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(Metal Oxide Semiconductor Field Effect Transistor) The most popular and widely used type of field effect transistor (see FET). MOSFETs are either NMOS (n-channel) or PMOS (p-channel) transistors, which are fabricated as individually packaged discrete components for high power applications as well as by the hundreds of millions inside a single chip (IC).

The "MO" and "FE"
The "metal oxide" (MO) in MOS comes from the first devices that used a metal gate over oxide (silicon dioxide). Subsequently, poly-crystalline silicon was used for the gate, but MOS was never renamed. The "field-effect" (FE) in FET is the electromagnetic field that is generated when the gate electrode is energized, causing the transistor to turn on or off.

In NMOS transistors, the silicon channel between the source and drain is of p-type silicon. When a positive voltage is placed on the gate electrode, it repulses the holes in the p-type material forming a conducting (pseudo n-type) channel and turning the transistor on. A negative voltage turns the transistor off. With a PMOS transistor, the opposite occurs. A positive voltage on the gate turns the transistor off, and a negative voltage turns it on. NMOS transistors switch faster than PMOS.

When an NMOS and PMOS transistor are wired together in a complementary fashion, they become a CMOS (complementary MOS) gate, which causes no power to be used until the transistors switch. CMOS is the most widely used microelectronic design process and is found in almost every electronic product. See power MOSFET, n-type silicon, bipolar transistor, chip and FET.

MOSFETs in a Chip
Note the channel underneath the gate electrode. When voltage is applied to the gate, it creates a "field" with the opposite charge.

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