Junction field-effect transistor (JFET) - definition
Junction field-effect transistor (JFET) - definition
The junction field-effect transistor (Junction Field-Effect Transistor, JFET) is a semiconductor active element, belonging to the family of Unipolar transistors, whose operation is based on controlling current flow by means of an electric field. Unlike Bipolar transistors, the JFET is voltage-controlled and the carriers dominating conduction are charges of one type - electrons in n-type structures or holes in p-type structures.
The JFET structure consists of a semiconductor channel, through which current flows from the drain (D) to the source (S), and a gate electrode (G), forming a p-n junction with the channel. Under normal conditions, the channel is conductive, and the application of a negative voltage to the gate (for n-type JFETs) causes the barrier layer to expand and the active cross-section of the channel to decrease. This has the effect of reducing the drain current. When the gate voltage reaches the so-called cut-off voltage, the channel is completely blocked and current flow ceases.
A characteristic feature of the JFET is its high input impedance, resulting from the fact that the gate of the p-n junction is negatively polarised and draws virtually no current. This makes transistors of this type ideal for low-power signal amplifier input circuits, selective circuits and analogue filters. They are also characterised by a low noise floor, which is important in audio and measurement circuits.
The operation of the JFET is linear over a wide range of parameters, and its current-voltage characteristics depend on the length and width of the channel and the value of the gate voltage. Parameters such as maximum drain current, cut-off voltage or resistance of the channel determine its applications in specific electronic circuits.
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