A physical unclonable function (PUF) array includes a plurality of PUF transistor cells each of which includes at least one inverter. An input and an output of the at least one inverter are shorted to a first reference node. There is adjustment circuitry for adjusting a reference voltage of the first reference node, and measurement circuitry for measuring a trip point of the at least one inverter. If the trip point is close to the reference voltage then bits of the at least one inverter are defined as unstable.