Uses of Jfet - Function

Function

JFET operation is like that of a garden hose. The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric charge through a JFET is controlled by constricting the current-carrying channel. The current also depends on the electric field between source and drain (analogous to the difference in pressure on either end of the hose).

Construction of the conducting channel is accomplished using the field effect: a voltage between the gate and source is applied to reverse bias the gate-source pn-junction, thereby widening the depletion layer of this junction (see top figure), encroaching upon the conducting channel and restricting its cross-sectional area. The depletion layer is so-called because it is depleted of mobile carriers and so is electrically non-conducting for practical purposes.

When the depletion layer spans the width of the conduction channel, "pinch-off" is achieved and drain to source conduction stops. Pinch-off occurs at a particular reverse bias (V_GS) of the gate-source junction. The pinch-off voltage (V_p) varies considerably, even among devices of the same type. For example, V_GS(off) for the Temic J201 device varies from -0.8V to -4V. Typical values vary from -0.3V to -10V.

To switch off an n-channel device requires a negative gate-source voltage (V_GS). Conversely, to switch off a p-channel device requires positive V_GS.

In normal operation, the electric field developed by the gate blocks source-drain conduction to some extent.

Some JFET devices are symmetrical with respect to the source and drain.