Definition

MOSFET (metal-oxide semiconductor field-effect transistor)

Part of the Electronics glossary:

Also see gallium arsenide field-effect transistor .

MOSFET (metal-oxide semiconductor field-effect transistor, pronounced MAWS-feht ) is a special type of field-effect transistor ( FET ) that works by electronically varying the width of a channel along which charge carriers ( electron s or hole s) flow. The wider the channel, the better the device conducts. The charge carriers enter the channel at the source , and exit via the drain . The width of the channel is controlled by the voltage on an electrode called the gate , which is located physically between the source and the drain and is insulated from the channel by an extremely thin layer of metal oxide.

There are two ways in which a MOSFET can function. The first is known as depletion mode . When there is no voltage on the gate, the channel exhibits its maximum conductance . As the voltage on the gate increases (either positively or negatively, depending on whether the channel is made of P-type or N-type semiconductor material), the channel conductivity decreases. The second way in which a MOSFET can operate is called enhancement mode . When there is no voltage on the gate, there is in effect no channel, and the device does not conduct. A channel is produced by the application of a voltage to the gate. The greater the gate voltage, the better the device conducts.

The MOSFET has certain advantages over the conventional junction FET, or JFET. Because the gate is insulated electrically from the channel, no current flows between the gate and the channel, no matter what the gate voltage (as long as it does not become so great that it causes physical breakdown of the metallic oxide layer). Thus, the MOSFET has practically infinite impedance . This makes MOSFETs useful for power amplifiers. The devices are also well suited to high-speed switching applications. Some integrated circuits ( IC s) contain tiny MOSFETs and are used in computers.

Because the oxide layer is so thin, the MOSFET is susceptible to permanent damage by electrostatic charges. Even a small electrostatic buildup can destroy a MOSFET permanently. In weak-signal radio-frequency ( RF ) work, MOSFET devices do not generally perform as well as other types of FET.

This was last updated in March 2011
Posted by: Margaret Rouse

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