On improving reliability of delay based Physically Unclonable Functions under temperature variations

Physically Unclonable Functions (PUFs) are a special class of circuits used for challenge-response authentication. The challenge-response pair for PUFs should be mathematically unpredictable, but must be reliable and remain unvarying. The reliability of PUFs implemented in CMOS circuits is frequently compromised by environmental conditions such as voltage and temperature. In this paper, we propose two methods for improving the reliability of delay based PUFs, by reducing temperature sensitivity. The first method focuses on improving the gate overdrive (V_GS - V_t(T)), by operating the PUF at an optimized supply voltage (V´_DD), also called as ZTC (Zero Temperature Coefficient) voltage. The optimum supply voltage for a 24 stage PUF is almost 23% lower than the nominal supply voltage in 45nm CMOS technology. The second method exploits the negative temperature coefficient (TCR) property of n⁺ and p⁺ polysilicon placed as source feedback resistors. A 16% improvement in reliability has been demonstrated for both the methods. Moreover, we also demonstrate that these design optimizations do not compromise the PUF uniqueness.