Physically Unclonable Functions (PUF) are security primitives to combat Integrated Circuit (IC) cloning and counterfeiting. The response of the PUF is expected to be stable under environmental fluctuations (e.g., temperature and voltage fluctuation). Our analysis indicate that conventional arbiter PUF experiences significant variations due to environmental fluctuation degrading quality. In this paper we propose a novel Schmitt-Trigger (ST) based PUF that exploits the susceptibility of ST to process variations to realize high-quality robust arbiter type PUF. Extensive simulations show significant improvement of quality metrics e.g., inter and intra-die hamming distance and NIST tests with the proposed ST-PUF at the cost of area, power and throughput overhead. Based on this observation, we propose a family of low-overhead ST-PUF flavors that amplify the effect of process variations and achieve similar quality as ST-PUF. We also propose the concept of selective challenge pruning to screen the unstable challenge-response pairs for improving the quality and stability further.