A 24-GHz-band, three-dimensional microwave monolithic integrated circuit (MMIC) bi-phase modulator, that is a combination of an active 0 splitter and a SPDT switch, is proposed and demonstrated. Comparing conventional K-band bi-phase modulators, this modulator is broadband, significantly MMIC compatible, and shrinks its chip area. A fabricated bi-phase modulator exhibits an area of 1.0 0.7 mm 2 and a 1-dB bandwidth of nearly 10 GHz. The SPDT switch is optimized in control-gate's time constant so that a very clear bi-phase modulated waveform can be generated from 1 Gbps PN code. The insertion loss and the phase imbalance are 6.4 dB+ 0.6 dB and within 10 , respectively, between 22 GHz and 32 GHz.Index Terms-Active splitter, bi-phase modulator, microwave monolithic integrated circuit (MMIC), 1 Gbit/s.

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