JFET

Junction FETs are normally-on devices turned off by reverse-biasing the gate junction.

Intermediate

A JFET channel conducts at $V_{GS} = 0$ ; making the gate more negative (N-channel) pinches the channel off at $V_{GS(off)}$ :

$I_D = I_{DSS}\left(1 - \frac{V_{GS}}{V_{GS(off)}}\right)^2$

JFETs have very low noise and high input impedance, so they appear in audio preamps, instrumentation input stages and as simple constant-current sources.

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