© 2020 IEEE. In this letter, a frequency-tunable rectifier in the $C$-band designed by a hybrid semiconductor integrated circuit (HySIC) concept is proposed. A GaAs monolithic microwave integrated circuit (MMIC) and a Si radio frequency integrated circuit (RFIC) were utilized as the HySIC configuration in the rectifier design. For the purpose of initial confirmation of this design validity, the GaAs and Si chips were fabricated and packaged onto the copper tungsten plate with gold plating. As measured results, frequency-tunable range from 3.82 to 4.55 GHz was measured. Maximum radio frequency (RF)-direct current (dc) conversion efficiency and output dc power in the measured power range from-10.0 to 17.8 dBm were 28.7% and 17.3 mW, respectively.