What is a MOSFET

MOSFET stands for ‘Metal Oxide Semi Conductor Field Effect Transistor’. It is a three terminal device that is used in electronic circuits such as voltage and power amplifiers, switch mode power supplies, constant current source etc. The three terminals of a MOSFET are Source, Drain and Gate.

To understand the working of a MOSFET, it is important first of all to know about the internal construction of the device.  Now there are two categories of MOSFET available in the market: – Enhancement only MOSFET (E-MOSFET) and Depletion Enhancement MOSFET (DE-MOSFET). These further come in two types – N-channel type and P-channel type.

The source and drain of an N-channel MOSFET are highly doped n-type regions diffused in a p-type substrate. Now these two regions can either be connected by a physical N-channel which is the case in DE-MOSFET or by an induced N-channel in the substrate due to the application of a positive voltage upon the gate terminal which is the case in E-MOSFET. The gate terminal in both cases is a strip of metal that is isolated from the channel by a layer of Silicon Dioxide. It is for this very reason that these devices are also known as Isolated Gate FET’s or IGFET’s. The construction of a P-Channel MOSFET is simply the replacement of N-type regions by P-type regions in the above description.

In normal mode of operation of an N-Channel MOSFET, the source is made negative with respect to the drain terminal. This causes a drain current to flow through the channel whose direction is from Drain to Source. Now application of a positive voltage on the gate terminal will increase the width of the channel and the drain current will increase while the application of a negative voltage on the gate terminal will decrease the width of the channel thereby reducing the drain current. In an E-MOSFET, as the channel is an induced one, the only mode of operation possible is that of a positive gate voltage i.e. drain current will flow only when the gate is kept at a positive potential.

In a MOSFET, a small fluctuation in gate voltage results in a large fluctuation in drain current. It is this property that makes a MOSFET useful for amplification purposes where the signal voltage can be applied on the gate terminal and an amplified faithful version of it can be obtained at the source or drain terminal.

Another property of MOSFET is that if the gate voltage is kept constant, then the drain current that increases with increase in drain-source voltage initially tends to reach a saturation point. Any further increase in drain-source voltage doesn’t raise the drain current. This makes the MOSFET useful as a constant current source.

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