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The output voltages of the thermoelectric power sensor generally have the frequency-dependent characteristic, which influences the measurement accuracy of microwave power. To explain the microwave frequency-dependent characteristic, the electrothermal conversion principle, and the thermoelectric conversion principle are researched. This thermoelectric power sensor is designed and fabricated using microelectromechanical systems technology and GaAs monolithic microwave integrated circuit (MMIC) process, and an economy package solution is given for this thermoelectric power sensor. This power sensor is measured at X-band with input power in the 20 dBm (100 mW) range before and after package. Over the 100-mW dynamic range, the maximum relative error of the power measurement is 5.9% before calibration. After calibration, the maximum relative error becomes 0.96%, and the power measurement is almost independent of the microwave frequency interference. The sensitivity is about 0.16 and 0.21 mV/mW with excellent linearity before and after package, respectively. According to the measurement results, the feasibility of direct back-side attaching with the chip on the carrier brings an economy package solution for the thermoelectric power sensor. Furthermore, in addition to excellent linearity and improved frequency-dependent characteristic, another significant advantage is that this power sensor can be integrated with MMICs and other planar connecting circuit structures with zero dc power consumption.