Exploiting short channel effects and multi-Vt technology for increased robustness and reduced energy consumption, with application to a 16-bit subthreshold adder implemented in 65 nm CMOS

When using standard multi-V_t CMOS processes when making logic gates, often for example Low-V_t (LVT), or Standard-V_t (SVT) or High-V_t (HVT) transistors are used within one and the same basic logic building block, like for example a NAND or NOR circuit. We show, to the contrary, how a combination of different types within a single logic circuit may be exploited to reduce energy consumption and increase robustness towards process variations. Additionaly, Reverse Short Channel Effects (RSCE) are exploited by using non-minimum gate lengths for increased robustness agains process variations. Also, a recently proposed technique using very regular layouts accompanying the above mentioned techniques in a 16-bit adder implemented in 65 nm CMOS. Chip measurements using Sub-/Nearthreshold supply voltages demonstrate the functionality of the adder for a voltage range of 119 mV to 350 mV. Simulations show that by increasing gate lengths to 200 nm instead of the minimum 60 nm, may increase the footprint area of logic gates by only 12 %, while at the same time reducing probability of failure by up to several orders of magnitude. Simultaneously, energy per operation is reduced, when compared to conventional design methods using minimum, or relatively short, gate lengths.