MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is also called Metal Oxide Semiconductor Field Effect Transistor, referred to as MOS tube, which is a kind of field effect transistor. MOSFET has become the most widely used semiconductor device at present, and is widely used in various electronic equipment, including power supplies, computers, TVs, etc.

Structure of MOSFET

MOSFET basically consists of three parts: active region (source), drain region (drain) and gate region (gate). In an N-channel MOSFET, on a P-type substrate (substrate), N-type deposits form the source and drain regions, and an insulating material (usually silicon oxide) is deposited between them to form the gate. By changing the voltage of the gate, the carrier concentration in the channel is changed, thereby changing the conductance between the source and the drain.

How MOSFETs work

In an N-channel MOSFET, when the gate voltage (Vgs) is higher than the threshold voltage (Vth), an N-type conduction channel is formed between the source and drain. In this case, the electrons on the channel can freely flow from the source to the drain, and the entire device changes from the blocking state to the conducting state. When the source-drain voltage is large enough, even if the gate voltage is increased, the source-drain current will no longer increase, and the MOSFET is in a saturated state at this time.
Classification of MOSFETs
According to the channel material type and the insulating gate type, there are two types: N-channel and P-channel; according to the conduction mode: MOS tubes are divided into depletion type and enhancement type, so MOS field effect transistors are divided into N-channel depletion type and enhancement type. Type; P-channel depletion type and enhancement type four categories: N-channel consumption type, N-channel enhancement type, P-channel consumption type, P-channel enhancement type.

Main Characteristics of MOSFETs

High input impedance: There is an insulating layer between the gate electrode of the MOS tube and the source and drain regions, and only a weak gate current, so the input impedance of the MOSFET is very high, close to infinity.

Low output impedance: Since the MOSFET is a voltage-controlled device, the current between its source and drain can change with the change of the input voltage, so its output impedance is very small.

Constant current: When the MOSFET works in the saturation region, even if the source-drain voltage changes, its current is almost unchanged, so the MOSFET has a good constant current.
Applications of MOSFETs

Switching circuit: Because MOSFET has the characteristics of fast switching speed, low power consumption, and low driving voltage, it is widely used in switching circuits, especially in high-frequency switching power supplies.

Analog circuits: For example, in operational amplifiers, the high input impedance of MOSFETs is beneficial to increase the input impedance of operational amplifiers.

Digital circuits: MOSFETs are widely used in highly integrated digital logic circuits, microprocessors, memory chips, etc.

The above is a preliminary introduction to the basic knowledge of MOSFET. MOSFET is a deep and extensive knowledge, and there is still a lot of in-depth knowledge that needs to be understood through professional books and studies.