A novel waveguide-coupled germanium p-i-n photodiode is demonstrated which combines high responsivity with very high -3 dB bandwidth at a medium dark current. Bandwidth values are 40 GHz at zero bias and more than 70 GHz at -1 V. Responsivity at 1.55 µm wavelength ranges from 0.84 A/W at zero bias to 1 A/W at -1 V. Room temperature dark current density at -1 V is about 1 A/cm2. The high responsivity mainly results from the use of a new, low-loss contact scheme, which moreover also reduces the negative effect of photo carrier diffusion on bandwidth.

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Fundamental limiting factors regarding high-speed performance of broadband depletion-type silicon (Si) Mach-Zehnder modulators (MZMs) are studied. Optical and electrical measurements of MZMs with traveling wave electrodes (TWE) reveal significant dependences between optoelectrical bandwidth and design parameters. An equivalent circuit model is implemented to fit measured modulator characteristics. Using the model, the limits of TWE depletion-type Si MZM systems are studied under the requirement of specific driving voltage. By comparing phase shifters with different doping concentration or junction position, we explore the fundamental optical and electrical tradeoffs which are limiting high-speed operation.

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