Editorial Feature

What Are CMOS Sensors?

Continue reading to learn more about complementary metal-oxide-semiconductor (CMOS) sensors, highlighting their working principle, advantages over CCD sensors, applications, challenges, and future trends.

The circuit board with the CMOS sensor lies on the circuit diagram

Image Credit: Andrew Berezovsky/Shutterstock.com

The concept of CMOS sensors was first proposed in the late 1960s, but commercial manufacturing didn't become feasible until the 1990s due to limitations in microfabrication technology. Today, these sensors are widely used in consumer electronics, automotive systems, surveillance, and medical imaging.1

Understanding CMOS Technology

The fundamental building block of a CMOS sensor is the pixel, consisting of a photodiode and three transistors: a reset transistor to pre-charge the photodiode, a source-follower transistor to sense the signal voltage and a row-select transistor.

These sensors typically operate at voltages ranging from 1.8V to 5V, resulting in lower power consumption and extended battery life. Additionally, they can be monolithically integrated with readout and signal processing electronics on the same chip, enabling compact and cost-effective solutions.

Moreover, CMOS sensors leverage the semiconductor chip manufacturing technology, allowing the integration of millions of transistors on a single chip. As a result, a large array of pixels can be created, each with its readout transistors and photodiode, along with ancillary electronics for addressing the array, buffering the analog video signal, and even digitizing it for processing or display.2

Basic Structure and Operation

A CMOS sensor's fundamental structure and operation is based on the functioning of PN-junction photodiodes. When reverse-biased at a voltage below the avalanche breakdown voltage, these photodiodes generate a photo-current proportional to incident light intensity. This photo-current is initially too small to measure directly and is integrated over time by exposing the photodiode to light for a specific duration.

The integrated charge provides a stronger and more reliable signal, particularly for weak or noisy signals. However, it is essential to prevent saturation at high light intensities and minimize dark current (generated without light) to maximize the device's sensitivity.

During operation, a two-dimensional array of photodetectors captures incident light intensity, with each photodetector converting charge to voltage. This voltage signal is then amplified and routed to an output amplifier via row and column select switches.

An analog-to-digital converter (ADC) digitizes the amplified signal. During readout, pixel values from a row are transferred in parallel to storage capacitors before being sequentially read out.3

In an active-pixel sensor (APS), each pixel includes an amplifier and a photodiode, while a passive-pixel sensor (PPS) lacks the integrated amplifier. In a digital-pixel sensor (DPS), each pixel incorporates its analog-to-digital converter and memory block, resulting in digital output values proportional to light intensity.4

Advantages Over CCD Sensors

Faster Data Rates

CMOS sensors leverage individual pixel amplification, direct access to pixel signals and parallel pixel readout, enabling faster data transfer rates than the bucket-brigade style of data transfer used in CCDs.1

Lower Cost and Power Consumption

CMOS sensors are significantly cheaper than CCDs, as they can be manufactured using semiconductor equipment. Furthermore, their digital circuitry and lower voltage operation substantially reduce power consumption (<1/3 the power of CCD), making them ideal for battery-powered devices.5

On-Chip Integration

The integration of logic circuitry during manufacturing allows CMOS sensors to incorporate image readout and signal processing circuits on the same chip, enabling applications such as image recognition and artificial vision.

Windowing Capabilities

CMOS technology offers the unique ability to read out a portion of the image sensor, allowing for elevated frame or line rates in specific regions of interest. This capability is particularly valuable for high-precision object tracking and motion capture applications.6

Applications in Imaging and Sensing

The versatility and performance of CMOS sensors have propelled their adoption across a wide range of applications, including:

Medical Imaging

High-speed spectral optical coherence tomography (OCT) instruments powered by CMOS cameras have revolutionized in vivo imaging of the eye's anterior segment, enabling detailed real-time observation and enhanced diagnostic capabilities.

These sensors can detect particles with velocities up to approximately 500 μm/s and diameters between 5-15 μm, offering a low-cost, practical solution for enhancing digital cytometric capabilities in cell detection and analysis.5

Automotive Industry

CMOS image sensors play a crucial role in car safety systems, leveraging technologies like 3D time-of-flight (ToF) imaging for object detection and collision avoidance. They are also employed in rangefinders for distance measurement, contributing to improved vehicle safety and pedestrian protection.

Digital Photography

CMOS sensors have revolutionized digital cameras, enabling noise-free designs, shot noise elimination, low-light imaging, digital watermarking, and real-time 3D imaging capabilities.

Space Applications

CMOS APSs have become pivotal in aerospace applications, such as star trackers for spacecraft attitude determination and pico-satellite imaging systems, offering reduced power consumption, higher sample rates, and improved angular rate limits compared to traditional CCD counterparts.5

Challenges and Limitations

Despite their numerous advantages, CMOS sensors still face some challenges and limitations:

Limited Fill Factor and Quantum Efficiency

In-pixel electronics and metal bus tracks can reduce the fill factor (light-sensitive area) by 30% and quantum efficiency, resulting in lower sensitivity. This compromise is due to light reflection from aluminum bus lines and the loss of photon-induced electrons within in-pixel transistor electronics.

