MOS tube is a metal-oxide-semiconductor field effect transistor, or metal-insulator-semiconductor.

The source and drain of MOS transistor can be switched, and they are both N-type regions formed in P-type backgate. In most cases, the two regions are the same, and even if the two ends are reversed, the performance of the device will not be affected. Such a device is considered to be symmetrical.

Bipolar transistor amplifies a small change in input current and outputs a large change in output current. The gain of bipolar transistor is defined as the ratio of output to input current (beta).

Another transistor, called FET, converts the change of input voltage into the change of output current. The gain of FET is equal to its transconductance, which is defined as the ratio of the change of output current to the change of input voltage. N-channel and P-channel are common in the market, and low-voltage MOS tubes are common in P-channel.

FET affects the current flowing through the transistor by projecting an electric field on an insulating layer. In fact, no current flows through this insulator, so the GATE current of FET tube is very small.

The most common FET uses a thin layer of silicon dioxide as the insulator under the GATE electrode. This kind of transistor is called metal oxide semiconductor (MOS) transistor, or metal-oxide-semiconductor field-effect transistor (MOSFET). Because MOS transistors are smaller and more energy efficient, they have replaced bipolar transistors in many applications.

Advantages of MOS tube

It can be used for amplification. Because the input impedance of the FET amplifier is very high, the coupling capacitance can be small, and electrolytic capacitors are not needed.

Very high input impedance is very suitable for impedance transformation, and it is often used for impedance transformation in the input stage of multistage amplifier.

Can be used as a variable resistor;

Can be conveniently used as a constant current source;

Can be used as an electronic switch;

The flexibility in circuit design is great, the gate bias can be positive or negative or zero, the triode can only work under positive bias, and the electron tube can only work under negative bias; In addition, the input impedance is high, which can reduce the load of signal source and is easy to match with the previous stage.

Schematic diagram of MOS tube structure principle

Structure and symbol (taking N-channel enhancement as an example)-two N-type regions with higher concentration are diffused on a piece of P-type silicon with lower concentration as the drain and source, and the semiconductor surface is covered with silicon dioxide insulation layer and an electrode is led out as the gate.