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A 30-130 GHz ultra broadband direct-conversion binary phase shift keying (BPSK) modulator using a 0.5-mum enhancement/depletion-pseudomorphic high-electron mobility transistor (E/D-PHEMT) process is presented in this letter. The BPSK modulator was designed using a modified reflection-type topology with E-mode PHEMT devices. An advantage for the E-mode PHEMT process is positive gate bias, and therefore the bias circuit for the modulation would be less complicated. Moreover, the BPSK modulator demonstrates an error vector magnitude of within 5.5%, an adjacent channel power ratio of better than -35 dBc, and an on-off isolation of greater than 20 dB from 30 to 130 GHz. The chip size of the BPSK modulator is only 0.8x0.7 mm2. To the best of the authors' knowledge, this work is the highest operation frequency with the widest bandwidth among all the reported monolithic microwave integrated circuit-based BPSK modulators.