Mosfets

Mosfet stands for metal oxide semi-conductor field effect transistor. MOSFETS can be n type or p type as for JFETS and each type is available in enhanced or depleted mode. The most popular is th n type enhanced mode version and we explain how this works below.

The diagram shows an n channel enhanced mode MOSFET as having four terminals. Most have just three terminals with the body connected to the source internally. The p type body material is very lightly doped whilst the n material is heavily doped.

The gate terminal is connected to a metal plate that has between it and the body a very thin layer of the insulator that is silicon oxide. The construction is such that electrons have moved across the n to p junctions to form energy stable depletion zones that are wider in the p region because of its light doping. So if we apply a voltage between drain and source no current will flow.

When we apply a voltage to the metal plate of the gate no current will flow because of the insulator but it does draw high numbers of electrons out of the plate creating many locations with desires for the energies that electrons can give them. Those desires are in the form of photon energy flows that reach through the insulator and into the source, drain and body materials encouraging electrons to move toward those energy desires.

The body p material also desires electrons and the source and drain have lots of them. The voltage produced desire at the gate means more electrons move from source and drain into the p material beneath the silicon oxide layer creating a widening and extending depletion area there. At what is termed the threshold gate voltage a depletion zone exists between drain and source and much of it has electrons in excess of its needs. Any further increase in gate voltage and the p material depletion zone has an excess of electrons throughout its length and as such is an effective conductor that will allow current to pass from drain to source.

As with the JFET, the drain to source current draws electrons out of the drain and pushes them into the source and so shapes the p depletion zones with their electron excesses as per the diagram. As for the JFET a saturation current results .

Though not as popular as enhanced mode MOSFETS, both p and n versions are available in depletion mode types. Such devices are doped at the time of manufacture so that a current flow channel exists between drain and source. Circuit symbols for the enhanced type are as in the diagram with a note explaining how they are changed to indicate a depletion mode mosfet.

The manufactured doping of the depletion mode type means that even with no voltage between gate and source a current will flow between drain and source if a voltage is applied between them. An increasing positive voltage on the gate will increases this current flow whilst a negative voltage on the gate will diminish that current and if sufficiently negative stop such flow.

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