We report the fabrication and experimental characterization of field-effect transistor-based silicon optical waveguide modulators. The entire modulation scheme is realized through the so-called carrier injection or plasma dispersion effect. The spin-on-dopant (SOD) method was conducted at 1000 °C in a mixture of nitrogen/oxygen ambient to separately pattern the heavily doped source (n+), gate (p+), and drain (n+) regions. The corresponding p- and n-type dopant profiles were determined using the spreading resistance probe (SRP) technique, where the average dopant concentrations of both the heavily doped p+ and n+ regions were in the neighborhood of 2-4×1020 cm-3. The resultant dopant concentrations and diffusion depths were found to be critically dependent on the diffusion time and temperature. An enhancement in the modulation efficiency of modulators was realized when the corresponding rib waveguide width and modulation length became respectively wider and longer. Furthermore, the modulators thus fabricated showed ultra-sensitivity in drain-source voltage (VDS), where a modulation depth of nearly 100% at VDS of less than 5 V was achieved with and without the biasing gate current applied. Finally, the rise and fall times extracted from the dynamical transmitted intensity measurement were in a range spanning from 350 to 550 μs.
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