What Are FETs?

FETs, or field-effect transistors, are three-terminal semiconductor devices that control current flow between a source and a drain terminal by applying an electric field at a gate terminal. Because the gate draws negligible current, FETs are classified as voltage-controlled devices, distinguishing them from bipolar junction transistors, which are current-controlled. This high input impedance makes FETs particularly well suited for amplifier front ends, switched logic, and analog interfaces where loading the driving circuit would degrade performance.

The field-effect principle was described theoretically by Julius Edgar Lilienfeld in 1925, but practical devices did not emerge until the 1960s, when advances in semiconductor surface passivation enabled reliable gate insulation. FETs now dominate both digital and analog integrated circuit design, with billions of MOSFET devices fabricated on a single chip in modern processors and memory arrays.

Operating Principle and Device Structure

A FET operates by modulating the conductivity of a semiconductor channel through the transverse electric field created by the gate voltage. In an n-channel device, a positive gate voltage attracts electrons to the channel region, increasing its conductivity and allowing current to flow from drain to source. In a p-channel device, a negative gate voltage performs the equivalent function with holes as the majority carriers. The relationship between gate voltage and drain current is characterized by the transconductance, a key parameter in small-signal amplifier design. The gate electrode is separated from the channel by an insulating layer in metal-oxide-semiconductor FETs (MOSFETs), while in junction FETs (JFETs) the gate is a reverse-biased p-n junction that controls channel width by varying the depletion region.

MOSFET Scaling and Integration

The MOSFET is the most widely manufactured electronic component in history. ScienceDirect's overview of field-effect transistors describes how progressive scaling of gate length from micrometers to the sub-10-nanometer range has followed Moore's Law, enabling successive generations of denser and faster integrated circuits. As gate lengths shrank below 100 nm, short-channel effects including threshold voltage roll-off and drain-induced barrier lowering required structural innovations such as silicon-on-insulator substrates, high-k gate dielectrics replacing silicon dioxide, and fully depleted fin-shaped channels in FinFET geometries. Gate-all-around nanowire FETs represent the architecture now in production at the 3 nm node.

Variants and Specialized Devices

Beyond standard MOSFETs, several FET variants address specific performance requirements. High-electron-mobility transistors (HEMTs) exploit a two-dimensional electron gas formed at a heterojunction interface, typically AlGaN/GaN or InAlAs/InGaAs, to achieve carrier mobilities far exceeding those in bulk silicon. These devices are preferred for microwave and millimeter-wave power amplification in radar, satellite, and 5G base station applications. Thin-film transistors (TFTs) use amorphous or polycrystalline semiconductor layers deposited on glass or plastic substrates for flat-panel display backplanes. Organic FETs extend the device concept to carbon-based semiconductors processed at low temperatures, enabling flexible electronics. Tunnel FETs, under research development, use quantum mechanical band-to-band tunneling to achieve steeper subthreshold slopes than the 60 mV/decade limit that constrains conventional MOSFET switching.

Applications

FETs have applications across a wide range of systems and technologies, including:

• CMOS logic gates and static RAM in microprocessors and system-on-chip devices
• Power switching in motor drives, inverters, and DC-DC converters using SiC and GaN power FETs
• Low-noise amplifier front ends in cellular and satellite receivers
• Biosensor platforms where gate voltage shifts indicate molecular binding events
• Pixel switching in active-matrix liquid crystal and OLED display panels