  We demonstrate the generation of higher-order modulation formats using
silicon-based inphase/quadrature (IQ) modulators at symbol rates of up to 100
GBd. Our devices exploit the advantages of silicon-organic hybrid (SOH)
integration, which combines silicon-on-insulator waveguides with highly
efficient organic electro-optic (EO) cladding materials to enable small drive
voltages and sub-millimeter device lengths. In our experiments, we use an SOH
IQ modulator with a {\pi}-voltage of 1.6 V to generate 100 GBd 16QAM signals.
This is the first time that the 100 GBd mark is reached with an IQ modulator
realized on a semiconductor substrate, leading to a single-polarization line
rate of 400 Gbit/s. The peak-to-peak drive voltages amount to 1.5 Vpp,
corresponding to an electrical energy dissipation in the modulator of only 25
fJ/bit.
