A silicon optical modulator of a metal–oxide–semiconductor (MOS) capacitor based on free-carrier absorption has been proposed and successfully developed. In this report, its modulation performance with accumulation-mode absorption instead of inversion-mode absorption is described expecting better absorption performance due to the relatively smaller carrier mobility of holes. The device consists of an 8-mm-long and 1.5-µm-thick (110) silicon-on-insulator core and a surrounding clad of 1.5-µm-thick buried oxide underneath and doped polycrystalline silicon on top. Infrared light absorption by accumulation carriers is not sufficiently strong in the 1.55-µm-infrared-wavelength regime. However, an optical response of 13% at a gate voltage of -30 V is obtained. Preliminary analysis has also been conducted for the potential of the optical modulator based on accumulation-carrier absorption. It is predicted that this modulator will be more effective in deeper-infrared regions unlike other modulators such as the Mach–Zehnder interferometer.
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