JFET (junction field-effect transistor)
A JFET is a transistor whose conducting channel is squeezed by a reverse-biased PN junction instead of an insulated gate. Picture a garden hose with a clamp on the side: tighten the clamp (apply gate voltage) and you pinch the flow of water (current from drain to source). Because the gate is a reverse-biased junction, it draws almost no current — giving the JFET an enormously high input resistance.
That near-infinite input impedance and very low noise made JFETs the classic front-end for the first stage of sensitive amplifiers, audio mixers and electrometers — anywhere you must sense a tiny signal without loading down its source. Unlike the MOSFET, a typical JFET is 'normally on' (depletion mode): you have to apply gate voltage to turn it off, not on.
JFET vs MOSFET in one line: both are voltage-controlled and high-impedance, but the JFET's gate is a junction (never forward-bias it) while the MOSFET's gate is fully insulated by oxide — which is why MOSFETs scale to billions on a chip and JFETs stayed a discrete, low-noise specialty.