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Why JFET has steady current at Pinch off voltage ?

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usmansa1

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I was studying the JFET. In the books and other material it is mentioned that when the voltage between the drain and source is increased then drain current is also increased. After a certain point (Pinch Off) the drain current becomes constant and will remain constant even if the drain voltage is increased. If we see the physical structure of the device the depletion region will start to increase when the voltage between the drain and source is increased. After the same certain point (Pinch off Point)depletion region touches and then current cannot move forward.
My question here is that then how the current becomes steady or current will pass when the depletion regions are closed. Doesn't the current become zero ?
If the depletion region is closed by Voltage between drain and source then what is the role of voltage between gate and source. Although I can see the steady current value is changed at different Voltage gate to source and a logic of faster build of depletion region (Through Voltage to Source and Drain to source) also comes in mind but what is the main logic behind it ?
Before replying please consider me as basic learner. Thanks
 

audioguru

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Your books are wrong because the datasheets of Jfets show that they do not conduct much current at pinch-off.
The datasheets show a graph of constant drain-source current even when the gate-source voltage is zero and at ANY gate-source voltage.
Years ago "current diodes" were made. They produced a constant current at almost any drain-source voltage. They were a Jfet with the gate connected to the source.
 

crutschow

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At the pinch-off gate voltage the channel starts to close.
That's not the same as the cutoff voltage where the channel is closed and the current is near zero.
At pinch-off, the current increases until all the available current carriers are occupied. Above that the current can't significantly increase so the current is steady with a change is drain-source voltage (the saturated region)..
 

audioguru

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Datasheets for two Jfets I saw show cutoff voltage, not pinch-off voltage. The also show the drain-source current when the gate-source voltage is zero.
 
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