Abstract: A physically unclonable function includes a flash memory, a current comparator and a controller. The flash memory includes a plurality of memory cells. A method of operating the physically unclonable function circuit includes the controller setting the plurality of memory cells to an initial data state, the controller setting the plurality of memory cells between the initial data state and an adjacent data state of the initial data state, the current comparator reading a first current from a memory cell in a first section of the plurality of the memory cells, the current comparator reading a second current from a memory cell in a second section of the plurality of the memory cells, and the current comparator outputting a random bit according to the first current and the second current.