Junction Field-Effect Transistors (JFET)

Junction field-effect transistors (JFET) consist of a semiconductor channel in which the width and the conductivity of the channel is controlled by the space-charge region associated with the p-n region. In an n-channel device, the junction field effect transistor channel contains an n-type semiconductor material, enabling the charges to move along the channel as a negative electron charge. In a p-channel device, the channel contains p-type material causing the charges to move end-to-end as a positive charge. In each type of device, the source puts charges into the JFET conduction channels while the drain removes them.

JFET devices comprise of a gate, drain, and channel source. The gate element of junction field-effect transistors is the main body of the device and may resemble the base of a transistor. The drain element corresponds to an emitter, and the channel source element corresponds to a collector. The JFET structure of these devices has a gate, resembling a solid bar, in the center with two deposits of either the n-type or p-type material dispersed on either side. On a portion of the gate in the middle between the deposits of material is the channel connecting the source and the drain.

Junction field-effect transistors provide two areas of operation: low-drain source voltages and high-drain voltages. At low-drain-source voltages, junction field-effect transistors resist variable values controlled by an applied gate-source voltage. High voltage JFET operation passes current values dependent on applied gate-source voltage, providing voltage-controlled source for most circuits. All JFETs operate in depletion mode where the voltage being applied decreases the current flow from source to drain.

Junction field-effect transistors are commonly used as JFET switches to control signals at reasonable voltage levels without requiring a positive supply. JFET switches are similar to signal or analog switches, in that the switch handles signals and not power. Switches made from junction field-effect transistors are frequently designed for applications where a DC to medium frequency analog signal needs to be controlled. JFET switches only require enough voltage to achieve pinch-off at the channel, very high resistance in cutoff, and small resistance between drain and source when conducting.

• AN211A: Field Effect Transistors in Theory and Practice, Courtesy of Motorla
  There are two types of field-effect transistors, the Junction Field-Effect Transistor (JFET) and the "Metal-Oxide Semiconductor" Field-Effect Transistor (MOSFET), or Insulated-Gate Field-Effect...
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• Advanced Features of InfraTec Pyroelectric Detectors (.pdf)
  In 2003 InfraTec added pyroelectric detectors with integrated CMOS Operational Amplifiers (OpAmp) to our established family of detectors with integrated Junction Field Effect Transistors (JFET)....
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