Performance Analysis of Subthreshold 32-Bit Kogge-Stone Adder for Worst-Case-Delay and Power in Sub-micron Technology - VLSI Design and Test - page 102

Fig. 1. VTC as a function of supply voltage for a minimum sized inverter at 27 0 C in UMC
90nm process

2.3 Process Variations

As mentioned above, the subthreshold current is exponentially dependent on the tran-
sistor's threshold voltage. Accordingly, process variations that substantially affect the
threshold voltages can cause a large variance in the behavior of subthreshold circuits.
In extreme cases, this can even cause certain circuits to malfunction, and so, special
means need to be taken to deal with this issue. Fig. 2 shows the VTC of a minimum
sized inverter in UMC90nm process for VDD at 200mV. A substantial shift of the
VTC in SF and FS corner.

Fig. 2. VTC of an inverter at various process corners