CMOS vs CCD

In the current age of AI, imaging systems are evolving to replace the human eye. Imaging is an exponentially growing mode of data acquisition and collection. Imaging is used to gather all kinds of data such as age, gender, height etc. of people standing at signal, using a vending machine, visiting a shop, crossing the road, passing by a kiosk, registering at some conference, ordering a coffee, boarding a flight etc. Apart from people, imaging also helps collect data on vehicles, packages, animals, birds, fishes, medicines, groceries and so on.

In this digital era, let’s keep our focus on digital sensors. Digital imaging sensors can be categorized into 2 types, CCD and CMOS, based on its building blocks. We shall delve into the 2 types of digital sensors that are in the market.

CCD (Charge Couple Devices) sensors are based on MOS technology with MOS capacitor as the building block. Each cell of a CCD image sensor is an analog device and it can be considered as a photo sensor. When light strikes the chip it is held as a small electrical charge in each photo sensor. CCD are current driven device. It collects charge in its pixels (basically capacitive bins), then physically shifts the charge on the imager surface to the output for sampling and digital conversion.

CMOS (Complementary Metal Oxide Semiconductor) sensors are also based on MOS technology, however, with MOSFET amplifiers as the building blocks. In contrast to CCD, CMOS as voltage-driven technology. CMOS image sensor has an amplifier for each pixel compared to the few amplifiers of a CCD. Light striking the pixel creates a voltage proportional to intensity. The voltage is sampled at the pixel and digitized on the imager.

Differences

1. Charge Reading – CMOS gives more flexibility in terms of charge reading because,

a. CCD – In CCD device, the charge is transported across the chip and read at one

corner of the array. The ADC converter turns each pixel value into a digital value.

b. CMOS – In CMOS, there are transistors at each pixel that amplify and move the

charge using traditional wires. Thus, each pixel is individual.

2. Manufacturing – CMOS is inexpensive as manufacturing process is simpler

a. CCD - CCDs use a special manufacturing process to create the ability to transport

charge across the chip without distortion.

b. CMOS - CMOS chip use traditional manufacturing processes, the same process

used to manufacture microprocessors, to create the chip.

3. Quality and Noise – CCD sensors are known for its higher quality.

a. CCD – The high-end production process and the sensor architecture enables CCD

to produce high quality, low noise images.

b. CMOS – CMOS production is done in a traditional manner and the architecture

makes it susceptible to noise.

4. Sensitivity – CCD sensors have higher sensitivity per pixel

a. CCD - CCD architecture has the photodiode well exposed to the photons. Thus,

the sensitivity is higher.

b. CMOS - CMOS circuitry involves multiple layers of semiconductor material to take

care of charge conversion and transport. Thus, photons needs not always hit the

photodiode which in turn reduces the effective sensitive that can be achieved.

5. Power Consumption – CMOS is more power conservative than CCD.

a. CCD – CCD uses process that consumes lot of power.

b. CMOS – CMOS is known for low power consumption. It consumers as much as

100 times lower power than CCD.

Summary

CCD sensors are used in application areas that focusses on highest quality image at high resolution and at the same time ensure highest sensitivity.

CMOS sensors are used in application areas requiring cost effective, power efficient imaging sensor options. CMOS has been evolving its capability to have higher pixels per sensor and are on the verge of nearing parity with CCD in some application areas.

Hope this article was helpful. Please feel free to get in touch with us in case of any queries.

We are expecting questions like – resolutions supported by CMOS, power consumed by CMOS, different architecture in CMOS.

Cheers

Sarvesh Rajagopal

sarvesh@regami.solutions