inversion layer electrons are supplied quickly from the source electrodes, in MOS capacitor they are produced much more slowly by thermal generation through the carrier generation and recombination centers in depletion region. On the other hand, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in body is known as threshold voltage.
This structure with P type body is basis of the N type MOSFET, which needs the addition of an N-type source and drain regions.
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