An equivalent model can be built to describe the OFF-state leakage and make this easier for understanding and calculation. Both the OFF-state transistors and reverse-biased diodes can be regarded as large resistors, as shown in Figure 2-7. Other resistance for example, line resistance is neglected for simplification in this case.

Figure 2-7 Replacing Transistors and Diodes With Resistors

From the output side, most of the leakage from input is sunk by the mid-point of transistors and cannot be seen at the output. Therefore, the paths linked to input can be removed as in Figure 2-8.

Figure 2-8 Removing Leakage Paths Linked to Input

The paths linked to the same pin (VDD/VSS) can be combined and the final equivalent resistor model is shown in Figure 2-9.

Figure 2-9 Equivalent Resistor Model of OFF Channel

To calculate the value of equivalent resistors R_off,up and R_off,down, connect the output to VDD and VSS respectively, and measure the current flowing out of the output.

Figure 2-10 Calculating Value of Equivalent Resistors

The R_off,up and R_off,down can be calculated as the following:

Due to the symmetry of channel structure, a reasonable assumption is that R_off,up and R_off,down are of the same order of magnitude, for example, if R_off,up is tens of MΩ, R_off,down can also be assumed to be tens of MΩ.