Glossary term
JFET
A junction field-effect transistor whose channel current is controlled by reverse-biasing a p-n junction gate.
Definition
deviceA JFET is a field-effect transistor in which a reverse-biased p-n junction gate controls current through a semiconductor channel.
JFET stands for junction field-effect transistor. It is a voltage-controlled, normally-on device: with zero gate-source bias, current can flow through the channel; applying reverse gate-source voltage depletes the channel and reduces current. JFETs are used in low-noise amplifiers, high-input-impedance analog front ends, current sources, RF circuits, switches, and sensor interfaces.
A JFET controls current through a semiconductor channel using an electric field from a reverse-biased gate junction. In an n-channel JFET, electrons carry current from drain to source. The gate-channel p-n junction is reverse biased, widening the depletion region and narrowing the conducting channel. As gate-source voltage becomes more negative, drain current decreases. At pinch-off, the channel is effectively closed.
Unlike a bipolar transistor, which is current-controlled at its input, a JFET is primarily voltage-controlled and has very high input impedance because the gate junction is reverse biased. Some leakage current still exists, and it increases with temperature.
Operating behaviour
At small drain-source voltage, the JFET behaves like a voltage-controlled resistor. At higher drain-source voltage, it enters a saturation or constant-current region where drain current is controlled mainly by gate-source voltage. The relationship is commonly approximated by Shockley’s equation for long-channel devices:
where I_{DSS} is drain current at zero gate-source voltage and V_P is pinch-off voltage.
Applications
JFETs are valued for low noise, high input impedance, simple biasing, and useful analog behaviour. They appear in microphone preamplifiers, electrometer inputs, sensor buffers, RF amplifiers, analog switches, constant-current diodes, and operational amplifier input stages. They are less common than MOSFETs in many digital and power applications, but they remain useful where low leakage, low flicker noise, and analog linearity matter.
Common mistakes
A common mistake is assuming a JFET gate can be driven like a MOSFET gate. The JFET gate is a p-n junction and should normally remain reverse biased. Forward-biasing it can inject current, distort operation, or damage the device. Another mistake is ignoring parameter spread. Pinch-off voltage and I_{DSS} vary widely between devices, so precision circuits usually require feedback, selection, or calibration.