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We have demonstrated a voltage reference circuit with ultra-low power by a standard 0.5 μm complementary metal oxide semiconductor (CMOS) N-well process. Two voltages with opposite temperature coefficient are added to produce a reference voltage which is approximately equal to the threshold voltage of a metallic oxide semiconductor field effect transistor (MOFET) at absolute temperature of zero degree. In addition, a current that is proportional to the square of absolute temperature, which can accurately compensate the pre-exponential factor of subthreshold current is introduced. Experimental results show that the proposed circuit can provide a reference voltage with a temperature coefficient of 5.54 ppm/°C over a range from -20 to 85°C, a total supply current of 123 nA, as well as an ultra-low power of only 0.209 μW under a supply voltage of 1.7 V at room temperature.