We demonstrate a silicon Mach-Zehnder modulator with L-shaped PN junctions to improve the modulation efficiency. The L-shape doping profile of the PN junction is obtained through multiple ion implantations with proper dose and energy. The depletion region of the PN junction has a strong overlap with the waveguide optical mode, resulting in improved modulation efficiency. We optimize the doping conditions to get a balanced performance in terms of modulator efficiency, insertion loss, and bandwidth. The measurement of the fabricated modulator reveals that the π phase shift voltage is 2.6 V for a 3-mm-long modulation arm. The modulation efficiency thus is Vπ·Lπ = 0.78 V·cm. The static extinction ratio is about 30 dB. The on-chip insertion loss is around 8.1 dB at zero bias. The EE 3-dB bandwidth is beyond 30 GHz at -2 V DC bias. Modulation of 32 Gb/s on-off keying (OOK) and binary phase-shift keying (BPSK) signals is successfully achieved.