Junction Field Effect Transistors-JFET


Junction field effect transistors 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.