Q.Explain what happens to the pn junctions when the gate is reverse biased.

Reverse biasing the gate acts on the gate-channel pn junctions, and this is exactly how the gate controls the drain current. …

[!TLDR]

Reverse biasing the gate widens the depletion regions of the gate-channel junctions, which narrows the channel and reduces the drain current.

In a JFET the pn junctions between the gate and the channel are kept reverse biased during operation. Under reverse bias, electrons and holes move away from the junction, leaving behind immobile positive ions on the n-side and immobile negative ions on the p-side; this ion region is the depletion region, which contains no free charge carriers. As the reverse gate voltage is increased, the depletion regions grow thicker and extend further into the channel, reducing the width of the conducting path. The narrower channel has a higher resistance, so the drain current decreases. If the reverse bias is made large enough, the two depletion regions extend right across the channel and touch each other, closing the ch …