Working of JFET- The two P-N junctions at the sides form two depletion layers. The current conduction by charge carrier i.e. electrons in case of N-channel JFET is thought the channel between two depletion layers and out of drain.
The width and hence resistance of this channel can be control by changing the input voltage Vgs. The greater the reverse voltage Vgs the wider will be the depletion layer and narrow will be the conducting channel.
The narrow channel means greater resistance. Hence source to drain current decreases thus JFET is operated on the principle that width and resistance of conduction channel can be varied by changing reverse voltage Vgs.
When a voltage Vds is applied between the source and drain terminal, and the voltage on the gate is Zero, the two P-N junctions at the sides of the bar establish depletion layers. The elected will flow from source to drain through a channel between depletion layers.
The size of these layers determines the width of the channel and hence current conduction through the bar.
When a reverse voltage Vgs is applied between gate and source the width of the depletion layer is increased.
This reduces the width of the conducting channel thereby increasing the resistance of the N-type bar.
So the current from source to drain is decreased. On the other hand, if the reverse voltage on gate decreases the width of the depletion layer also decreases. This increases the width of conducting channel and hence source to drain current.
So in JFET current from drain to source can be controlled by applying potential across drain terminal.
Also Read-Difference between BJT and FET