What is V
GS,off of a FET Transistor?
VGS,off is the Cutoff Voltage of a FET transistor, which is the voltage necessary to turn the transistor off.
For JFETs or depletion MOSFETs, the transistor is normally on, meaning normally conducting current from drain to source.
However, a specific voltage applied to the gate-source region of the transistor will turn the transistor off, so that no current flows from drain to source. This is the cutoff voltage, or VGS,off
At this voltage, the transistor enters the cutoff region.
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 JFETs and depletion MOSFETs, 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 and depletion MOSFETs, 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.