Energy Efficient Sub/Near-Threshold Ripple-Carry Adder in Standard 65 nm CMOS

Ali Asghar Vatanjou,  Trond Ytterdal,  Snorre Aunet


This manuscript includes chip measurements for a 32-bit Ripple-Carry Adder (”RCA”), demonstrating functionality for a supply voltage ("Vdd") down to 84 mV. The low Vdd might be the lowest reported for comparable CMOS circuitry, not depending on special Schmitt-trigger based logic or body biasing. Two 32- bit ripple-carry adders are implemented in 65 nm CMOS, having all gate lengths of 60 nm and 80 nm, respectively. The implementation having 80 nm gate lengths exploits secondary effects like the Reverse Short Channel Effect (”RSCE”) to provide lower energy per operation, compared to the 60 nm implementation, when operated down to subthreshold supply voltages. Dimensioning for symmetric noise margins, and using minority-3 circuits and inverters only with regular layouts, contribute to the ultra low Vdd potential. According to simulations, the energy per operation could be down to about 1.5 fJ/bit for the implementation based on L = 80 nm. For delays in the 20 ns to 110 ns range, the energy consumption for the RCA having L = 60 nm, was from 18.5 to 47 % higher than the RCA having L = 80 nm. The area was 9.7 % less for the L = 80 nm implementation, compared to the L = 60 nm RCA.