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What is MOSFET? MOSFET Structure and Working Principle.

What is MOSFET?

 

Metal oxide semiconductor field effect transistor (MOSFET, MOS-FET) is a field effect tube with an insulated gate. Features: There is a silicon dioxide insulating layer between the metal gate and the channel, which has a high input resistance. It is usually divided into N-channel and P-channel MOSFET. The three poles of the MOSFET: source (S), gate (G), and drain (D). The middle arrow indicate the substrate. If the arrow points inward, it indicates an N-channel MOSFET (NMOS), and the arrow outwards indicates a P-channel MOSFET (PMOS).

 

P-channel MOSFET

 

The ytterbium drain and source are heavily doped p + regions, and the substrate is n-type. Current flows due to the flow of positively charged holes (also called p-channel MOSFET). When we apply a negative gate voltage, the electrons existing under the oxide layer are repelled and then pushed down into the substrate, and the depletion region is occupied by the combined positive charge associated with the donor atom. The negative gate voltage also attracts holes from the p + source and drain regions into the channel region.

 p-channel-mosfet

N-channel MOSFET

 

The hafnium drain and source are heavily doped n + regions, and the substrate is p-type. Current flows due to the flow of negatively charged electrons (also called n-channel MOSFET). When we apply a positive gate voltage, the holes existing under the oxide layer are repelled, and the holes are pushed down into the combined negative charge associated with the acceptor atom. A positive gate voltage also attracts electrons from the n + source and drain regions into the channel, thereby forming electrons to the channel.

n-channel-mosfet

Working principle of MOSFET

 

There are two back-to-back PN junctions between the drain D and the source S of the enhanced MOSFET. When the gate-source voltage VGS = 0, even if the drain-source voltage VDS is added, there is always a PN junction in the reverse biased state, and there is no conductive channel between the drain and source (no current flows), so the drain at this time Current ID = 0.

 

At this time, if a forward voltage is applied between the gate and the source, that is, VGS> 0, an electric field is generated in the SiO2 insulation layer between the gate and the silicon substrate and the gate points to the P-type silicon substrate. The physical layer is insulated. The voltage applied to the gate VGS cannot form a current, and a capacitor is formed on both sides of the oxide layer. VGS is equivalent to charging this capacitor and forming an electric field. As VGS gradually increases, the gate is affected by the gate. The positive voltage attracts a large number of electrons on the other side of the capacitor and forms an N-type conductive channel from the drain to the source. When VGS is greater than the turn-on voltage VT of the tube (generally about 2V), The N-channel tube starts to conduct, forming a drain current ID. We call the gate-source voltage at the beginning of the channel formation as the turn-on voltage, which is generally represented by VT.

 

Controlling the size of the gate voltage VGS changes the strength of the electric field, and the purpose of controlling the drain current ID can be achieved. This is also an important feature of the MOSFET using the electric field to control the current, so it is also called a field effect tube.


Applicaions:


1. MOSFET are used in amplifiers. Because the input impedance of FET amplifiers is high, the coupling capacitance can be smaller, and no electrolytic capacitor is necessary.

2. DC motor

3. MOSFET can be used as variable resistors.

4. MOSFET can be conveniently used as a constant current source.

5. MOSFET can be used as electronic switches.

6. Inverter

7. Digital circuit

5. Passive components (resistors, capacitors and inductors)

6. Electronic DC Relay


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