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The integration of nanophotonics components with advanced complementary metal-oxide-semiconductor (CMOS) electronics requires drive voltages as low as 1 V for enabling next-generation CMOS electrophotonics transceivers. Slow-light propagation has been recently demonstrated as an effective mechanism to enhance the modulation efficiency in free-carrier-based electrooptical silicon modulators. Here, we exploit the use of slow light to reduce the driving voltage of carrier-depletion-based Mach-Zehnder modulators. The slow-light phase shifter consists of a p-n junction positioned in the middle of a corrugated waveguide. A modulation efficiency as high as VπLπ ~ 0.6 V·cm is achieved, thus allowing data transmission rates up to 10 Gb/s with a 1.5-Vpp drive voltage and an insertion loss of ~12 dB. The influence of the drive voltage on the modulation speed as well as the variation of the insertion losses with a group index is also analyzed and discussed.