What is a field effect transistor (FET)?

A field effect transistor (FET) is a type of transistor that uses an electric field at its gate to control the current flowing between its source and drain terminals. Among all transistors, FET devices stand out because the gate draws almost no current. FETs are also unipolar devices, relying on only one type of charge carrier — electrons in N-channel devices or holes in P-channel devices.

For the wider family of transistors that FETs belong to, see what is a transistor.

How does a FET work?

A FET works by using the voltage on its gate to set up an electric field inside the device. In a MOSFET, the gate sits on a thin insulating oxide above the channel — the origin of the name metal-oxide-semiconductor field effect transistor. When the gate voltage passes the threshold, a conducting channel forms in the semiconductor material and current flows from drain to source. In a JFET the same field effect is produced instead by a reverse-biased junction.

Either way, the gate sets the current without drawing meaningful current itself. 

Main types of FETs

This is a short introduction to field effect transistors and their main families. There are several types of field effect transistors, but two dominate real-world designs: the MOSFET and the JFET. Both are voltage-controlled, three-terminal devices that differ mainly in how the gate is built. Other types of FETs — such as the MESFET and HEMT — handle specialised high-frequency requirements.

A MOSFET is also called an insulated-gate field effect transistor (IGFET), after its insulated metal gate, while the related IGBT (insulated-gate bipolar transistor) blends FET and bipolar behaviour for high-power switching.

MOSFET: a voltage-controlled semiconductor

A MOSFET is a voltage-controlled device in which an electric field at the gate controls the current between the source and drain terminals. The gate is isolated from the semiconductor channel by a thin layer of silicon dioxide, so no DC current flows into it. Applying a voltage above the gate threshold voltage VGS(th) creates a conduction channel, allowing drain-to-source current to flow.

Because the gate draws no steady-state current, MOSFETs are far more efficient to drive than BJTs — particularly at high switching frequencies where driver losses accumulate rapidly.

N-channel MOSFETs are preferred in most power applications. For equivalent die area, N-channel devices achieve lower on-resistance (RDS(on)) than P-channel equivalents, because electrons have higher mobility than holes in silicon. P-channel MOSFETs are used in high-side switching without a bootstrap circuit, and in complementary half-bridge and H-bridge topologies.

MOSFETs come in two major types:

Structurally, a MOSFET is a metal-oxide-semiconductor FET built on a doped substrate: an N-channel FET, for example, forms its channel in P-type semiconductor material on a P-type substrate.

JFET (junction field effect transistor)

A JFET (junction FET, short for junction field effect transistor) is also voltage-controlled, but uses a reverse-biased PN junction as its gate rather than an insulated oxide. This makes JFETs depletion-mode devices — they conduct by default and are turned off by applying a gate voltage. They are available in N-channel JFET and P-channel versions, with the N-channel type being the more common.

Junction field effect transistors are not widely used in power electronics, but their very low noise and high input impedance make them the go-to choice for sensitive analog work, where signal integrity matters more than switching efficiency.

Because a JFET conducts at zero gate voltage, it behaves much like a depletion-mode MOSFET — both are "normally on" devices that need a gate voltage to turn off, which is why the two are often compared.

Characteristics of field effect transistors

Although different types of FETs vary in construction, they share a set of defining characteristics:

  • Voltage control — the gate-source voltage sets the channel current, while the gate itself draws almost no current
  • Very high input impedance — the gate barely loads the stage driving it
  • Unipolar operation — current is carried by only one type of charge carrier
  • Fast switching — low gate-drive power allows operation at high frequencies

In operation, a MOSFET moves through three regions:

    • Cutoff: Gate voltage below threshold voltage VGS(th); device is OFF
    • Linear (ohmic) region: Device behaves like a variable resistor.
    • Saturation region: Device supports constant current flow for amplification.

    Applications of field effect transistors

    FETs appear across almost all of modern electronics because they switch efficiently, switch quickly, and present a very high input impedance. MOSFETs are the dominant choice for power and digital electronics, while JFETs fill specialized roles in sensitive analog applications.

    MOSFETs dominate in:

    • Power switching and DC-DC conversion
    • Motor control (H-bridges, inverters)
    • Digital logic (CMOS)
    • Battery management systems
    • High-frequency applications (hundreds of kHz to MHz)

    JFETs, by contrast, are chosen for low-noise analog work — such as audio preamplifiers, instrumentation, and high-impedance sensor front-ends — where signal integrity matters more than switching power.

    For the power side in more detail, see how FETs used in power electronics below.

    Advantages and disadvantages of FETs

    FETs bring a strong set of advantages, balanced by a few trade-offs that mostly matter at the extremes of cost, speed, and power. 

    Compared with BJTs, FETs offer the following advantages and disadvantages:

    Advantages of FET

    • Faster switching speeds
    • Very high input impedance, so they barely load the previous stage
    • Low gate-drive power requirements
    • Low on-resistance (MOSFETs) for high efficiency as switches
    • Easy integration — billions of FETs fit on a single CMOS chip

    Disadvantages of FET

    • Sensitive to electrostatic discharge (ESD) — the thin gate oxide of a MOSFET is easily damaged by static
    • The gate capacitance must be charged and discharged every cycle, which costs energy at very high switching frequencies
    • Often more expensive than a simple BJT for low-power, low-cost tasks
    • Performance such as on-resistance (RDS(on)) shifts with temperature, so high-power designs need careful thermal management

    Which side weighs more depends on the application: for high-frequency power conversion and digital logic the advantages clearly dominate, while for a simple, low-cost, low-power switch a BJT can still be the better choice.

    Recommended documents

    Frequently asked questions about Field Effect Transistors (FETs)

    What are the main terminals in FETs (gate, source, and drain)?

    For MOSFETs and JFETs:

    • Gate (G): voltage-controlled input
    • Source (S): reference terminal
    • Drain (D): output current terminal

    FETs are among the most important electronic components in modern technology. As compact, efficient switches they form the logic in digital circuits — every processor and memory chip is built from MOSFETs in CMOS technology.

    As power devices they make energy conversion efficient across a huge range of electronic devices and electronic applications, from phone chargers to electric-vehicle drivetrains.

    Because they can be manufactured by the billion on a single chip, FETs underpin essentially all of today's computing and power electronics.

    A bipolar junction transistor (BJT) conducts using both electrons and holes, whereas unipolar FETs (MOSFETs, JFETs) use only one carrier type. The practical consequence is that a BJT is current-controlled while a FET is voltage-controlled — giving FETs near-zero gate current, very high input impedance, and faster switching, while BJTs keep an edge in low-cost, low-power roles.

    For the full side-by-side comparison, see MOSFET vs Transistor (BJT).

    Both are voltage-controlled FETs, but a JFET gate is a PN junction (normally on, depletion mode) while a MOSFET gate is an insulated oxide layer (normally off, enhancement mode). MOSFETs are more versatile; JFETs are preferred in ultra-low-noise analog circuits.

    MOSFET transistors dominate many power electronics systems, including:

    • DC-DC converters
    • Motor control circuits
    • H-bridge drivers
    • Battery management systems

    Because MOSFETs switch efficiently at high frequencies, they are widely used in energy-efficient power conversion systems.