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In this study, highly miniaturized and low-impedance on-chip passive components were developed using a novel microstrip-line structure employing periodically perforated ground metal (PPGM) on a GaAs monolithic microwave integrated circuit (MMIC). The proposed microstrip-line structure exhibited a much lower characteristic impedance and shorter guided wavelength than the conventional structure. Using the microstrip line with PPGM, a highly miniaturized rat-race and branch-line coupler with low-port impedances were developed for K/Ka-band MMIC applications. The miniaturized rat-race and branch-line coupler were fabricated on a GaAs substrate with a height of 100 μm, and their sizes were 0.375 and 0.25 mm2, respectively, which are less than 10% of the sizes of conventional ones. The miniaturized rat-race and branch-line coupler exhibited good RF performances from 20 to 30 GHz. In addition, in this study, highly miniaturized on-chip filters and biasing components were also realized using the microstrip-line structure with PPGM. The above results indicate that a microstrip-line structure with PPGM is a promising candidate for applications to highly miniaturized passive components on GaAs MMICs, and it will enable the development of fully integrated MMICs, including all passive components.