The working principle and characteristics of field effect transistors

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The working principle and characteristics of field effect transistors

Posted Date: 2024-02-07

1. How FET works

Transistors that work on the principle of electric field control of current are called field effect transistors (FET: Field effect transistor). It forms a PN junction inside and has three terminals: D, S, and G. These three terminals are called drain (D), source (S), and gate (G) respectively. As shown in the figure, if a voltage is applied between the drain and the source, a current Id flows. Next, when a reverse voltage is applied between the gate and the source, the depletion layer expands, the current channel (channel) narrows, and the current is limited.

The current Id is called the drain current. By changing the voltage between the gate and the source, the thickness of the depletion layer can be changed, thereby changing the drain current.

2. Types and characteristics of FETs

As shown in the figure, an FET with an N-type semiconductor channel is called an N-channel FET, and an FET with a P-type semiconductor channel is called a P-channel FET. In an ordinary transistor, current is carried by electrons and holes, which "have two polarities", so it is called a bipolar transistor. In contrast, N-channel FET uses electrons to carry current, and P-channel FET uses holes to carry current, so it is also called a unipolar transistor.

Ordinary transistors are current-controlled components. In contrast, FETs are voltage-controlled components. FET has the characteristics of high input impedance and low noise.

In the figure, the gate uses a PN junction, so it is called a junction FET. In contrast, a field effect transistor in which a very thin oxide insulating film (SiO2) with good insulation properties is prepared on the surface of the semiconductor, and metal is evaporated to form the gate is called a MOS FET. The evaporation mentioned here refers to heating metal in a vacuum. The metal melts and evaporates and then adheres to the surface of the semiconductor to form a thin film. MOS is the abbreviation of Metal-Oxide Semiconductor, which means metal oxide semiconductor. The diagram lists the graphical symbols of junction FET and MOS FET.

Review Editor: Huang Fei

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