CMOS vs CCD Image Sensors: Differences Explained

Both sensors turn light into electric charge in each pixel; they differ in how that charge is read out. A CCD moves the charge pixel by pixel to a single output amplifier. A CMOS sensor converts charge to voltage inside each pixel and reads many pixels in parallel. CMOS sensors are cheaper, faster and use less power, and they can integrate ADCs and image processing on the same chip, so they have replaced CCDs in almost all consumer products.

AspectCMOS image sensorCCD image sensor
ReadoutEach pixel has its own amplifier; signals are read out in parallel, usually through column ADCsCharge is shifted pixel by pixel through the array to one or a few output amplifiers
ManufacturingStandard CMOS-type process, built in common semiconductor fabsSpecial process designed for charge transfer
On-chip functionsADCs, timing and image processing can be integrated; stacked designs add logic and memoryMostly only the sensor; supporting circuits are separate chips
Power consumptionLowHigh, because charge transfer needs high-speed clocks and several supply voltages
Speed (frame rate)High, thanks to parallel readoutLower, limited by the serial transfer
Noise and uniformityOnce worse, now comparable or better thanks to pinned photodiodes and correlated double samplingTraditionally low noise and uniform pixels
ShutterRolling shutter is common (fast subjects can look skewed); global shutter versions need extra pixel circuitryGlobal shutter is natural in many designs
Blooming and smearLess proneMore prone, because excess charge can spill into neighboring pixels
CostLow, thanks to mass productionHigher
Typical usesSmartphones, digital cameras, automotive cameras, webcamsScientific and astronomical cameras, some industrial, medical and space imaging

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

CMOS
Almost everything today: phones, cameras, cars, security and industrial cameras. It is cheap, fast and flexible, with backside-illuminated and stacked versions improving sensitivity and speed.
CCD
Special cases that value its long track record of low noise and uniformity, such as some scientific, astronomy and space instruments.

FAQ

Why did CMOS sensors replace CCDs?

CMOS sensors are cheaper, faster and use less power, and they can integrate ADCs and image processing on the same chip. Improvements in noise and sensitivity removed the main advantage of the CCD.

What is a rolling shutter?

A rolling shutter reads the rows of a CMOS sensor one after another, so fast-moving subjects look skewed. A global shutter exposes all pixels at once but needs extra circuitry in each pixel.

Is CCD still used?

Yes, in some scientific, astronomical, industrial and space imaging where its long record of low noise and uniformity matters.

What is blooming?

Blooming happens when a bright pixel overflows and spills charge into neighboring pixels. CCDs are more prone to it than CMOS sensors.

Check your understanding

Compared with CCD image sensors, which is a general advantage of CMOS image sensors?

  1. APixel charges are transferred in sequence and read out at a single output
  2. BPeripheral circuits are easy to integrate on the same chip, and power consumption is low
  3. CThey cannot be manufactured with CMOS processes
Show answer and explanation

Answer: B. Peripheral circuits are easy to integrate on the same chip, and power consumption is low

CMOS image sensors give each pixel its own amplifier and are made in standard CMOS processes, so ADCs and image-processing circuits integrate easily on the same or a stacked chip, with low voltage, low power and fast readout. CCDs shift charge sequentially to a single output amplifier; they offer low noise but require high drive voltages and dedicated processes, and CMOS now dominates.

Which correctly describes how a CCD image sensor reads out its signal?

  1. APixel charges are transferred in sequence like a bucket brigade and converted to voltage at the output
  2. BEach pixel's amplifier converts charge to voltage, and pixels are read row by row
  3. CLight is converted directly into a digital signal
Show answer and explanation

Answer: A. Pixel charges are transferred in sequence like a bucket brigade and converted to voltage at the output

A CCD shifts stored charge pixel to pixel like a bucket brigade by sequencing electrode voltages, converting it to voltage at a single output amplifier. Transfer efficiency exceeds 99.999%, and the single amplifier minimizes pixel-to-pixel variation, giving low noise. But CCDs need drive voltages above 10 V and dedicated processes, consume more power and read out slowly, so CMOS has replaced them except in niches such as astronomy.

Why do fast-moving subjects appear distorted with the rolling shutter method of CMOS image sensors?

  1. ABecause exposure and readout timing are offset row by row
  2. BBecause all pixels are exposed at the same time
  3. CBecause of the color filter arrangement
Show answer and explanation

Answer: A. Because exposure and readout timing are offset row by row

A rolling shutter exposes and reads pixels row by row, so top and bottom rows are captured several to tens of milliseconds apart. Fast-moving subjects appear skewed and flickering lights produce bands. A global shutter exposes all pixels simultaneously, avoiding distortion, and is used in industrial inspection and automotive cameras, but it needs in-pixel charge storage, making pixels larger.

  1. What is the purpose of correlated double sampling (CDS) used in CMOS image sensor readout?
  2. Which correctly describes a stacked CMOS image sensor?
  3. What is the main advantage of a backside-illuminated (BSI) CMOS image sensor?

Practice Image sensors and optical devices questions

Key terms explained

CMOS image sensor
An image sensor made with a CMOS process that has an amplifier in each pixel. It is low-power, can integrate peripheral circuits, and dominates smartphone cameras and more.
CCD image sensor
An image sensor that shifts pixel charges along like a bucket brigade and converts them to voltage at an output stage. It offered low noise and high image quality but used more power, and CMOS sensors have replaced it in most uses.
Stacked CMOS image sensor
An image sensor whose pixel array and signal-processing circuitry are made on separate chips and bonded together. Each can use the most suitable process, making the chip smaller and processing faster.

Related comparisons