Junction Field-Effect Transistors (JFET) are three-terminal semiconductor devices that use an electric field to control the electrical conductivity of a channel. Composed of a channel of either n-type or p-type semiconductor (named N-JFET or P-JFET respectively), the transistors operate by creating a depletion region through which charge may or may not flow, the state of which can be controlled by adjusting the voltage applied to the terminals. The current flowing through the channel is controlled by the gate-source voltage. The basic functionality of a JFET involves “pinching-off” the current flow through the channel by expanding the depletion region, and it’s this foundational process that gives rise to its alternate name—the voltage-controlled resistor.

JFETs bear great significance in modern electronic devices, particularly those requiring amplification, such as signal and audio amplifiers. They have the advantage of being relatively immune to radiation, an attribute making them suitable for spatial and radiation-heavy applications. Although bipolar junction transistors (BJTs) are typically more widely used, JFETs’ advantage of low input current (or gate leakage current) and high input impedance makes them useful in high input impedance circuit applications, such as the input stages of oscilloscopes and multimeters.