Working of a JFET after increase in depletion region

[rgujju]

Hey Guys,

I am trying to figure out a n-channel JFET works. Consider the following case

Vgs = 0V and Vds is positive

At this point there is some current flowing say Id. and there is depletion region formed at the two PN junctions. Now as I increase Vds the depletion region becomes larger. I understand up to this point.

But now I do not understand why does the current increase as Vds becomes more positive.

As the Vds becomes more positive, the depletion region becomes bigger hence the n-channel becomes smaller which should increase the resistance and hence the current should decrease right? But this does not happen.

What is wrong in my understanding?

[David Hess]

The current only increases in saturation when Vds is low.  Once the Vds is high enough, pinch off occurs and the drain current becomes constant, which is how constant current "diodes" work.

[T3sl4co1l]

Yes, you have it right.  At first the depletion region has no effect and the channel resistance remains near Rds(on).  As voltage goes up (on the order of the gate cutoff voltage, for obvious? reasons), the channel starts to shrink and Rds(on) rises.

Eventually, the depletion region fills the channel, and only a narrow thread of current flow remains; this is the constant current region.

Tim