Image sensors and optical devices

In a CMOS image sensor, what problem occurs when charge transfer from the photodiode to the floating diffusion is incomplete?

  1. ACharge from the previous frame remains and affects the next image (image lag)
  2. BThe color filter changes color
  3. CThe pixel count decreases
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Answer: A. Charge from the previous frame remains and affects the next image (image lag)

In a four-transistor pixel, all charge accumulated during exposure is supposed to be moved through the transfer gate to the floating diffusion. If charge is left behind, for example by a potential barrier, it adds to the next frame's signal as image lag and also contributes noise. Complete transfer is achieved by designing the photodiode's potential profile.

  • A. Correct. Leftover charge causes image lag.
  • B. Transfer problems do not change the filter color.
  • C. The pixel count does not change.
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