what is basic different betweren cut-off voltage and potential voltage

In short, threshold voltage(cut off) is a specific form of a potential barrier. Let`s say point a is at a potential Va and point b is is at a potential Vb, where Vb>Va. Then the potential barrier between them is Vb - Va.Threshold voltage is more specifically used in case of a diode or a transistor. The gate bias (in case of a MOSFET) or forward bias (in case of a PN junction diode) required to overcome a certain potential barrier to turn ON the device.However, in the recent short channel devices, tunneling can occur before you reach the threshold voltage. That`s altogether a different issue, though...... Regards....Arun

Pinch off voltage: Pinch off voltage is the drain to source voltage after which the drain to source current becomes almost constant and JFET enters into saturation region and is defined only when gate to source voltage is zero.Vgs (off) voltage: It 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.