Circuit design requires devices of different types - NMOS and PMOS
transistors - to be closely matched in order to be used in
complementary circuits. Matching devices exhibit an identical
electrical behavior when discounting for the sign of the electrical
currents and voltages. More precisely, the respective threshold
voltages are identical.
Due to process variations and different process sequences
for the N and P type devices, the distributions of these electrical
parameters do not coincide automatically. In a fashion similar to the
one described in the previous section, statistical simulation can be
used to establish a set of nominal process parameters that minimizes
statistically the differences between the two groups.