Comparison of the picosecond characteristics of silicon and silicon-on-sapphire metal-semiconductor-metal photodiodes

Abstract
The picosecond characteristics of silicon-based metal-semiconductor-metal (MSM) diodes with submicrometer finger spacing and width were studied. Diodes made on both bulk silicon and silicon-on-sapphire (SOS) substrates were measured by a subpicosecond electro-optic sampling system. The response of bulk-silicon MSM diodes was strongly dependent on the wavelength of the excitation light because of the change in penetration depth. The response of SOS diodes, on the other hand, had a weak dependence on wavelength since the thickness of the silicon layer limits the depth of photogenerated carriers. The response of a 200 nm SOS diode has a full-width at half-maximum of 4.5 and 5.7 ps with blue- and red-light excitations. The external quantum efficiency of SOS diodes was also determined at several selected wavelengths.