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Definition: MOSFET

(Metal Oxide Semiconductor Field Effect Transistor) The most popular type of field effect transistor (see FET). MOSFETs are fabricated as a "discrete" transistor packaged individually for high power applications as well as chips with hundreds of millions of transistors.

The "MO" in MOS and "FE" in FET
The "metal oxide" (MO) 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) is the electromagnetic field that is generated when the gate electrode is energized, causing the transistor to turn on or off.

MOSFETs come in negative channel and positive channel varieties (see below). When NMOS and PMOS transistors are wired together, 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.

NMOS and PMOS Transistors
When voltage is applied to the gate in a MOSFET transistor, it creates a "field" with the opposite charge. In an NMOS transistor, holes are repulsed in the p-type silicon forming a conductive n-type channel, and current flows from source to drain. With PMOS, the opposite occurs, and voltage on the gate impedes the current flow. NMOS transistors switch faster than PMOS, but PMOS is more immune to noise. For more details, see n-type silicon.