Junction Field-Effect Transistors
A JFET controls current through a semiconductor channel using the electric field from a reverse-biased PN junction. Because the gate junction is normally reverse biased, gate current is very small.
N-channel JFET
An N-channel JFET contains an N-type conductive channel between source and drain. P-type gate regions form junctions beside the channel. Reverse bias on the gate widens the depletion regions and narrows the channel.
As the gate becomes more negative relative to the source, drain current is reduced. At a sufficient reverse gate voltage, the channel can be nearly pinched off.
Normally on
A conventional JFET is a depletion-mode device: with VGS = 0, it normally conducts. Applying reverse gate bias reduces the current.
High input resistance
Because the gate junction is reverse biased, a JFET draws very little DC gate current. This made JFETs particularly useful in high-impedance inputs, instrumentation, microphone preamplifiers, sensor interfaces, and low-noise analog circuits.
Shockley's transfer relationship
A useful approximate model for an N-channel JFET is:
IDSS and pinch-off-related voltage vary widely among individual devices. JFET circuits are therefore often designed with self-bias or feedback rather than assuming one exact device current.
Self-bias
A resistor in the source lead can automatically create negative gate-to-source bias as drain current flows. This provides useful local feedback and stabilizes the operating point.
JFET as voltage-controlled resistor
At small drain-source voltages, the channel can act approximately like a voltage-controlled resistance. JFETs have been used in analog switches, automatic gain control, attenuators, and variable-resistance circuits.
Noise
Selected JFETs can have very low voltage noise and extremely small input current noise, making them useful with high-impedance sensors, condenser microphones, photodiode circuits, and precision front ends.
JFET and MOSFET
Both are field-effect transistors, but the JFET gate is a semiconductor junction while the MOSFET gate is insulated by an oxide or other dielectric. The insulated gate lets MOSFETs reach even higher DC input resistance and makes dense integrated construction practical.