Combining the four components, the threshold voltage can now be computed as

Body Effect : The transistors in a MOS device seen so far are built on a common substrate. Thus, the substrate voltage of all such transistors are equal. However, while one designs a complex gate using MOS transistors, several devices may have to be connected in series. This will result in different source-to-substrate voltages for different devices. For example, in the NAND gate shown in Figure 1.5, the nMOS transistors are in series, whereby the source-to-substrate voltage V_SB of the device corresponding to the input A is higher than that of the device for the input B.

Under normal conditions ( V_GS > V_th ), the depletion layer width remains unchanged and the charge carriers are drawn into the channel from the source. As the substrate bias V_SB is increased, the depletion layer width corresponding to the source-substrate field-induced junction also increases. This results in an increase in the density of the fixed charges in the depletion layer. For charge neutrality to be valid, the channel charge must go down. The consequence is that the substrate bias V_SB gets added to the channel-substrate junction potential. This leads to an increase of the gate-channel voltage drop.