

How Does a FET Transistor Work? Principle, Types & Differences
A Field-Effect Transistor (FET) is a semiconductor device widely used for controlling current in electronic circuits. FETs operate by applying an electric field to a channel within the device, which affects its conductivity and regulates how much current can flow between two terminals called the source and drain.
Compared to other types of transistors, FETs offer high input impedance and low noise, making them suitable for amplification and switching purposes. Understanding how FETs work is essential for students learning about electronics and semiconductor devices.
What is a Field-Effect Transistor (FET)?
A FET has three terminals: the source (S), the drain (D), and the gate (G). The current flows between source and drain through a semiconductor channel. The gate terminal is used to control this flow.
Applying a voltage to the gate produces an electric field, which either attracts or repels charge carriers (electrons or holes) within the channel. Whether carriers are attracted or repelled depends on the polarity of the gate voltage.
This electric field changes the channel's conductivity. By adjusting the gate voltage, one can modulate and control the current passing between the source and drain, making the FET an important voltage-controlled device in electronics.
Characteristics and Operation of FETs
FETs are known as unipolar devices because the current is carried by only one type of charge carrier (either electrons or holes), unlike bipolar transistors which use both. This property often results in simpler and more stable operation, with reduced noise.
A key feature of FETs is their very high input impedance. This means they draw minimal current from the input signal, preventing signal loss and distortion. This makes FETs ideal for use as amplifiers and buffers in sensitive circuits.
FETs are also efficient for low-power and battery-operated devices due to their minimal power requirement at the gate terminal.
Feature | FET | Bipolar Junction Transistor (BJT) |
---|---|---|
Type | Unipolar (one charge carrier) | Bipolar (two charge carriers) |
Control Mechanism | Voltage-controlled (gate voltage) | Current-controlled (base current) |
Input Impedance | Very high | Low to moderate |
Noise | Low | Higher |
Types of Field-Effect Transistors
There are two main types of FETs: JFET (Junction Field-Effect Transistor) and MOSFET (Metal Oxide Semiconductor Field-Effect Transistor).
- JFET: Has a channel of semiconductor material, with source and drain at either end. The gate is a PN junction placed perpendicular to the channel. Applying a voltage to the gate widens the depletion region, narrowing the channel and reducing current. The channel type can be n-channel or p-channel, depending on which type of charge carrier is used.
- MOSFET: Similar in layout, but the gate is separated from the channel by a thin insulating layer (often silicon dioxide). The electric field from the gate changes the channel's conductivity without direct contact. MOSFETs are divided into two further subtypes:
• Enhancement-mode: Channel is normally off; a voltage is required at the gate to turn the device on.
• Depletion-mode: Channel is normally on; applying a voltage to the gate can reduce or switch off the channel current.
The choice between JFET and MOSFET depends on the application’s power and speed requirements. For more details, visit MOSFET resource and JFET overview.
Type | Control Element | ON/OFF Nature | Usage |
---|---|---|---|
JFET | PN junction gate | Current controlled by depletion layer | Amplifiers, analog switches |
MOSFET (Enhancement) | Insulated gate | Normally OFF | Digital circuits, fast switches |
MOSFET (Depletion) | Insulated gate | Normally ON | Special analog, RF circuits |
Advantages and Limitations of FETs
- Very high input impedance—ideal for amplifying weak signals in measurement devices.
- Low noise—makes FETs suitable for low-noise amplifiers.
- Low power consumption—important for battery-powered devices.
- Fast switching speed—enables use in digital circuits and power supplies.
- Good temperature stability—performance remains consistent across temperature changes.
- Certain MOSFETs can handle high voltages—suited to power electronic applications.
However, FETs can be limited by sensitivity to static electricity, high input capacitance (affecting bandwidth), and lower power handling than some other transistors. Their gain is also typically lower than that of bipolar transistors, making them less suited for some high-gain applications.
Stepwise Approach to FET Problem Solving
- Identify the FET type (JFET or MOSFET) and channel type (n or p).
- Write relevant relationships for channel current (use manufacturer or textbook equations as applicable).
- Apply correct polarity and magnitude for the gate voltage as per the question.
- Use the voltage or resistance information to calculate output characteristics.
- Interpret the result, focusing on the impact of high input impedance and operational mode.
Key FET Formulas
Formula | Description |
---|---|
ID = IDSS [1 − (VGS / VGS(off))]2 | JFET drain current equation (parabolic dependence on gate voltage) |
gm ∝ ID (varies with channel & gate voltage) | Transconductance relationship |
Example: Calculating JFET Drain Current
Suppose a JFET has IDSS = 8 mA and VGS(off) = -4 V. Find ID if VGS = -2 V.
