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What is a MOSFET?

By John Sunshine
Updated May 16, 2024
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A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a semiconductor device. A MOSFET is most commonly used in the field of power electronics. A semiconductor is made of manufactured material that acts neither like a insulator nor a conductor. An insulator is a natural material that will not conduct electricity, such as a dry piece of wood. A conductor is a natural material that conducts or passes electricity. Metals are the most common examples of conductors. Semiconductor material from which devices like a MOSFET are made exhibit both insulation like properties and conduction like properties. Most importantly, semiconductors are designed such that the conduction or insulation properties can be controlled.

The transistor is perhaps the best known semiconductor device. Early transistors use a technology referred to as bi-polar material. Pure silicon can be doctored or "corrupted"--a process that is referred to as "doping". It is possible to make either p type (positive) material or n type (negative) material depending upon material used to "dope" or corrupt the pure silicon. If you combine p type material and n type material, you have a bipolar device. The transistor is a basic example of a bipolar device. The transistor has three terminals, the collector, the emitter, and the base. The current in the base terminal is used to control the flow of current between the emitter and the collector.

MOSFET technology is an enhancement on bipolar technology. Both n and p type material are still used but metal oxide insulators are added to provide some performance enhancements. There are still typically only three terminals but they now have the following names, the source, the drain, and the gate. The field effect portion of the name refers to the method used to control the electron or current flow through the device. The current is proportional to the electrical field developed between the gate and the drain.

One other very significant enhancement over bipolar technology is that a MOSFET has a positive temperature co-efficient. This means that as the temperature of the device increases its tendency to conduct current decreases. This feature allows the designer to easily use it in parallel to increase the system's capacity. A bipolar deice has the opposite effect. With MOSFET technology, devices in parallel will naturally share current between them. If one device tries to conduct more than its share it will heat up and the tendency to conduct current will decrease causing the current through the device to decrease until all devices are again sharing evenly. Bipolar devices in parallel, on the other hand, increase in temperature if one device starts to conduct more current. This means more current will switch to this device which will result in a further increase in temperature, and a further increase in current. This is a runaway condition that quickly destroys the device. For this reason it is much more difficult to connect bipolar devices in parallel and the reason MOSFET devices are now the most popular power semiconductor type transistor.

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Discussion Comments

By jlovley — On Feb 17, 2011

Does anyone know what is required to pass the ASTM electric scooter protocol F2641 section which states "Field Effect Transistor (FET) Short Sensing - Controller shall be able to compare power transistor versus speed controller/throttle to prevent any power to the motor, if and when the FET is shorted"?

I am designing an electric scooter, but cannot determine what ASTM is looking for. Any help would be greatly appreciated. The more detail, the better!

By jackcare — On Jun 16, 2009

would a mosfet be used as a water or moisture detector. the one in question is a disc the size of a penny with a paint like covering in the center, but it only has two connections: one to the center of the disc and one to the edge of the disc.

By anon26122 — On Feb 08, 2009

Yes that is a great description. I was still wondering what a *mosfet* actually does. From the above description it seems to control electrical current flowing through a circuit, but I am not clear on how it does that. Can any one explain that simply, particularly mentioning how the three terminals (the source, the drain, and the gate) are involved? Thanks heaps! Haydn

By anon13858 — On Jun 05, 2008

Superb. I went to Wikipedia to understand mosfets, and again was reminded that is had become useless to anybody wanting to learn something they don't already know.

Well written, packs all the fundamental info into few words. Thanks so much.

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