The ultrafast characteristics of crystalline-silicon metal-semiconductor-metal (MSM) photodiodes with finger widths and spacings down to 200 nm, subjected to femtosecond optical pulse excitations, was measured with a subpicosecond electro-optic sampling system. Electrical responses with fullwidth at half-maximum (FWHM) as short as 3.7 ps, at a corresponding 3 dB bandwidth of 110 GHz, were generated by violet-light excitation. These diodes are the fastest silicon photodetectors reported to date. Detailed bias and light-intensity dependence of the diode response has been measured. These results are used to obtain the velocity-field relation of electrons in silicon and to demonstrate the ideal transit-time-limited response of the diodes.