CMOS Sensors

Glossary

CMOS Sensors

updated: 2025-07-13

CMOS (complementary metal-oxide-semiconductors) sensors are most common in today electronic cameras. They belong to the category of active-pixel sensors (APS), where each cell consists of a photodetector and one or more transistors.

Compared with CCD (charged coupled device) sensors, CMOS sensors have a couple of advantages:

less expensive to produce
image capturing and sensing elements can be combined into the same chip
better control of blooming (=bleeding of electrons from one cell to neighboring cells)

but also some disadvantages, like

Rolling shutter effect: The sensor captures one row at a time, which leads to distorted images of fast-moving objects.
Older CMOS sensors produced images with more noise than CCD sensors. However, this has not been the case for newer CMOS sensors since at least 2020.
The active circuitry in CMOS pixels occupies an area of the surface that is not light-sensitive, which reduces the device's photon-detection efficiency.Most modern astro cameras use back-illuminated sensors, which can mitigate this problem.. However, frame-transfer CCDs also have about half the area dedicated to non-sensitive frame store nodes. Therefore, the relative advantages depend on the types of sensors being compared.

Internal structure of a pixel

The standard CMOS APS pixel consists of a photodetector (pinned photodiode), a floating diffusion, and a 4T cell, which consists of four complementary metal–oxide–semiconductor (CMOS) transistors: a transfer gate, a reset gate (Mrst), a selection gate (Msel), and a source-follower readout transistor (Msf).

Attribution: By Gargan - Own work, Public Domain