CMOS Architecture

A CMOS imager is fundamentally a photodiode array with simple buffer circuitry embedded within each pixel. The pixels are addressed row at a time using a vertical scanner structure at the edge of the imaging area. The selected row can then either be reset, which clears any photo-charge accumulated on the photodiodes, or read-out, which involves connecting each pixel's output to peripheral buffer circuitry via column bus structures. Further on-chip peripheral circuitry can be used to digitise the voltage signals giving a fully digital sensor. Other CMOS devices operated at lower frame rates give an analogue output.


Schematic 4 x4 pixel CMOS sensor

Photomicrograph of a small region

of a CMOS sensor

Since the photodiodes must share silicon "real estate" with the unit cell transistors and bus structures, the light sensitivity of CMOS sensors is generally less than CCDs.