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We propose a novel ultralow-power, high-sensitivity, bias-free sub-threshold process variation sensor for monitoring the random variations in the threshold voltage. The proposed sensor characterizes the threshold voltage mismatch between closely spaced, supposedly identical transistors using the exponential current-voltage relationship of sub-threshold operation. The sensitivity of the proposed sensor is 2.3× better than the previous sensor reported in the literature which utilizes above-threshold operation. To further improve the sensitivity of the proposed sensor, an amplifier stage working in the sub-threshold region is designed. This enables 4× additional increase in sensitivity. A test-chip containing an array of 128 PMOS and 128 NMOS devices has been fabricated in 65-nm bulk CMOS process technology. A total of 28 dies across the wafer have been fully characterized and the random threshold voltage variations are reported here.