the JFET
/ JAY-fet /
The JFET, or junction field-effect transistor, is the FET's older cousin. Instead of an insulated metal gate, its gate is a reverse-biased PN junction pressed against the conducting channel. At zero gate voltage the channel is wide open and current flows freely, so a JFET is naturally a depletion-mode (normally-on) device. As you push the gate more negative (for an N-channel JFET), the reverse-biased junction grows a depletion region that squeezes the channel narrower, choking the current, until at the pinch-off voltage the channel closes entirely.
Because the gate junction is held in reverse bias, it draws only a tiny leakage current, giving the JFET a very high input resistance like its MOSFET cousin, but without the ultra-fragile gate oxide. That ruggedness, plus naturally low noise, makes JFETs prized for the front end of sensitive amplifiers and instruments, and as simple constant-current sources and analog switches. The trade is that they cannot be packed densely or scaled the way MOSFETs can.
For a beginner, the JFET is mainly worth knowing as the conceptual bridge: same field-effect idea, gate controls a channel by voltage, almost no gate current, but with a junction gate that is always depletion-mode rather than an insulated gate that is usually enhancement-mode. In modern designs the MOSFET has displaced the JFET almost everywhere except a few low-noise and discrete niches, so you will meet MOSFETs constantly and JFETs occasionally.
An N-channel JFET with its gate tied to its source conducts a fixed current (its IDSS): wire it that way and you have a cheap two-terminal constant-current diode with no supply or threshold to worry about.
A JFET is normally on, choked off by a reverse-biased gate.
A JFET gate is a reverse-biased junction, not an insulator. Forward-bias it and you destroy the high input resistance and can damage the part: keep the gate junction reverse-biased.