Capacitor based physical unclonable function

The present disclosure relates to a PUF apparatus for generating a persistent, random number. The random number is determined by selecting one or more PUF cells, each of which comprise a matched pair of capacitors that are of identical design, and determining a value that is accurately and reliably indicative of a random manufacturing difference between them, based in which the random number is generated. The random manufacturing differences between the capacitors creates the randomness in the generated random number. Furthermore, because the random manufacturing difference should be relatively stable over time, the generated random number should be persistent.