A 373-F² 0.21%-Native-BER EE SRAM Physically Unclonable Function With 2-D Power-Gated Bit Cells and VSS Bias-Based Dark-Bit Detection

This article presents a highly stable SRAM-based physically unclonable function (PUF) using enhancement- enhancement (EE)-structure bit cells for native stability improvement. The PUF bit cells are power-gated 2-D and are normally in the OFF state, which largely reduces power and is beneficial to attack tolerance. In addition, a dark-bit detection technique based on a lightweight integrated VSS-bias generator is implemented in order to screen out potentially unstable bit cells (dark bits) induced by supply voltage/temperature (VT) variations and other factors. Measured native bit error rate (BER) of prototype chips fabricated in 130-nm standard CMOS is 0.21% at 0.8 V and 23 °C, which is 14× better compared with the conventional SRAM-based PUF. After masking the detected dark bits, no bit error (3339 bits × 500 evaluations) appeared at the worst VT corner across 0.8 to 1.4 V and -40 °C to 120 °C. This technique also eliminated all unstable bits in the accelerated aging test. Both the data before and after dark-bit masking have passed all applicable NIST SP 800-22 randomness tests. The measured operational energy at 0.8 V is 128 fJ/bit and the standby power is 0.44 pW/bit, thanks to the 2-D power-gating scheme. The nMOS-only bit cell is highly compact, with a normalized bit cell area of 373 F2.