2.2.2 Threshold Voltage

2.2.2 Threshold Voltage

The threshold voltage V=VT, corresponding to the onset of the strong inversion, is one of

the most important parameters characterizing metalinsulatorsemiconductor devices. As

discussed above, strong inversion occurs when the surface potential s y becomes equal to

2j b . For this surface potential, the charge of the free carriers induced at the insulator –

semiconductor interface is still small compared to the charge in the depletion layer, which

is given by

constant potential. Assuming that the inversion layer is grounded, VB biases the effective

junction between the inversion layer and the substrate, changing the amount of charge in

the depletion layer. In this case, the threshold voltage becomes

T FB b s a ( b B ) i V = V + 2j + 2e qN 2j - V c (2.18)

Note that the threshold voltage may also be affected by socalled fast surface states at the

semiconductoroxide interface and by fixed charges in the insulator layer. However, this

is not a significant concern with modern day fabrication technology.

As discussed above, the threshold voltage separates the subthreshold regime, where the

mobile carrier charge increases exponentially with increasing applied voltage, from the

abovethreshold regime, where the mobile carrier charge is linearly dependent on the

applied voltage.