1. A field effect tube (FET) is a semiconductor device that controls the electric field effect of the input loop to control the current of the output loop, and 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.

2. The working principle of the FET is "the ID of the channel flowing between the drain and source electrode, and the ID of the reverse-biased gate voltage controlled by the pn junction between the gate and the channel".

More correctly, the width of the ID flowing through the path, i.e., the channel cross-sectional area, is controlled by changes in pn junction reverse-bias, resulting in changes in depletion layer expansion. In the unsaturated region of VGS=0, the expansion of the represented transition layer is not large, and according to the electric field of VDS applied between the drain and the source electrode, some electrons in the source region are pulled away by the drain electrode, that is, there is a current ID flowing from the drain to the source electrode.

The transition layer extending from the gate to the drain will form part of the channel as a blocked type, ID saturated. This state is called pinch break. This means that the transition layer is blocking part of the channel, not cutting off the current.

Because there is no free movement of electrons and holes in the transition layer, it almost has insulation characteristics in an ideal state, and the current is usually difficult to flow. However, at this time, the electric field between the drain and source electrode is actually two transition layers near the lower part of the drain and gate, and the high-speed electrons pulled by the drifting electric field pass through the transition layer. The saturation phenomenon of ID is generated because the intensity of drift electric field is almost constant.

Secondly, the VGS changes in the negative direction, so that VGS=VGS(off), at which time the transition layer roughly becomes a state covering the whole area. Moreover, most of the electric field of VDS is added to the transition layer, and the electric field that pulls the electrons in the direction of drift is only a very short part near the source pole, which makes the current can not flow.

3. Function:

1. Field effect tubes can be used for amplification. Because the input impedance of the FET amplifier is high, the coupling capacitance 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 for impedance conversion in the input stage of multistage amplifiers.

3. Fets can be used as variable resistors.

4. Fets can be conveniently used as constant current sources.

5. Fets can be used as electronic switches.