The present disclosure relates to a PUF apparatus and method for generating a persistent, random number. The generated number is random in that each particular instance of PUF apparatus should generate a randomly different number to all other instances of PUF apparatus, and is persistent in that each particular instance of the PUF apparatus should repeatedly generate the same number, within acceptable error correction tolerances. The persistent, random number is determined by selecting one or more PUF cells, each comprising a matched pair of transistors that are of identical design, and comparing an on-state characteristic of the pair (e.g., turn-on threshold voltage or gate-source voltage). The difference in on-state characteristic of each selected pair of transistors is caused by random manufacturing differences between the transistors. This causes the randomness between each different instance of PUF apparatus, and should be relatively stable over time to provide persistence of the generated number.