I see quite a few publications on Physically Unclonable Functions implemented using regular silicon IC techniques; for example A 0.19pJ/b PVT-variation-tolerant hybrid physically unclonable function circuit for 100% stable secure key generation in 22nm CMOS (10 authors at Intel), at 2014 IEEE International Solid-State Circuits Conference, with free preview there. The device is summarized as
..creating a golden key at first array power-up during tester-time operation and thereafter recreating this golden value from the inherently noisy raw PUF value during regular field operation. The golden key is used to compute an Error Correction Code signature, which is stored on-die as fuses. During regular operation, error correction circuits mix raw PUF bits with ECC fuse values to regenerate the golden key with 100% accuracy.
A reference for that principle is given as: FPGA Intrinsic PUFs and Their Use for IP Protection (4 authors at Philips), in proceedings of CHES 2007.
I wonder in what sense these things are unclonable.
In particular, I fail to understand why it would not be possible to replace the noisy PUF array and its sensors by ROM with content the (probed) raw PUF value, replace the ECC data/signature by ROM too, letting the error-correction circuitry do its thing; or replace the whole thing by a small ROM or wires defining the golden key.