Vth is the voltage differrene between Gate and source you should maintain to have a current flow from drain to source in N Mosfet. When ever your Vgs will be more than Vth, mosfet will behave like closed switch.
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide) and the substrate (body) of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction. In other words it is the gate source voltage below which drain current is effectively zero. But this not the case in submicron technology. There is also significant amount of current flow below threshold voltage. This is called sub-threshold leakage. Threshold voltage - Wikipedia, the free encyclopedia **broken link removed** **broken link removed**
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