Structures and methods for deriving stable physical unclonable functions from semiconductor devices

1. A semiconductor device including circuitry configured to derive a physical unclonable function, the circuitry comprising:

a plurality of bitcells, wherein: each bitcell in the plurality of bitcells comprises a differential transistor pair including a plurality, fewer than three, of transistors,

each bitcell outputs a ‘1’ value when a threshold voltage of a first transistor in the plurality of transistors exceeds a threshold voltage of a second transistor in the plurality of transistors, and

each bitcell outputs a ‘0’ value when the threshold voltage of the second transistor exceeds the threshold voltage of the first transistor; and

sense amplifier circuitry configurable to read values from the plurality of bitcells, the sense amplifier circuitry comprising:

differential sense amplifier circuitry having a current stage and a voltage stage, the current stage having a first leg coupled to a bit line of the first transistor and a second leg coupled to a bit line of the second transistor, the voltage stage comprising a comparator coupled between the first leg and the second leg, and

margin circuitry configurable (i) to selectably bias reading of the plurality of bitcells toward one of ‘0’ values and ‘1’ values by a current generator coupled to a respective one of the first leg and the second leg, and configured to selectably inject at any one time a margining current into at most one of the first leg and the second leg, (ii) to identify addresses of bitcells having a stable ‘1’ value when the margin circuitry is configured to bias reading of the plurality of bitcells toward ‘0’ values, and (iii) to identify addresses of bitcells having a stable ‘0’ value when the margin circuitry is configured to bias reading of the plurality of bitcells toward ‘1’ values.

2. The semiconductor device of claim 1 , further comprising non-volatile memory configurable to store the addresses of the bitcells having the stable ‘1’ value and the addresses of the bitcells having the stable ‘0’ value.

3. The semiconductor device of claim 2 , wherein the non-volatile memory is a one-time programmable memory.

4. The semiconductor device of claim 3 , further comprising processing logic configured to read, without applying any bias, the values of the bitcells whose addresses have been stored, and to derive a stable unique bit string from the values of the bitcells whose addresses have been stored.

5. The semiconductor device of claim 4 , further comprising conditioning logic configured to process the stable unique bit string to derive a unique device secret key.

6. The semiconductor device of claim 5 , wherein the conditioning logic is configured to process the stable unique bit string by encryption.

7. A method for deriving a physical unclonable function from a semiconductor device that has a plurality of bitcells, each bitcell being readable as one of a ‘0’ value and a ‘1’ value and comprising a differential transistor pair including a plurality, fewer than three, of transistors, the method comprising:

selectably biasing reading of the plurality of bitcells toward ‘0’ values by injecting a margining current into a first leg of a differential sense amplifier coupled to at least one bitcell of the plurality of bitcells, to identify addresses of bitcells having a stable ‘1’ value; and

at a time different from when selectably biasing reading of the plurality of bitcells toward ‘0’ values, selectably biasing reading of the plurality of bitcells toward ‘1’ values by injecting a margining current into a second leg of a differential sense amplifier coupled to the at least one bitcell of the plurality of bitcells, to identify addresses of bitcells having a stable ‘0’ value.

8. The method of claim 7 , further comprising storing the addresses of the bitcells having the stable ‘1’ value and the addresses of the bitcells having the stable ‘0’ value.

9. The method of claim 8 , further comprising:

reading, without applying any bias, the values of the bitcells whose addresses have been stored; and

deriving a stable unique bit string from the values of the bitcells whose addresses have been stored.

10. The method of claim 9 , further comprising processing the stable unique bit string to derive a unique device secret key.

11. The method of claim 10 , wherein processing the stable unique bit string comprises applying a function to the stable unique bit string.

12. The method of claim 11 , wherein applying a function to the stable unique bit string comprises encrypting the stable unique bit string.

13. A method of deriving a unique device secret key from a semiconductor device that has a plurality of bitcells, each bitcell being readable as one of a ‘0’ value and a ‘1’ value and comprising a differential transistor pair including a plurality, fewer than three, of transistors, the method comprising:

selectably biasing reading of the plurality of bitcells toward ‘0’ values by injecting a margining current into a first leg of a differential sense amplifier coupled to at least one bitcell of the plurality of bitcells, to identify addresses of bitcells having a stable ‘1’ value;

at a time different from when selectably biasing reading of the plurality of bitcells toward ‘0’ values, selectably biasing reading of the plurality of bitcells toward ‘1’ values by injecting a margining current into a second leg of the differential sense amplifier coupled to the at least one bitcell of the plurality of bitcells, to identify addresses of bitcells having a stable ‘0’ value; and

storing the addresses of the bitcells having the stable ‘0’ value and the addresses of the bitcells having a stable ‘1’ value.

14. The method of claim 13 , further comprising processing the unique device string to derive the unique device secret key.

15. The method of claim 14 , wherein processing the unique device string comprises applying a function to the unique device string.

16. The method of claim 15 , wherein applying a function to the unique device string comprises encrypting the unique device string.