However, thinning and back-illumination have been proposed, with some sensors achieving quantum efficiencies comparable to CCD counterparts.2

Higher Noise Levels

CMOS sensors can be more susceptible to various noise sources, especially in low-light conditions, due to multiple active devices in the readout path and fabrication inconsistencies.4

Limited Dynamic Range

Low-voltage CMOS sensors cannot achieve linear voltage swings outside transistor threshold regions, resulting in a dynamic range of about 5000 (significantly less than CCD sensors).

Researchers are exploring switched-sensitivity, non-linear pixel designs and more complex five- and six-transistor configurations to enhance dynamic range.2

Future Trends and Innovations

Increased Resolutions and Pixel Densities

One of the most significant trends is the continuous increase in resolutions and pixel densities, enabling the capture of images with unprecedented detail and visual clarity. Modern sensors already offer resolutions exceeding 250 megapixels, catering to the demands of industrial inspection systems, aerial imaging, and mapping platforms.7

Miniaturization and Power Efficiency

Driven by consumer demand for compact and energy-efficient devices, CMOS sensor development has focused heavily on miniaturization and reduced power consumption. System-on-chip (SoC) platforms have advanced dramatically, combining sensors with processing, memory, and other peripheral devices on a single chip, further improving power efficiency and facilitating adoption in portable devices and the Internet of Things (IoT) applications.8

Custom-stacked CMOS Sensors

Stacked CMOS image sensor technology, which separates pixel transistors and photodiodes onto different substrate layers, promises wider dynamic range, reduced noise, and the opportunity for customized CMOS chips designed for specific applications.

This innovation opens up new possibilities for faster cameras, high-speed imaging, and the integration of intelligent functions directly into the chip, such as noise reduction and image processing algorithms.9

As CMOS image sensor technology continues to evolve, it is poised to play an increasingly important role in various applications, from consumer electronics and automotive to medical and scientific research.

CCD vs CMOS – Which is Better?

References and Further Reading

  1. Tokyo Electron Limited. (2024). What is a CMOS Image Sensor? [Online]. Available at: https://www.tel.com/museum/exhibition/principle/cmos.html
  2. Waltham, N. (2013). CCD and CMOS sensors. In: Huber, M.C.E., Pauluhn, A., Culhane, J.L., Timothy, J.G., Wilhelm, K., Zehnder, A. (eds) Observing Photons in Space. ISSI Scientific Report Series, vol 9. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-7804-1_23
  3. Theuwissen, A. J. (2008). CMOS image sensors: State-of-the-art. Solid-State Electronics52(9), 1401-1406. https://doi.org/10.1016/j.sse.2008.04.012
  4. Dr. Steve Arar. (2020). Introduction to CMOS Image Sensors. [Online]. Available at: https://www.allaboutcircuits.com/technical-articles/introduction-to-cmos-image-sensors/
  5. Drăgulinescu, A. (2012). Applications of CMOS image sensors: state-of-the-art. In Advanced Topics in Optoelectronics, Microelectronics, and Nanotechnologies VI (Vol. 8411, pp. 331-340). SPIE. https://doi.org/10.1117/12.966391
  6. Litwiller, D. (2001). Ccd vs. cmos. Photonics spectra35(1), 154-158. https://courses.cs.duke.edu/fall11/cps274/papers/Littwiller01.pdf
  7. Canon. (2024). CMOS Image Sensor-Ultra-High 250MP Resolution. [Online]. Available at: https://asia.canon/en/campaign/cmos-image-sensors/ultra-high-250mp/ultra-high-250mp-resolution
  8. John Butler. (2023). Image Sensor Developments Usher in the Future of Imaging. [Online]. Available at: https://www.automate.org/vision/industry-insights/image-sensor-developments-usher-in-the-future-of-imaging
  9. Possibility Editorial. (2023). The Future of CMOS is Stacked. [Online]. Available at: https://possibility.teledyneimaging.com/the-future-of-cmos-is-stacked/

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Owais Ali

Written by

Owais Ali

NEBOSH certified Mechanical Engineer with 3 years of experience as a technical writer and editor. Owais is interested in occupational health and safety, computer hardware, industrial and mobile robotics. During his academic career, Owais worked on several research projects regarding mobile robots, notably the Autonomous Fire Fighting Mobile Robot. The designed mobile robot could navigate, detect and extinguish fire autonomously. Arduino Uno was used as the microcontroller to control the flame sensors' input and output of the flame extinguisher. Apart from his professional life, Owais is an avid book reader and a huge computer technology enthusiast and likes to keep himself updated regarding developments in the computer industry.

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