Definition of Threshold voltage

The threshold voltage of a MOSFET is usually defined as the gate voltage where a depletion region forms in the substrate (body) of the transistor. In an NMOS the substrate of the transistor is composed of p-type silicon which has more positively charged electron holes compared to electrons. When a voltage is applied on the gate, an electric field causes the electrons in the substrate to become concentrated at the region of the substrate nearest the gate causing the concentration of electrons to be equal to that of the electron holes, creating a depletion region.