A method for detecting unreliable bits in transistor circuitry includes adjusting a value of a variable capacitor coupled to a physical unclonable function (PUF) cell of a transistor circuit. The adjusting includes tilting the PUF cell to either a zero or one state: if the PUF cell changes its state during the tilting it is deemed unstable, and if the PUF cell does not change its state during the tilting it is deemed stable.