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

Thread Starter

usmansa1

Joined Jan 22, 2017
23
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 dont 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.
 

crutschow

Joined Mar 14, 2008
24,749
1) At pinch off voltage the depletion region blocks the channel so how does the current still flow or it remain constant.
Current flows above the pinchoff region but not below.
Read this.

Look at the characteristics curves below for a typical JFET, which sweeps the collector voltage from 0V to 5V, for various values of Vgs.
A Vgs=0 reaches about 12mA at a Vd of about 3V.
At the pinch-off voltage of Vgs=-3V, the current drops to essentially zero.

upload_2019-5-14_17-1-24.png
 

Thread Starter

usmansa1

Joined Jan 22, 2017
23
That is my question, at pinch off voltage channel is closed so how come the current flow if the channel is closed.
 

Thread Starter

usmansa1

Joined Jan 22, 2017
23
Current flows above the pinchoff region but not below.
Read this.

Look at the characteristics curves below for a typical JFET, which sweeps the collector voltage from 0V to 5V, for various values of Vgs.
A Vgs=0 reaches about 12mA at a Vd of about 3V.
At the pinch-off voltage of Vgs=-3V, the current drops to essentially zero.

View attachment 177413
Simulation shows the same response and I already read Wiki. The real question is that why the response is like this
 

Jony130

Joined Feb 17, 2009
5,157
That is my question, at pinch off voltage channel is closed so how come the current flow if the channel is closed.
The short answer because this is how a JFET works. As you can see if Vds > pinch off voltage the transistor starts to behaviors just like a constant current source controlled via Vgs voltage.
 
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