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CMOS implementation of Physically Unclonable Functions (PUFs) facilitates a number of applications ranging from digital rights management, device authentication, counterfeit detection/prevention and cryptographic key generation. Key expectations from such PUF circuits are high (i) uniqueness and (ii) reliability. Uniqueness refers to differentiated responses over challenge-response pairs. Reliability demands unvarying responses under varying environmental conditions such as temperature, supply voltage and noise. This paper describes two methods for achieving the above goals in a ring oscillator based PUF. The first method exploits the negative temperature resistance property of n+ and p+ polysilicon placed as source feedback resistors to de-sensitize ring oscillators to temperature variations. The second method uses an optimized supply voltage (V'DD) to reduce the temperature sensitivity of delay based ring oscillator PUFs. We report an improvement in reliability of 16% by combining these methods. Further, we propose a temperature-invariant ring oscillator PUF architecture based on Serial-Input Serial-Output (SISO) topology. In the proposed design, the relative phase difference between two ring oscillators is translated to a digital response bit. We show that this phase difference based response generation is superior to frequency based response generation in terms of area and power.