Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) are gained popularity recently because of the advantages, such as ability of on-chip signal processing, and low cost and low power consumption, compared to other types of image devices. In addition, chip industry's steady march to finer sub-micron nodes, combined with adding more features on a per-pixel basis, continues to push CIS solutions ahead of charge-coupled devices (CCDs). CIS technology makes it possible to integrate imaging, timing, and readout functions all on the same device. CIS technology also enables practical system-on-a-chip solutions that serve an expanding array of display-centric applications.
Dark current is one of the important parameters used to characterize the performance of an image sensor. Dark current (leakage current) is electric current that flow through photosensitive devices, such as photodiodes, when no photons are entering the devices. As the pixel sizes decrease, the amount of photons received by the photodiodes of CIS also decreases. As a result, the effect of dark current becomes more pronounced. Therefore, minimizing dark current is a critical issue for advanced CIS. It is within this context the following disclosure arises.