Why do high-speed silicon modulators often use the depletion type, which applies reverse bias to the pn junction of the phase shifter to vary the depletion width?

Answer: C. The refractive index changes as majority carriers move in and out, so it is not limited by minority-carrier lifetime and can operate fast

In the depletion type, the magnitude of the reverse bias varies the depletion width, moving majority carriers in and out to change the refractive index. Its response speed is set mainly by the junction capacitance and resistance, giving bandwidths of several tens of GHz. The PIN type, which injects carriers under forward bias, produces a larger index change and higher efficiency, but the injected minority carriers take on the order of nanoseconds to disappear by recombination, making high speed difficult as is. The depletion type can be said to trade efficiency for speed. Avalanche multiplication does not amplify light.