FET

A field effect transistor is a three-terminal semiconductor device that uses an electric field to control the flow of current through a conductive channel. Unlike bipolar transistors, which are current-controlled, FETs switch and amplify by applying voltage to the gate terminal, which modulates conductivity between the source and drain terminals. This voltage-controlled behavior makes FETs useful for high-impedance signal sensing and switching applications where gate current draw must be minimal.

Two main families exist: JFETs (junction FETs) and MOSFETs (metal-oxide-semiconductor FETs). JFETs use a reverse-biased pn-junction as the gate and were developed first; they are depletion-mode devices, meaning current flows in the off-state. MOSFETs, which dominate modern electronics, use an insulated gate separated by a thin oxide layer. MOSFETs come in enhancement-mode (normally off) and depletion-mode variants, with both n-channel and p-channel polarities available. Power MOSFETs operate at higher voltages and currents than their signal-level counterparts, with on-resistance measured in milliohms.

Why FETs matter in industrial circuits

In switching applications, FETs offer low on-resistance (especially power FETs), fast switching speeds, and no reverse recovery charge like bipolar devices. This makes them ideal for DC-DC converters, motor drivers, and power supplies. As amplifiers, FETs provide very high input impedance (gigaohms range for MOSFETs), which is essential in instrumentation and sensor conditioning where source impedance is high or loading must be avoided. The transconductance (gm) parameter specifies gain in small-signal amplification.

Failure modes include gate oxide breakdown in MOSFETs from overvoltage or electrostatic discharge, threshold voltage drift over temperature (more pronounced in JFETs), and thermal runaway in power FETs if not properly heat-sinked. Body diode conduction in MOSFETs during reverse bias must be accounted for in switching circuits. Gate charge (Qg) and switching time are critical in high-frequency power applications; gate drivers are often required to supply the current needed to charge and discharge the gate capacitance quickly.

The name reflects the device operation: an electric field, created by voltage applied to the gate, modulates or depletes the carrier population in the channel, thus controlling current flow. This is fundamentally different from the base current control in bipolar transistors. In industrial automation, FETs are found in solid-state relays, PWM controllers, analog multiplexers, and any application requiring a voltage-driven switch or amplifier with minimal input current.