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MOSFET Basic working principle?
Questions about MOSFET
By Anonymous on 28 March, 2005 - 7:26 pm

Dear all,

Can anyone explain to me basic working principle of MOSFET (Metal oxide semiconductor field effect Transistor)?

Does mosfet has the same principle like any other Transistor, where it function as a switch when there is voltage input given on them and the switch is active?

What cause Mosfet active? & what is the differences with ordinary transistor?

Thank You very much....

By Thomas Hergenhahn on 29 March, 2005 - 9:31 pm

It takes too long to answer all your questions. Physical principal is completely different between bipolar "normal" transistor and MOSFET. Both are basically not switches but current (bipolar) or voltage (FET) controlled current sources.

Please see Google for some page about semiconductor basics.

By Steve Myres, PE on 29 March, 2005 - 11:17 pm

The MOSFET has a "gate" terminal as the control instead of a "base" terminal in a bipolar transistor. Little current flows in the gate, control is by virtue of the electrical field generated in the gate (hence "field effect"), so the FET is primarily a voltage controlled device, as opposed to a transistor, which responds to current in the base.

By controlcraft on 29 March, 2005 - 11:48 pm

An FET is what is known as a gate device, vs. a junction device. In a junction transistor, the flow of current crosses junction boundaries between differently doped semiconductor materials (p or n types). Junction effects can distort signal quality and limit power.

On the other hand, an FET is controlled by regulating the electrical field across a channel, like pinching or opening a hose and preventing or allowing flow, respectively. Because of this property, FETs are great for large current flow. They can also be designed to be biased to close or open, so that a bias to the gate electrode will either stop or enable current flow.

By Priya Dharsini on 10 September, 2007 - 9:46 am

Dear all,

MOSFET is Metal Oxide field effect Transistor. It consists of source, drain and gate. V can look like a communication system where source is the transmitter and drain is the receiver and gate to be the channel. V supply voltage to all the three juctions and a depletion layer is formed below the source and drain while positive voltage is applied to the gate. The positive charges will repel on the substrate layer and there forms the channel. And then conduction takes places from source to drain.

There is a lot to be explained...

By Eric Thielking on 11 September, 2007 - 12:28 am

As was stated above:

MOSFET is a voltage controlled current device. Great for using to control larger loads, with a just a low power voltage.

Voltage on the gate, changes current through the drain-source circuit.

High input impedance, low output impedance.