We present a broadband 2x2 electro-optic silicon switch with an ultralow switching power and fast switching time based on a Mach-Zehnder interferometer (MZI). Forward-biased p-i-n junctions are employed to tune the phase of silicon waveguides in the MZI, to achieve a π-phase switching power of 0.6 mW with a drive voltage 0.83 V with a MZI arm length of 4 mm. The 10%-90% switching time is demonstrated to be 6 ns. Optical crosstalk levels lower than −17 dB are obtained for an optical bandwidth of 60 nm. The free carrier induced silicon refractive index change is extracted from the experimental results for the concentration range from 10^16 to 10^17 cm^−3. We find that at the concentration of 10^16 cm^−3, the index change is about twice that calculated by the commonly used index change equation.