A 75 GHZ SILICON METAL-SEMICONDUCTOR-METAL SCHOTTKY PHOTODIODE

The ultrafast characteristics of crystalline-silicon metal-semiconduct or-metal (MSM) photodiodes with 300 nm finger width and spacing were m easured with a subpicosecond electro-optic sampling system. Electrical responses with full width at half maximum as short as 5.5 and 11 ps, at corresponding 3 dB bandwidths of 75 and 38 GHz, were generated by v iolet and red photons, respectively. The difference is attributed to t he photon penetration depth which is much larger than the diode finger spacing at red, but smaller at violet. Light-intensity dependence was also examined at different wavelengths, indicating a linear relation and a higher sensitivity in the violet. These results not only demonst rated the fastest silicon photodetector reported to date, but also pin pointed the dominant speed-limiting factor of silicon MSM photodiodes. A configuration is suggested to improve the speed of these detectors at long wavelengths.