MOSFET (Metal-Oxide-Semiconductor FET)

MOSFET (Metal-Oxide-Semiconductor FET)

What is a MOSFET: -

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled, unipolar semiconductor device that controls the flow of current by applying voltage to the gate terminal. It is the most widely used type of field-effect transistor (FET), especially in digital and switching applications.

MOSFETs are foundational in modern electronics, used in microprocessors, memory chips, power supplies, amplifiers, and more. What makes MOSFETs special is their insulated gate, which offers very high input impedance, low power consumption, and excellent switching speed.

Structure of a MOSFET: -

MOSFETs consist of four terminals:

• Gate (G): Controls the current flow.
• Source (S): Provides carriers (electrons or holes).
• Drain (D): Collects carriers.
• Body (B): Substrate, usually connected to the source internally.

Layers: -

• The gate is separated from the channel via a skinny insulating layer of silicon dioxide (SiO₂).
• Under the oxide layer is the semiconductor channel, usually silicon.
• Metal contacts are used for gate, supply, and drain terminals.

Channel Types:

• n-kind channel: Electrons bring the present day.
• P-kind channel: Holes deliver modern-day.

Types of MOSFET: -

MOSFETs are categorised based on channel type and mode of operation:

1. Enhancement Mode MOSFET

1. No channel exists initially.
2. A voltage must be applied to create a conductive channel.
3.  Common in digital circuits.

2. Depletion Mode MOSFET

• A channel exists at zero gate voltage.
• Applying a voltage depletes the channel.
• Less commonly used than enhancement types.

Classification Table: -

Type	Channel	Mode
N-Channel Enhancement	Electrons	Enhancement
P-Channel Enhancement	Holes	Enhancement
N-Channel Depletion	Electrons	Depletion
P-Channel Depletion	Holes	Depletion

Operating Regions of MOSFET: -

A MOSFET is seen to exhibit three operating regions. Here, we will discuss those regions.

Cut-Off Region

The cut-off region is a region in which there will be no conduction, and as a result, the MOSFET will be OFF. In this condition, the MOSFET behaves like an open switch.

Ohmic Region

The ohmic region is a region where the current (I_DS)increases with an increase in the value of V_DS. When MOSFETs are made to operate in this region, they are used as amplifiers.

Saturation Region

In the saturation region, the MOSFETs have their I_DS constant in spite of an increase in V_DS , and this occurs once V_DS exceeds the value of the pinch-off voltage V_P. Under this condition, the device will act like a closed switch through which a saturated value of I_DS flows. As a result, this operating region is chosen whenever MOSFETs are required to perform switching operations.

MOSFET as a Switch: -

MOSFETs are commonly used as switches. The circuit below shows the configuration of the MOSFET when it is used as a switch.

In the circuit arrangement, an enhancement-mode N-channel MOSFET is used to switch a simple lamp “ON” and “OFF”. The input gate voltage V_gs is adjusted to an appropriate positive voltage to switch “ON” the device, and the voltage level is set to a negative value or zero to turn it “OFF”.

The switching characteristics for both N-channel and P-channel type MOSFETs are summarised in the table below:

MOSFET TYPE	VGS<<0	VGS=0	VGS>>0
N-channel Enhancement	OFF	OFF	ON
N-channel Depletion	OFF	ON	ON
P-channel Enhancement	ON	OFF	OFF
P-channel Depletion	ON	ON	OFF

Advantages of MOSFET: -

• High Input Impedance – Ensures negligible loading effect.
• Fast Switching Speed – Ideal for high-frequency circuits.
• Low Power Consumption – No gate current in steady state.
•  Scalability – Easily miniaturised in IC design.
• Thermal Stability – Less thermal runaway than BJTs.
•  Low Noise – Particularly important in audio applications.

Disadvantages of MOSFET: -

• Static Sensitivity – Especially enhancement types; can be damaged by ESD.
• Complex Drive Requirements – For high-speed switching, gate capacitance must be carefully managed.
• Lower Current Handling – Compared to BJTs (in similar package sizes).

Applications of MOSFET: -

MOSFETs are ubiquitous in electronic systems. Here are some key applications:

1. Digital Logic Circuits

Core building block in CMOS technology.

Used in processors, memory, FPGAs, microcontrollers.

2. Switching Power Supplies

Used in SMPS, DC-DC converters, inverters, and battery chargers.

3. Motor Controllers

Power MOSFETs control motors in robotics, drones, and EVs.

4. Audio Amplifiers

Linear MOSFETs are used for low-noise, high-fidelity amplification.

5. RF Amplifiers

High-speed MOSFETs work well in RF modulation/demodulation.

6. Analog Switches

Low ON resistance and high OFF resistance make MOSFETs ideal for analogue multiplexers and sample-and-hold circuits.

7. Protection Circuits

Used as ESD protection devices, overcurrent protection, and voltage clamping.

MOSFET vs BJT: -

Feature	MOSFET	BJT
Type	Voltage-controlled	Current-controlled
Input Impedance	Very high	Low
Speed	High	Moderate
Power Consumption	Low	Higher
Thermal Stability	Better	Lesser
Integration in ICs	Easier	Harder
Preferred Use	Digital & switching	Analog & linear gain

Real-World Examples: -

• Intel/AMD CPUs: Contain billions of MOSFETs in CMOS architecture.
• EV Battery Management Systems: Use MOSFETs for charge-discharge control.
• LED Drivers: Power-efficient MOSFETs used in dimmers and controllers.
• Home Inverters: Convert DC from batteries to AC using MOSFET bridges.
• Gaming Consoles: Use power MOSFETs for voltage regulation and speed control.

Conclusion: -

The MOSFET is certainly a modern tool in the subject of electronics. Its electricity performance, excessive-speed performance, and scalability have made it the spine of virtual and analogue circuit design. Whether you're managing a microprocessor, a motor driving force, or an energy inverter, it's rare that MOSFETs play an important role behind the curtain.

For electronics college students, hobbyists, and specialists alike, a deep knowledge of MOSFETs isn't just useful—it is critical. As electronics continue to evolve, the function of MOSFETs in allowing more compact, faster, and electricity-green technology will simply develop.