FETs

FETs (field-effect transistors) are voltage-controlled devices in which the current flowing from source to drain is controlled by an electric field produced by the voltage applied to the gate. They have three terminals: source, drain, and gate. The gate forms an electric field that modulates the conductivity of a channel between source and drain. Because the gate is insulated (in MOSFETs) or reverse-biased junction (in JFETs), the gate current is very small, giving high input impedance.

Major families include junction field-effect transistors (JFETs), and metal-oxide-semiconductor FETs (MOSFETs). MOSFETs are further categorized as enhancement-mode and depletion-mode, and as n-channel (NMOS) or p-channel (PMOS). In enhancement-mode devices, a gate voltage induces a conducting channel; in depletion-mode devices, a channel exists at zero gate bias and is depleted by gate voltage. Other variants include MESFETs (metal-semiconductor field-effect transistors) and HEMTs (high-electron-mobility transistors) used primarily in RF and high-speed circuits.

In JFETs, the channel current is controlled by widening or narrowing the depletion region with reverse-biased gate PN junction, yielding Ids versus Vgs characteristics. In MOSFETs, the gate voltage above (or below) threshold forms (or removes) an inversion layer that carries current; the Id–Vds and Id–Vgs relationships define regions of operation: ohmic/linear and saturation. Threshold voltage, transconductance, and output conductance describe performance. High input impedance and fast switching are notable advantages.

FETs are widely used for amplification, switching, and impedance buffering. In integrated circuits, MOSFETs dominate digital logic (CMOS) due to low static power and scalability; JFETs and RF devices such as MESFETs and HEMTs are common in specialized analog and RF applications. The technology has evolved to nanoscale MOSFETs with billions of transistors on a chip.