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In this paper, a ring oscillator-based CMOS temperature sensor with nano-watt power consumption is presented for RFID applications. Unlike conventional temperature sensors based on bandgap reference and ADC that consume large amount of power, the proposed sensor exploits the temperature dependence of the threshold voltage and carrier mobility of MOS transistors that affect the frequency of a ring oscillator. In order to maximize the temperature sensitivity and dynamic range, a supply voltage of 0.3 V is used, which allows the oscillator to operate in subthreshold, near-threshold and above threshold region under different temperature conditions. In order to handle process variation, the frequency of the oscillator can be digitally trimmed by both a capacitor bank and stacked transistors. Measured data from 0.13-mum CMOS test chips indicate that the proposed temperature sensor has a resolution of 0.4degC/LSB with a 10-bit digital output code over a temperature range of 8degC to 85degC. At 10 Hz of sampling frequency, the proposed sensor consumes 95 nW and occupies 0.04 mm2.