Determining the threshold voltage of a MOSFET means figuring out how much gate voltage is needed before a strong conducting channel forms at the semiconductor surface.

At low gate voltage the semiconductor surface is either slightly depleted or slightly accumulated and the device does not conduct much. As the gate voltage is increased the electric field through the oxide bends the energy bands in the semiconductor and pushes away majority carriers near the surface, creating a depletion region with fixed ionized dopants.

To reach threshold the gate voltage must first overcome any built in work function difference between the gate material and the semiconductor, then supply enough field to support the charge in the depletion region while also balancing any fixed charge or trapped charge in the oxide or at the interface.

When the surface potential is large enough that the concentration of minority carriers at the surface becomes comparable to the majority carrier concentration in the bulk, a thin layer of minority carriers appears along the surface, called the inversion layer.

The particular gate voltage where this strong inversion just begins is called the threshold voltage.

Its exact value depends on the substrate doping level, the oxide thickness, the amount of fixed or trapped charge, the chosen gate material, and any bias applied to the substrate through the body effect which can raise the threshold when the substrate is reverse biased.

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