Working principles of MOSFET

What is a MOSFET?

Mosfet Metal (or poly-silicon doped heavily to act like a metal)• Oxide (SiO2, Acts as an insulator.) . Semiconductor (One can selectively change the carrier type to n-type or p-type.) Field Effect (Device is controlled by an electric field as opposed to current.) Transistor (Three terminal device)


mosfet structure.JPG

(a) When VGS (Gate-source voltage) is not supplied
(b) When VGS (Gate-source voltage) is supplied

mosfet structure1.JPG

Figure 3: The Structure of an Enhancement Type MOSFET and its Operation

(a) When VGS (Gate-source voltage) is not supplied
(b) When VGS (Gate-source voltage) is supplied

The advantages of the lateral MOSFET are:

1. Low gate signal power requirement. No gate current can flow into the gate after the small gate oxide capacitance has been charged.

2. Fast switching speeds because electrons can start to flow from drain to source as soon as the channel opens. The channel depth is proportional to the gate volage and pinches closed as soon as the gate voltage is removed, so there is no storage time effect as occurs in bipolar transistors.

The major disadvantages are

1. High resistance channels. In normal operation, the source is electrically connected to the substrate. With no gate bias, the depletion region extends out from the Nadrain in a pseudo-hemispherical shape. The channel length L cannot be made shorter than the minimum depletion width required to support the rated voltage of the device.

2. Channel resistance may be decreased by creating wider channels but this is costly since it uses up valuable silicon real estate. It also slows down the switching speed of the device by increasing its gate capacitance.

MOSFET Experiment - Video:


MOSFET - Presentation:

[slideshare id=1583661&doc=metal-oxide-semiconductorfetmosfet-090615015822-phpapp02]

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