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We report on the experimental demonstration of a GaAs IQ modulator. The device consists of two “nested” Mach-Zehnder modulators for the inphase and quadrature component and is operated at a symbol rate of 25 GBd. Using QPSK, 16QAM, 32QAM and 64QAM, data rates of up to 150 Gbit/s were encoded on a single carrier in one polarization. The individual Mach-Zehnder modulators, and hence, the IQ-modulator have an electro-optic 3 dB bandwidth of 27 GHz and a 6 dB bandwidth larger than 35 GHz. The extinction ratio of the Mach-Zehnder exceeds 20 dB. The devices exhibit small footprint of 2 mm × 40 mm and can be integrated on large-area GaAs wafers using high-yield fabrication processes while providing performance similar to established lithium niobate devices.