What Are Field Effect Transistors?

Field effect transistors are three-terminal semiconductor devices that control the flow of current between a source and a drain terminal by varying an electric field applied at a gate terminal. Unlike bipolar junction transistors, which rely on minority carrier injection, FETs are majority-carrier devices: the gate voltage modulates the conductance of a channel without injecting current into it, giving FETs high input impedance, low power consumption in the off state, and favorable scaling characteristics as device geometries shrink.

The field effect principle was first proposed by Julius Lilienfeld in 1926, though practical devices did not emerge until the 1960s when planar silicon processing made it possible to reliably form thin insulating gate oxides. The metal-oxide-semiconductor FET (MOSFET) quickly became the dominant transistor type in digital integrated circuits, and it remains the device on which modern processor and memory fabrication is built. Compound semiconductor variants, including GaAs MESFETs and high-electron-mobility transistors (HEMTs), address applications at microwave and millimeter-wave frequencies where silicon MOSFETs cannot match the electron mobility of III-V materials.

Operating Principles and Device Types

All FETs share the same operating principle: a voltage applied to the gate creates an electric field that either attracts or repels charge carriers in the channel beneath it, either enhancing or depleting conductivity. Enhancement-mode devices are normally off and require a gate voltage to open the channel; depletion-mode devices are normally on and require a gate voltage to pinch the channel closed. The MOSFET, which places a thin silicon dioxide layer between the metal gate and the semiconductor, is the most common FET family. Junction FETs (JFETs) use a reverse-biased p-n junction instead of an oxide layer to deplete the channel. HEMTs form the channel at a heterojunction interface where the electron mobility is exceptionally high, making them the preferred device in low-noise amplifiers and power amplifiers at frequencies above 10 GHz. Detailed treatment of these device physics is covered in IEEE Xplore resources on FET device fundamentals.

FET Integrated Circuits

FET integrated circuits are the foundation of nearly all digital and mixed-signal electronics. CMOS (complementary metal-oxide-semiconductor) logic, which pairs n-channel and p-channel MOSFETs to minimize static power dissipation, is used in microprocessors, memory arrays, and system-on-chip devices. The ability to scale MOSFET gate lengths to a few nanometers while maintaining acceptable leakage and electrostatic control has been the central challenge of silicon technology development since the 1990s. FinFET structures, in which the channel wraps around a narrow silicon fin, improved electrostatic control below the 22 nm node and remain in production. Gate-all-around (GAA) nanosheet transistors, introduced at the 3 nm node by manufacturers including Samsung and TSMC, extend this control further. The International Roadmap for Devices and Systems maintained by IEEE's Rebooting Computing initiative tracks the anticipated trajectory of FET scaling and process node transitions.

Emerging Materials and Graphene Devices

Researchers have explored FETs built on materials beyond silicon and GaAs, driven by the need for higher mobility, lower operating voltage, and new functionalities. Graphene, a single atomic layer of carbon atoms arranged in a hexagonal lattice, attracted wide attention after the 2004 isolation experiments of Geim and Novoselov at the University of Manchester. Graphene FETs can operate at terahertz frequencies because of the extremely high carrier mobility in the material, though the absence of a bandgap limits on/off current ratios and complicates their use in digital logic. Two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS2) offer a natural bandgap and are under study for ultra-thin-body transistors. A survey of two-dimensional material transistor research is available through Nature Electronics and related IEEE publications.

Applications

Field effect transistors have applications in a wide range of disciplines, including:

• Microprocessors and memory chips in consumer electronics
• Radiofrequency amplifiers and switches in wireless communications
• Power management and motor control in automotive and industrial systems
• Biosensors for detecting biomolecules through gate surface functionalization
• Millimeter-wave imaging and radar front ends