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GCSE     Semiconductor     Comparison     Diode     LED     >MOSFET<     Photodiode     Rectifier and PSU     Transistor     Zener Diode    

Semiconductor MOSFET


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Small changes in gate potential produce large changes in the current flowing from source to drain.

AS and A Level Only ...

IDrain   =   gM ( VGS - 3 )        3 volts are needed to turn on the MOSFET    2 to 4 V in real life.

P   =   I2 R   =   IDrain2 RDrain Source        MOSFET power or heat dissipation.

MOSFET Pin Names

Metal Oxide Semiconductor Field Effect Transistor

Here is a water analogy of a MOSFET. Pressure on the hose depletes the available water under your foot so the flow is cut off. The gate in the diagram below is your foot.

MOSFET Water Analogy Pinch Off

Field effect transistors are three pin devices. The pins are called the SOURCE, GATE and DRAIN. (Don't confuse these voltage controlled devices with bipolar transistors which are current controlled.)

This diagram shows an N Channel MOSFET. The insulation between the gate and the channel is very thin and easily destroyed by electrostatic discharge. Anti-static precautions need to be taken when handling MOSFETS.

MOSFET Structure

The gate potential causes an electric field in the gate region of the MOSFET. This potential alters the number of N Type carriers in the gate region.

P Channel MOSFETS are available too. They work in the same way but a negative potential causes enhancement.

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