The component of the threshold voltage that offsets the depletion charge is then given by -Q_d /C_ox , where C_ox is the gate oxide capacitance per unit area, or C_ox = (ratio of the oxide permittivity and the oxide thickness).

A set of positive charges arises from the interface states at the Si-SiO₂ interface. These charges, denoted as Q_i , occur from the abrupt termination of the semiconductor crystal lattice at the oxide interface. More importantly, alkali metal ions like Na+ introduce positive charges in the fixed oxide. The component of the gate voltage needed to offset this positive charge (which induces an equivalent negative charge in the semiconductor) is –Q_i /C_ox where Q_i accounts for various fixed oxide and interface charges. On combining all the four voltage components, the threshold voltage V_T0 , for zero substrate bias, is expressed as

For non-zero substrate bias, however, the depletion charge density needs to be modified to include the effect of V_SB on that charge, resulting in the following generalized expression for the threshold voltage, namely

The generalized form of the threshold voltage can also be written as