We propose and demonstrate a fast and efficient silicon thermo-optic switch based on reverse breakdown of the pn junction. Benefiting from the direct heating of silicon waveguide by embedding the pn junction into the waveguide center, fast switching with on/off time of 330 and 450 ns and efficient thermal tuning of 0.12 nm/mW for a 20 μm radius microring resonator are achieved, indicating a high figure of merit of only 8.8 mW·μs. The results here show great potential for application in the future optical interconnects.