Statistical Model and Transistor Size Effect of Hot Carrier Injection for Stability Reinforced SRAM Physically Unclonable Function
Shufan Xu, Kunyang Liu, Kiichi Niitsu, Hirofumi Shinohara · IEEE Transactions on Device and Materials Reliability · 2025
Hot carrier injection (HCI) has been strategically leveraged to enhance the stability of SRAM physically unclonable functions (PUFs). Since the effects of HCI are not constant, exhibiting cell-to-cell variability, a comprehensive distribution model is essential to harness HCI effectively. This article presents a statistical distribution model of mismatch after HCI burn-in and examines the impact of transistor size of PUF on the distribution shape, yielding enhanced stability and shorter HCI burn-in time. The proposed mismatch model after HCI burn-in integrates the native distribution with a Poisson distribution for number of captured electrons and a Gamma distribution for the effect of captured electrons. Model calculations based on size effects reveal that over three times reduction in HCI burn-in duration by enhancing the size to quadruple times: a 15-min for quadruple-size transistor SRAM PUF compared to 46-min for single-size PUF. The model is confirmed by the real chip measurement. The PUFs with several sized transistors are fabricated in a 130-nm standard CMOS process. Experimental results show that quadruple-size transistor SRAM PUF reaches 1.82E09 unstable cell ratio after 18-min HCI burn-in, which align with the model based expectation. Furthermore, robust stability is exhibited even the worst VT corner (0.6V / 40), demonstrating zero bit error (BER7.81E08).