Comprehensive Introduction to MOSFET

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-driven device controlled by an electric field. It serves as a core component in both integrated circuits and discrete devices, and is widely used in various analog and digital circuit systems due to its simple structure, fast response, and ease of integration.

The development of MOSFET has significantly advanced technologies in power electronics, power management, computer chip design, and automotive electronics.

1. Detailed Structure

The core structure of a MOSFET consists of four basic regions:

• Source: The point where electrons or holes are injected; it serves as the current input terminal.
• Drain: The terminal through which carriers exit; it is the current output terminal.
• Gate: Controls whether a conductive channel forms by applying an external voltage; acts as a switch. It is typically covered by a thin oxide layer (SiO₂) with excellent insulation properties.
• Substrate: The foundational semiconductor material of the device, usually lightly doped P-type or N-type silicon.

Substrate Type and Channel Relationship:

• NMOS: P-type substrate + N-type source/drain; electrons are the conduction carriers.
• PMOS: N-type substrate + P-type source/drain; holes are the conduction carriers.
• Difference Between Enhancement and Depletion Modes:
• Enhancement Mode: Normally off; requires gate voltage to conduct.
• Depletion Mode: Normally on; becomes non-conductive when gate voltage is applied.

2. Operating Principle (Field-Controlled)

The operation of a MOSFET is primarily based on the field effect, which controls the distribution of charge carriers in the semiconductor.

• Enhancement-Mode NMOSFET: When a positive voltage is applied to the gate, the P-type substrate beneath the gate attracts electrons due to the electric field, forming an N-type inversion layer and establishing a conductive channel between source and drain.
• Enhancement-Mode PMOSFET: When a negative voltage is applied to the gate, holes are attracted to the surface of the N-type substrate, forming a P-type channel that allows conduction between source and drain.

Controlling On and Off States:

The switching behavior of a MOSFET is determined by the gate-source voltage (VGS):

When VGS ≤ Vth (threshold voltage), the channel forms and the device turns on.

When VGS ＜ Vth, the channel vanishes and the device turns off.

3. Major Classification Methods

MOSFETs can be categorized in various ways based on different criteria:

NMOS devices are more widely used in high-frequency and high-performance applications due to their higher electron mobility and lower on-resistance.

4. Typical Application Areas

MOSFETs are suitable for various scenarios ranging from low-power control to high-voltage driving:

• Digital Circuits and Logic Design: CMOS logic gates, composed of complementary NMOS and PMOS, form the foundation of microprocessors, microcontrollers, and memory chips.Modern SoCs (System-on-Chip) integrate billions of MOSFETs.
• Power Supply and Power Control: As efficient switching devices, they are widely used in DC-DC converters, switching power supplies (SMPS), inverters, and LED drivers.They support soft switching, low EMI, and high-efficiency designs.
• Analog Circuits: MOSFETs are used as voltage amplifiers, constant current sources, and analog switches.They perform excellently in audio amplifiers and power feedback loops.
• Automotive and Industrial Control: MOSFETs are widely used in automotive ECUs, motor controllers, and onboard chargers (OBCs).They are also applied in drive modules, PLCs, and relay replacements in industrial automation.
• Sensors and Detection Systems: MOSFETs are used to build signal reading circuits in CMOS image sensors, infrared sensors, and MEMS sensors.They serve as front-end amplifiers for high-sensitivity signal detection.

5. Conclusion

With excellent electrical performance, simple structure, and high controllability, MOSFETs have become indispensable components in the electronics industry.

From consumer electronics and communication equipment to industrial automation and new energy electric vehicles, their applications are ubiquitous.

By understanding their structure, principles, and application features, one can more effectively complete device selection, system design, and product optimization.