Why is the drain current steady after pinchoff in a JFET?

[Muhammad Usman]

TL;DR Summary

1) At pinch off voltage the depletion region blocks the channel so how does the current still flow or it remain constant.
2) What happens when we apply the gate voltage that completely stopped the drain current as depletion region for P-types already touched each other.

Hi,
I was studying JFET and it was written that at Pinch off voltage the drain current becomes constant. And when the gate to source voltage is applied. The more negative the voltage is the current will be decreased and eventually reached to 0. I don't understand below both points.
1) At pinch off voltage the depletion region blocks the channel so how does the current still flow or it remain constant.
2) What happens when we apply the gate voltage that completely stopped the drain current despite the fact the depletion region is already touched or blocked.

Please consider me as a basic learner while replying.

 

[Baluncore]

1) At pinch off voltage the depletion region blocks the channel so how does the current still flow or it remain constant.

It does NOT block the channel. The term pinch-off voltage is very misleading.

With the gate connected to the source, Vgs = 0, the pinch-off voltage is defined as the drain voltage, Vds, at which the channel changes from being resistive to being current limiting. The drain current, Id, is then at a maximum and is limited by the maximum channel width.

2) What happens when we apply the gate voltage that completely stopped the drain current despite the fact the depletion region is already touched or blocked.

Referring to the most common N-channel depletion mode JFET.

Gate voltages, Vgs, more positive than the source cannot further increase the width of the channel as it is then limited by the physical size of the gate structure.

As the gate voltage, Vgs, becomes more negative, the gate progressively increases in thickness. That progressively restricts the channel width, until the channel width approaches zero, when the drain current, Id, is reduced to the minimum.