MOSFET stands for metal oxide semiconductor field effect transistor. Like common transistors,it is also used as amplifying and switching device.The main difference from bi-polar transistor is that it’s gate (which is equivalent to base of a bipolar transistor) is electrically insulated.The metal-oxide insulation, commonly silicon di-oxide provides very high input resistance which makes the device suitable for different types of applications. MOSFETs comes in various packages.Here in this tutorial I am taking mosfet IRF450N as example. It’s a N-channel mosfet. Pinout is given on the datasheet.
To understand the working of a N-channel mosfet you have to make a very tiny and simple circuit. The source of the mosfet is connected with ground and the drain is connected to positive voltage source through a series current limiting resistor and a LED. The gate of the transistor is also connected to ground through a push button or switch. Note that , there is another resistance connecting gate of the mosfet and ground.This is called pull down resistor.It helps the circuit to avoid false triggering.
As you know that MOSFETs are voltage controlled Device.When the switch is open the gate of mosfet remains at ground potential because of the pull down resistor.In this condition mosfet does now allow current to pass from drain to source through load. As soon as the switch is closed gate of the mosfet gets positive voltage and turns on. Then only it allows current to pass through it. Here is a video demonstration for better understanding