A Physically Unclonable Function Using Soft Oxide Breakdown Featuring 0% Native BER and 51.8 fJ/bit in 40-nm CMOS

This paper presents a physically unclonable function (PUF) based on the randomness of soft gate oxide breakdown (BD) locations in MOSFETs, namely, soft-BD PUF. The proposed PUF circuit features a self-limiting mechanism that generates exactly one soft-BD spot in a pair of NMOS transistors. Highly stable “0” and “1” bits with an equal probability of 0.5 are extracted based on the locations of the generated BDs. A differential readout scheme is employed based on the proposed reference-free sense amplifier (SA), resulting in good current sensitivity and side-channel attack resilience. The soft-BD PUF, fabricated in a 40-nm CMOS process, comprises all essential periphery circuits. Measurements show that the soft-BD PUF has good data stability in a wide operating range. The native bit error rate is 0% for V DD = 1 V and above, shown by measuring 10k readout cycles among 10k PUF cells. Data stability degrades at lower supply voltage and higher temperature due to the conductivity of PUF cells and the offset of SAs. Under the nominal V DD of 0.9 V in this technology, the throughput is shown to be at least 40 Mb/s and the PUF readout consumes only 51.8 fJ/bit. The averaged hamming weight and hamming distance are 0.497 and 0.496, respectively, showing a good randomness and uniqueness. The resulting PUF data show good statistical properties by passing all the relevant tests in the NIST 800-22 suite.