CMOS Image Sensors vs. CCDs

CMOS image sensors are widely used in digital still cameras for capturing images. Their competitors are CCDs, accomplishing the same task. We analyze the pros and cons of these two options.

CMOS image sensors with an active-pixel architecture differ from CCD devices in that each pixel not only has the photo-sensitive element transforming incoming photons (light) into electrons, but also a circuitry sensing such generated electronic charge through a charge-to-voltage conversion. This is the major difference between CCD and CMOS image sensors and, from there, the rest of architecture differs accordingly.

We can see, from this simple fact, what are the pros and cons of one solution versus the other. For starters, the opaque sensing circuitry does not allow photons to reach the photoactive substrate, where electrons are generated. Therefore, given a pixel dimension, a CMOS sensor is inherently less sensitive to light thus requiring brighter scenes.

Another difference is that the charge-to-voltage amplifier is just one in a CCD and one per pixel in a CMOS. Therefore, CCD offers greater uniformity because it is not afflicted by process spread (millions of transistors can not statistically have the same electrical characteristics).

Contrary to CCDs, in CMOS sensors the analog to digital conversion is on-chip. The circuitry present in each pixel delivers an analog signal, which is fed to an analog to digital converter (ADC). This is yet another macroscopic difference between CCD and CMOS sensors: CMOS sensors have additional circuitry making their design effort and sizes higher, but permit a simpler off-chip circuitry.

Another difference is that CMOS sensors require much less energy and lower voltages to operate, thus leading to extended battery life for handheld applications.

Finally, an advantage of CMOS over CCD sensors is that they require a less dedicated process, thus reducing cost.

Although all the above-mentioned differences are real, CCD and CMOS image sensors have become closer and closer in their characteristics and performances. That is why both technologies still co-exist. Moreover, their unique strengths and weaknesses are exploited in different applications.