Why is silicon oxide used in MOSFETs

MOS field effect transistor (MOS-FET)

The designation MOS means Metal-Oxide-Semiconductor, which means something like metal-oxide-semiconductor component. The MOS-FET is also known as an IG-FET. This name comes from Insulated Gate and means insulated gate. This has to do with the structure of the MOS-FET.

There are a total of 4 MOS-FET types, a distinction being made between n-channel and p-channel, as well as enhancement and depletion types.

Structure of a MOS-FET

Basic structure of a MOS-FET (n-channel enhancement type).

The n-channel enhancement type field effect transistor consists of a p-conducting crystal, the substrate. Two n-conductive islands are doped into the substrate. The crystal is covered with silicon dioxide (SiO2) covered (insulating layer). However, the n-conducting islands are still exposed and led to the outside via contacts (S and D). An aluminum layer (Al) is vapor-deposited on the silicon dioxide as a gate electrode.

How a MOS-FET works (enhancement type)


This description of the MOS-FET refers to the enhancement type. There is also the impoverishment type.
The MOS-FET is always in the blocking state (therefore self-blocking) when there is no positive voltage between the gate and source connection.
If a positive voltage UGS applied, an electric field is created in the substrate. The electrons are attracted to the gate connection from the p-conducting substrate (many holes, very few electrons). They migrate to under the silicon dioxide (insulating layer).
The holes migrate in the opposite direction. The zone between the n-conducting islands mainly contains electrons as free charge carriers. There is now an n-conducting channel between the source and drain connection.
The conductivity of this channel can be determined by the gate voltage UGS Taxes.
The increase in the positive gate voltage leads to an enrichment of the channel with electrons. The channel becomes more conductive. The reduction in the positive gate voltage leads to a depletion of the channel with electrons. The channel becomes less conductive.
Because the silicon dioxide layer has an insulating effect between aluminum and substrate, no gate current I flowsG. Only one gate voltage U is used for controlGS needed. The control of the current ID. through the MOS-FET takes place without power.

Impoverishment principle

The MOS-FET described above is an enhancement type. It is self-locking. But there are also MOS-FETs as depletion types. They are self-conducting because they are already after voltage UDS are conductive. This is created by a weak n-doping between the n-conducting islands (source and drain). This MOS-FET only blocks completely when the gate voltage UGS is more negative than the voltage at the source connection.
The normally on MOS-FET is activated by a negative as well as a positive gate voltage UGS controlled.

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