Junction Field-Effect Transistor (JFET)

The junction field-effect transistor (JFET) is a three-terminal voltage-controlled semiconductor device. Current flows through a doped channel between two terminals called the source and the drain, while a third terminal, the gate, controls how much current passes. Unlike the bipolar junction transistor, the JFET is a unipolar device: conduction relies on majority carriers only. JFETs are made in two polarities, n-channel and p-channel, depending on the doping of the channel.

Control works through a reverse-biased p-n junction. As the reverse voltage on the gate increases, the depletion region around the junction widens and intrudes into the channel, narrowing the path available for current. At a sufficiently large reverse voltage, called the pinch-off voltage, the channel is effectively closed. Because the gate junction is reverse biased in normal operation, the gate draws only a very small leakage current, giving the JFET a high input impedance. The device is described as depletion-mode, since it conducts at zero gate voltage and is progressively turned off as gate bias increases.

JFETs are valued for low noise and high input impedance, which makes them useful as input stages in amplifiers, in analog switches, and as voltage-controlled resistors. The related term IGFET (insulated-gate field-effect transistor) refers instead to MOSFET-type devices, in which the gate is separated from the channel by an insulating oxide rather than a junction; the two should not be confused, although both are field-effect transistors that modulate a channel by an electric field.

Frequently asked questions

How does a JFET differ from a MOSFET?

A JFET controls its channel with a reverse-biased p-n junction, while a MOSFET (an IGFET) uses an insulated gate separated by an oxide layer. The MOSFET's gate has even higher input impedance and can operate in enhancement mode.

What is pinch-off voltage?

It is the gate-to-source voltage at which the channel is fully constricted by the depletion region, effectively stopping current flow between drain and source.

Why are JFETs used in amplifier input stages?

They offer very high input impedance and low noise, so they load the signal source lightly and add little of their own noise, which suits sensitive front-end amplification.