MOS tube structure principle

The advantage of the MOS
1. Great flexibility in circuit design, grid bias can be positive or negative or zero, triode can only work under positive bias, vacuum tube can only work under negative bias; In addition, the input impedance is high, which can reduce the load of the signal source and make it easy to match the front stage.
2. Can be used as an electronic switch;
3. It can be easily used as a constant current source;
4. Can be used as a variable resistance;
5. Very high input impedance is very suitable for impedance transformation and is often used for impedance transformation at the input stage of a multistage amplifier;
6. Can be used for amplification. Because of the high input impedance of field-effect tube amplifiers, the coupling capacitance can be small without the use of electrolytic capacitors;

Schematic diagram of MOS tube structure
Structure and symbol (take n-channel enhanced type as an example) -- Two high-concentration N-type areas are diffused on a low-concentration P-type silicon as drain and source, the semiconductor surface is covered with a silicon dioxide insulation layer and an electrode is drawn as a gate.

Fig.1

Other MOS tube symbols:

Fig.2 

Working principle (take N-channel Enhancement type as an example)

Fig.3

When V_GS=0, no matter what the polarity of V_DS is, there is always a PN junction that is inversely biased, so there is no conductive channel

V_GS = 0, I_D = 0

V_GS must be greater than 0 for the pipe to work

Fig.4

At V_GS >, an electric field perpendicular to the semiconductor surface is generated in Sio2 medium, repels multiple subholes in the P region and attracts fewer subelectrons. When THE V_GS reaches a certain value, the P-zone surface will form an inverse layer to communicate the N-zone on both sides and form a conductive channel

V_GS > 0 → g attracts electrons → invert layer → conductive channel

V_GS↑ → invert layer thickening → V_DS↑ → I_D↑

Fig.5

↑_D↑ → I_DS is small: V_DS when V_T ≥ V_GSV

V_T: Voltage on, V_GS when conduction begins under the action of V_DS, V_T = V_GS -- V_DS

Fig.6

increases to a certain value, the channel near the drain is clipped to form a clipping zone._DS is 0 and V_GSWhen V

V_DS↑ → I_D stays the same

MOS transistor step-down circuit
In the figure, Q27 is an N-channel MOS tube. The 1-pin of U22A outputs Q27 conduction at high voltage, and the V_CC-D_DR memory voltage is reduced to obtain the power supply of 1.2V-HT bus, while the 1-pin of U22A outputs Q27 at low voltage, and the voltage of 1.2V_HT bus is 0V.

Fig.7

MOS tube applications
1. Industrial field, stepper motor drive, electric drill tools, industrial switching power supply
2. New energy field, photovoltaic inverter, charging pile, UAV
3. In the field of transportation, on-board inverters, car HID stabilizers, electric bicycles
4. Green lighting field, CCFL energy-saving lamp, LED lighting power supply, metal halide lamp ballast