- JFET Definition: A Junction Field Effect Transistor (JFET) is a semiconductor device that controls electrical current using an applied voltage.
- Types of JFETs: There are two main types: n-channel and p-channel, each using different charge carriers (electrons or holes) and having opposite operating polarities.
- Operation Mechanism: The JFET operates by altering the width of the depletion layer via gate voltage, which affects the channel’s conductivity and the overall current flow.
- Application in Electronics: JFETs are commonly used in circuits as amplifiers and switches due to their high impedance and efficient current control.
- Unique Properties: The device features high input impedance and no gate current under ideal conditions, making it a robust component for various electronic applications.
A Junction Field Effect Transistor (JFET) is a depletion-mode field-effect transistor whose reverse-biased gate-channel p-n junction controls a conducting channel. N-channel and p-channel devices use opposite carrier types and voltage polarities. MOSFETs are another common field-effect transistor family.
Engineers describe a JFET as voltage controlled because gate-source voltage sets channel depletion and drain current. Channel current uses majority carriers, so the device is unipolar. The reverse-biased gate junction gives high input resistance, but a real device still has leakage current and capacitance.
Types of JFET
JFETs are classified as n-channel or p-channel. Their operating principles match after reversing carrier types, conventional-current directions and voltage polarities.
N Channel JFET
An n-channel JFET contains an n-type semiconductor channel with ohmic contacts at its ends. These contacts become source and drain according to the applied bias and device construction. Heavily doped p-type gate regions form p-n junctions along the channel and share the gate terminal. Silicon is common; compound-semiconductor field-effect structures may use related junction-gate designs, but not every GaAs FET is a JFET. 
P Channel JFET
A p-channel JFET reverses the dopants: it has a p-type channel and n-type gate regions. Its drain-source and gate-source polarities reverse relative to an n-channel device.
Source and drain: A geometrically symmetrical JFET may tolerate limited source-drain interchange, but many real devices are characterised and protected for one orientation. Use the datasheet pin roles and ratings.
Drain-source voltage creates a lateral electric field and majority-carrier drift current through the channel. In an n-channel JFET, conventional drain current normally enters a positive drain; the p-channel polarities reverse.
The gate-channel junction remains at zero or reverse bias in normal operation. Making an n-channel gate more negative relative to its source widens the depletion region, narrows the conducting channel and reduces drain current. A sufficiently negative gate-source voltage reaches cutoff. Increasing drain-source voltage also widens depletion near the drain until the channel pinches off and current enters a saturation region.
Pinch-off at the drain end does not mean zero drain current; it marks the start of the nearly constant-current region used for amplification. Cutoff is the separate condition in which gate bias depletes the channel enough to reduce current to leakage. JFET circuits use these regions for amplifiers, current sources, analog switches and high-input-resistance interfaces.
Properties of JFET
- Gate-source voltage controls depletion width and therefore channel current. The gate field gives the junction field-effect transistor its name.
- The gate-channel junction is normally reverse biased, which produces high input resistance.
- An ideal reverse-biased gate draws no current. A real JFET has temperature-dependent reverse leakage and displacement current through its capacitance.
- Electrons carry channel current in an n-channel device and holes carry it in a p-channel device. This majority-carrier operation makes a JFET unipolar.
