Field Effect Transistors (FETs) are unipolar semiconductor devices that use the effect of an electric field to control current flow. It has the characteristics of high input impedance, low noise, good thermal stability and simple manufacturing process. It is used in large scale and very large scale integrated circuits.

The working principle of the field effect transistor is said in one sentence, that is, "the ID flowing through the channel between the drain and the source is used to control the ID by the reverse biased gate voltage formed by the pn junction between the gate and the channel". To be more precise, the width of the path through which ID flows, that is, the cross-sectional area of the channel, is controlled by the change in the reverse bias of the pn junction, resulting in a change in the expansion of the depletion layer.

In the unsaturated region of VGS=0, because the expansion of the transition layer is not very large, according to the electric field of VDS applied between the drain and the source, some electrons in the source region are pulled by the drain, that is, from the drain A current ID flows to the source. The transition layer extending from the gate to the drain forms part of the channel as a plugging type, and the ID is saturated. This state is called pinch-off. This means that the transition layer blocks part of the channel, not the current being cut off.

Extended information:

1. Field effect tube can be used for amplification. Since the input impedance of the FET amplifier is high, the coupling capacitor can be small, and the electrolytic capacitor is not necessary.

2. The high input impedance of the FET is very suitable for impedance transformation. It is often used in the input stage of multi-stage amplifier for impedance transformation.

3. FETs can be used as variable resistors.

4. The FET can be conveniently used as a constant current source.

5. FETs can be used as electronic switches.

How FETs work

The working principle of the field effect transistor: the ID flowing through the channel between the drain and the source is used to control the ID by the reverse biased gate voltage formed by the pn junction between the gate and the channel.

Field Effect Transistor (Field Effect Transistor abbreviation (FET)) referred to as FET. There are mainly two types (junction FET—JFET) and metal-oxide semiconductor field effect transistor (metal-oxide semiconductor FET, referred to as MOS-FET). Conducted by majority carriers, also known as unipolar transistors.

It belongs to a voltage-controlled semiconductor device. It has the advantages of high input resistance (107~1015Ω), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown phenomenon, wide safe working area, etc., and has now become a powerful bipolar transistor and power transistor. competitor.