A very stable diode-based physically unclonable constant

Physically Unclonable Constants (PUC) are circuits used to embed unique secret bit-words in chips. We propose a simple PUC, employing two Schottkydiodes in reverse. The difference of the reverse currents of the two diodes is used to charge a capacitance. The charge stops when the two currents become equal. It is shown that this scheme has a single equilibrium point that depends discontinuously from the difference of the two saturation currents. The proposed scheme is studied both theoretically and by means of simulations (0.18 μm technology). It is shown that the proposed PUC is unbiased (inter distance ≈50%), very stable (intra distance from 2.8% to 1.5%) and temperature insensitive (only 0.3% of the cells changes output over a military temperature range). Energy required is predicted to be as small as 0.6 pJ/bit.