Reliability Improvement on SRAM Physical Unclonable Function (PUF) Using an 8T Cell in 28 nm FDSOI

An 8T cell is proposed to improve reliability and radiation tolerance of the static random access memory (SRAM) physical unclonable function (PUF) which is formed by adding two cascode pMOS transistors biased at the ON-state to standard 6T cell. Two custom SRAM PUFs in the same batch consisting of 6T cells and 8T cells using the 28 nm fully depleted silicon on insulator (FDSOI) process were used to evaluate the characteristics with respect to bit error rate (BER) and interchip Hamming distance (HD) with an increase in Co-60 total ionizing dose (TID), various supply voltages, ramp-up time, and temperature. According to the experimental result, the BER of the proposed SRAM PUF decreased by 2x under various conditions with better radiation tolerance compared with the 6T one. Both kinds of SRAM PUF have similar interchip HD closed to 50%, but the proposed prototype has less variation, thanks to better reliability. When supply voltage, ramp-up time, and temperature conditions were set to 400 mV, 100 ms, and 25 degrees C, respectively, the proposed SRAM PUF can achieve the lowest BER of only 0.72%. Some design tips of reliable and rad-hard SRAM PUF are given as design guidance.