VGS(th) is a MOSFET designer's parameter and defines the point where the device is at the threshold of turning on. The MOSFET VGS(th) or gate threshold voltage is the voltage between the gate and source that is needed to turn on the MOSFET. 3 Answers · Do not get confused between Gate Threshold Voltage (Vth) and Gate -Source Voltage(Vgs). The MOSFET gate-source threshold voltage (VGS-th) and maximum gate-source voltage. The MOSFET turns on when the gate-source voltage higher than the rated gate threshold voltage Vth is applied, as shown in the following figure. The threshold voltage, commonly abbreviated as Vth, of a field-effect transistor ( FET) is the minimum gate-to-source voltage VGS (th) that is needed to create a.