What is the Cutoff Voltage, VGS,off, of a JFET Transistor?

The Cutoff Voltage of a JFET is the voltage that is needed at the gate-source region of the JFET in order to turn it off.

At this point, the transistor enters the cutoff region.

For JFETs, the cutoff voltage is referred to as VGS,off, because it is the voltage which must be applied to the gate-source terminal to turn off the transistor.

The figure below shows the terminal the cutoff voltage must be applied to in the JFET:

The cutoff voltage is the particular gate-source voltage where the JFET begins to act like an open circuit. This is where the drain current ID which flows from the drain to the source ceases to flow and the transistor turns off.

For N-channel JFET, the cutoff region point is reached when the gate-source voltage, VGS, goes below a certain negative voltage, perhaps -4V. And for P-Channel JFETs, the cutoff region point is reached when VGS goes above a certain positive voltage, perhaps +4V or so. At this point, the current flowing from drain to source will cease and the transistor turns off.