Image sensors and optical devices

In the buried (pinned) photodiode of a CMOS image sensor, what is the main purpose of the heavily doped p+ layer at the surface?

  1. ATo act as a color filter
  2. BTo make the pixel larger
  3. CTo suppress dark current originating from surface interface states
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Answer: C. To suppress dark current originating from surface interface states

The interface between the silicon surface and the oxide has many interface states, which are a major source of dark current. Burying the photodiode's n layer away from the surface and covering the surface with a heavily doped p+ layer greatly reduces dark current from interface states. It also makes complete charge transfer easier.

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