A high-speed carrier-depletion silicon modulator based on a fringe field pn junction design is presented. Due to the strong fringe field, the size of heavily doped regions can be reduced and away from the waveguide core, whereas large modulation efficiency is still accomplishable. The VπL is 1.8 V-cm and the phase shifter loss is 1.3 dB/mm. The figure of merit (FOM), defined by the product of VπL and phase shifter loss, is estimated to be 23.4 dB-V. The modulation speed and depth are 11.8 GHz and 8.1 dB, respectively, which is mainly limited by the mobility of poly-Si.