/ˌɛf iː tiː/
noun — "transistors controlled by electric fields instead of currents."
FET (Field-Effect Transistor) is a type of transistor in which the current flowing between the source and drain terminals is controlled by an electric field applied to the gate terminal. Unlike bipolar junction transistors (BJTs) that rely on carrier injection and base current, FETs modulate conductivity through voltage applied to the gate, providing high input impedance, low power consumption, and excellent signal control. They are widely used in analog and digital circuits, RF amplification, switching applications, and integrated circuits.
Technically, FETs come in several types, including Junction FETs (JFETs), Metal-Oxide-Semiconductor FETs (MOSFETs), and high-performance variants like HEMT. The gate voltage controls the width of a conductive channel between source and drain, which in turn modulates the current. In MOSFETs, an insulated gate allows almost no direct current flow into the control terminal, yielding high input resistance and low leakage. FETs are classified as depletion-mode or enhancement-mode depending on whether the default channel is naturally conductive or requires voltage to turn on.
Key characteristics of FETs include:
- Voltage-controlled: gate voltage regulates current, unlike BJTs which require input current.
- High input impedance: minimally loads preceding circuits.
- Low power consumption: ideal for energy-efficient devices.
- Scalability: fundamental to modern CMOS integrated circuits.
- Variants for speed and frequency: including HEMT for RF and microwave applications.
In practical workflows, FETs are used in switching and amplification roles. In a microcontroller circuit, MOSFETs might switch power to motors or LEDs without significant voltage drop, while in RF applications, JFETs or HEMTs provide low-noise amplification of signals. Designers choose FET type based on frequency, voltage, and power requirements.
Conceptually, a FET acts like a water faucet: the gate voltage is the handle, controlling the flow of electrons (current) through a channel (pipe) between source and drain.
Intuition anchor: FET turns voltage into precise current control, forming the backbone of modern low-power and high-speed electronics.