Field Effect Transistor (FET)

What is a Field Effect Transistor (FET): -

A Field Effect Transistor (FET) is a voltage-controlled semiconductor device that uses an electric field to control the flow of current. Unlike Bipolar Junction Transistors (BJTs), which are current-controlled, FETs require very little input current, making them more energy-efficient.

The basic function of an FET is to control the conductivity of a semiconductor channel using an electric field generated by a voltage applied at its terminals.

Terminals of a FET: -

An FET consists of three main terminals:

• Source (S): The terminal through which carriers enter the channel.
• Drain (D): The terminal through which carriers leave the channel.
• Gate (G): Controls the flow of carriers by creating an electric field.

Some advanced FET configurations also have a fourth terminal called the Body (B) or Substrate, usually internally connected to the source in discrete components.

Types of FETs: -

FETs can be broadly classified into two major types:

1. Junction Field Effect Transistor (JFET)

• Uses a reverse-bias p-n junction to control current flow.
• Operates only in depletion mode.

Types:

• n-channel JFET
• p-channel JFET

2. Metal-Oxide-Semiconductor Field Effect Transistor(MOSFET)

Most widely used FET type.

• Uses an insulated gate (typically silicon dioxide).
• Operates in both enhancement and depletion modes.

Types:

• n-channel MOSFET
• p-channel MOSFET

FET Working: -

When a voltage is applied to the gate electrode, an electric field is created across the insulating layer, which in turn creates a depletion region inside the channel. The depletion region reduces the wide variety of free fee companies inside the channel, and as a consequence, the conductivity of the channel is reduced. This effect is called the sector impact, and it's the premise of the FET operation. In the case of an n-kind FET, a negative voltage is applied to the gate electrode, creating a depletion region in the channel, which reduces the go with the flow of electrons from the source to the drain. In comparison, a tremendous voltage is applied to the gate electrode of a p-type.

FET creates a depletion region that reduces the go with the flow of holes from the source to the drain. Thus, by way of varying the voltage applied to the gate electrode, the conductivity of the channel can be managed, and the glide of cutting-edge through the FET can be modulated.

To give an explanation for the operating principle of FET, the analogy of a water pipe and vessel may be used. In this analogy, the source of water can be taken into consideration because the source terminal of the FET, while the vessel that collects water can be analogous to the drain terminal of the FET. The gate terminal can be in comparison to the controlling tap that regulates the flow of water. Similar to how the tap modulates the drift of water, the voltage applied to the gate terminal controls the flow of contemporary from the supply to the drain terminal of the FET. Thus, the FET operates by controlling the glide of cutting-edge through the channel by way of modulating the wide variety of rate carriers in the channel and the usage of the voltage applied to the gate terminal.

Advantages of FETs: -

• High input impedance → minimal loading on the preceding stage.
• Low power consumption → ideal for battery-powered devices.
• Simple construction → easy integration in ICs.
• Thermal stability → better performance in varying temperatures.
• Noise immunity → suitable for sensitive applications.

Disadvantages of FETs: -

• Static sensitivity: Especially MOSFETs are vulnerable to static discharge.
• Lower gain compared to BJTs.
• Non-linear behaviour in some configurations.
• Limited bandwidth in high-frequency applications compared to BJTs.

Applications of FETs: -

1. Amplifiers

• FETs are ideal for small-signal amplification in audio and RF circuits.

2. Switching Circuits

• Used in logic gates, digital ICs, and switching regulators.

3. Analog Signal Processing

• In mixers, modulators, and sample-and-hold circuits.

4. Voltage Controlled Devices

1. Like voltage-controlled resistors, oscillators, and filters.

5. Integrated Circuits (ICs)

• MOSFETs are the foundation of CMOS technology in microprocessors and memory devices.

6. Power Electronics

• Power MOSFETs are used in motor controllers, inverters, and SMPS.

FET vs BJT: -

Feature	FET	BJT
Control	Voltage-controlled	Current-controlled
Input Impedance	High	Low
Power Consumption	Low	Moderate
Thermal Stability	Better	Poorer
Noise	Low	Higher
Applications	Digital, low-noise analog	High-gain analog

Real-World Examples: -

• Smartphones and Laptops: Billions of MOSFETs are used in processors and reminiscence.
• Television Sets: FETs used in tuner circuits and photo processors.
• Automobiles: Used in sensors, engine-manipulating devices (ECUs), and infotainment systems.
• Solar Inverters: Power MOSFETs switch DC to AC effectively.
• Industrial Robotics: FET-based motor drivers control speed and torque.

Conclusion: -

Field Effect Transistors (FETs) are the cornerstone of modern electronics. Their high performance, low power requirement, and versatility make them suitable for everything from primary signal amplification to complex virtual common sense. Understanding how FETs work and where they may be applied is critical for any pupil or engineer operating in electronics.

As we keep moving toward more compact, quicker, and power-green technology, FETs—especially MOSFETs—will remain at the heart of innovation. Mastering the fundamentals of FETs will give you a strong foundation for deeper exploration into analog digital circuit design.