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Advanced Electronics - Field Effect Transistor (FET)

Junction Field Effect Transistor Tutorial

Junction Field Effect Transistor

The field effect transistor (FET) is made of a bar of N type material called the SUBSTRATE with a P type junction (the gate) diffused into it. With a positive voltage on the drain, with respect to the source, electron current flows from source to drain through the CHANNEL.

If the gate is made negative with respect to the source, an electrostatic field is created, which squeezes the channel and reduces the current. If the gate voltage is high enough the channel will be "pinched off" and the current will be zero.

The FET is voltage controlled, unlike the transistor which is current controlled.

This device is sometimes called the junction FET or JUGFET or JFET. If the FET is accidentally forward biased, gate current will flow and the FET will be destroyed.

To avoid this, an extremely thin insulating layer of silicon oxide is placed between the gate and the channel. The device is then known as an insulated gate FET, or IGFET or metal oxide semiconductor FET (MOSTFET)

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