Using ID = 8 [1 - (-2 / -4)]2 = 8 [1 - 0.5]2 = 8 × (0.5)2 = 8 × 0.25 = 2 mA.
Thus, the drain current is 2 mA for the given conditions.
Applications and Next Steps
- Use FETs as preamplifiers and voltage buffers in audio equipment.
- Implement FETs for fast electronic switching in digital devices.
- Choose FETs for high-fidelity or measurement instruments where input impedance and signal purity are critical.
- For more concepts, refer to Semiconductors overview and BJT topic for comparison.
Practice drawing and analyzing FET circuits to build confidence. Attempt problems from Vedantu practice sets for thorough preparation.
FAQs on Field Effect Transistor (FET) – Principle, Symbol, and Uses
1. What is an FET (Field Effect Transistor)?
An FET (Field Effect Transistor) is a type of semiconductor device that controls current flow using an electric field applied to the gate terminal.
- FETs have three main terminals: Source (S), Drain (D), and Gate (G).
- The current flowing from source to drain is modulated by the voltage applied to the gate.
- FETs are known for high input impedance and low noise, making them widely used in amplifiers and switching circuits.
2. How does a Field Effect Transistor (FET) work?
An FET operates by controlling the conductivity of a semiconductor channel using the electric field produced at the gate terminal.
- Applying a voltage to the gate generates an electric field that changes the width of the channel.
- This modulation controls current flow between the source and drain terminals.
- FETs are voltage-controlled devices, unlike BJTs which are current-controlled.
3. What are the main differences between FET and BJT?
FETs and BJTs differ in their working mechanism, input impedance, and carrier type.
- FET: Unipolar device (uses single type of charge carrier), voltage-controlled, high input impedance, low noise.
- BJT: Bipolar device (uses both electrons and holes), current-controlled, lower input impedance, higher gain but more noise.
4. What are the main types of FET?
The two major types of FETs are JFET (Junction Field Effect Transistor) and MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
- JFET: Gate is a p-n junction controlling channel width.
- MOSFET: Has an insulated gate and is used in digital circuits and power devices.
- Both can be further classified as n-channel or p-channel types.
5. What is the difference between JFET and MOSFET?
JFETs use a p-n junction gate, while MOSFETs use an insulated gate structure.
- JFETs have high input impedance and are mainly used in analog amplifiers.
- MOSFETs have even higher input impedance, support faster switching, and are widely used in digital circuits and power electronics.
- MOSFETs come in enhancement-mode and depletion-mode variants.
6. Why is FET called a unipolar device?
FETs are called unipolar devices because current conduction occurs due to one type of charge carrier only—either electrons (n-channel) or holes (p-channel).
This makes their operation simpler compared to bipolar devices, which use both electrons and holes for conduction.
7. What are the main applications of FET in electronics?
FETs are used in a wide range of applications due to their high input impedance and fast switching capability.
- Low-noise amplifiers and buffer amplifiers
- Switches in analog and digital circuits
- MOSFETs in microprocessor and memory circuits
- Voltage-controlled resistors in oscillators and measuring instruments
8. What is Shockley’s equation for JFET drain current?
Shockley’s equation expresses the drain current (ID) of a JFET as a function of gate-source voltage (VGS):
ID = IDSS[1 – (VGS / VGS(off))]2
- Where IDSS is the maximum drain current, VGS(off) is the cutoff voltage.
- This equation is critical for solving FET-based numerical problems in exams.
9. Why is FET preferred over BJT in some applications?
FETs are preferred when high input impedance, low noise, and thermal stability are required.
- FETs draw negligible input current, making them ideal for sensitive signal amplification.
- Lower noise levels improve performance in audio and measurement circuits.
- Better thermal stability helps ensure reliable operation over wide temperature ranges.
10. How can you identify the symbol and terminals of an FET in a circuit diagram?
FET symbol consists of three terminals—Source (S), Drain (D), and Gate (G)—with an arrow indicating channel type.
- For n-channel FETs, the arrow points inward (toward the channel).
- For p-channel FETs, the arrow points outward (away from the channel).
- Correctly labeling the terminals helps analyze and solve circuit questions accurately.
11. What are the advantages of FETs?
FETs offer several key advantages in electronics:
- Very high input impedance, reducing loading effect on previous stage
- Low power consumption makes them suitable for battery-powered devices
- Low noise operation ideal for sensitive amplifiers
- Fast switching speeds, especially in MOSFETs, benefit digital and power applications
12. What precautions should be taken while handling MOSFETs?
MOSFETs are sensitive to static electricity and can be damaged by improper handling.
- Always use antistatic wrist straps or grounding while handling MOSFETs.
- Avoid touching the gate terminal with bare hands.
- Store MOSFETs in antistatic bags and avoid high voltages near gate terminals.

















