Active-pixel sensor
An active-pixel sensor (APS) is an
CMOS sensors emerged as an alternative to charge-coupled device (CCD) image sensors and eventually outsold them by the mid-2000s.[4]
The term active pixel sensor is also used to refer to the individual pixel sensor itself, as opposed to the image sensor.[5] In this case, the image sensor is sometimes called an active pixel sensor imager,[6] or active-pixel image sensor.[7]
History
Background
While researching
At
A key element of the modern CMOS sensor is the
Passive-pixel sensor
The precursor to the APS was the passive-pixel sensor (PPS), a type of
Passive-pixel sensors were being investigated as a
Active-pixel sensor
The active-pixel sensor consists of active pixels, each containing one or more
The
In the early 1990s, American companies began developing practical MOS active pixel sensors. In 1991, Texas Instruments developed the bulk CMD (BCMD) sensor, which was fabricated at the company's Japanese branch and had a vertical APS structure similar to the Olympus CMD sensor, but was more complex and used PMOS rather than NMOS transistors.[2]
CMOS sensor
By the late 1980s to early 1990s, the
In 1993, the first practical APS to be successfully fabricated outside of Japan was developed at NASA's Jet Propulsion Laboratory (JPL), which fabricated a CMOS compatible APS. It had a lateral APS structure similar to the Toshiba sensor, but was fabricated with CMOS rather than PMOS transistors.[1] It was the first CMOS sensor with intra-pixel charge transfer.[2]
In 1999, Hyundai Electronics announced the commercial production of a 800x600 color CMOS image sensor based on 4T pixel with a high performance pinned photodiode with integrated ADCs and fabricated in a baseline 0.5um DRAM process.
Photobit's CMOS sensors found their way into webcams manufactured by Logitech and Intel, before Photobit was purchased by Micron Technology in 2001. The early CMOS sensor market was initially led by American manufacturers such as Micron, and Omnivision, allowing the United States to briefly recapture a portion of the overall image sensor market from Japan, before the CMOS sensor market eventually came to be dominated by Japan, South Korea and China.[24] The CMOS sensor with PPD technology was further advanced and refined by R. M. Guidash in 1997, K. Yonemoto and H. Sumi in 2000, and I. Inoue in 2003. This led to CMOS sensors achieve imaging performance on par with CCD sensors, and later exceeding CCD sensors.[2]
By 2000, CMOS sensors were used in a variety of applications, including low-cost cameras,
The video industry switched to CMOS cameras with the advent of high-definition video (HD video), as the large number of pixels would require significantly higher power consumption with CCD sensors, which would overheat and drain batteries.[24] Sony in 2007 commercialized CMOS sensors with an original column A/D conversion circuit, for fast, low-noise performance, followed in 2009 by the CMOS back-illuminated sensor (BI sensor), with twice the sensitivity of conventional image sensors.[26]
CMOS sensors went on to have a significant cultural impact, leading to the mass proliferation of
In 2012, Sony introduced the stacked CMOS BI sensor.[26] There have been several research activities ongoing in the field of image sensors. One of them is the quanta image sensor (QIS), which might be a paradigm shift in the way we collect images in a camera. In the QIS, the goal is to count every photon that strikes the image sensor, and to provide resolution of less than 1 million to 1 billion or more specialized photoelements (called jots) per sensor, and to read out jot bit planes hundreds or thousands of times per second resulting in terabits/sec of data. The QIS idea is in its infancy and may never become reality due to the non necessary complexity that is needed to capture an image [28]
Boyd Fowler of
By the late 2010s CMOS sensors had largely if not completely replaced CCD sensors, as CMOS sensors can not only be made in existing semiconductor production lines, reducing costs, but they also consume less power, just to name a few advantages. (see below)
HV-CMOS
HV-CMOS devices are a specialty case of ordinary CMOS sensors used in high-voltage applications (for detection of high energy particles) like CERN Large Hadron Collider where a high-breakdown voltage up to ~30-120V is necessary.[31] Such devices are not used for high-voltage switching though.[31] HV-CMOS are typically implemented by ~10 μm deep n-doped depletion zone (n-well) of a transistor on a p-type wafer substrate.[31]
Comparison to CCDs
APS pixels solve the speed and scalability issues of the passive-pixel sensor. They generally consume less power than CCDs, have less image lag, and require less specialized manufacturing facilities. Unlike CCDs, APS sensors can combine the image sensor function and image processing functions within the same
Advantages of CMOS compared with CCD
A primary advantage of a CMOS sensor is that it is typically less expensive to produce than a CCD sensor, as the image capturing and image sensing elements can be combined onto the same IC, with simpler construction required.[32]
A CMOS sensor also typically has better control of blooming (that is, of bleeding of photo-charge from an over-exposed pixel into other nearby pixels).
In three-sensor camera systems that use separate sensors to resolve the red, green, and blue components of the image in conjunction with beam splitter prisms, the three CMOS sensors can be identical, whereas most splitter prisms require that one of the CCD sensors has to be[dubious ] a mirror image of the other two to read out the image in a compatible order. Unlike CCD sensors, CMOS sensors have the ability to reverse the addressing of the sensor elements. CMOS Sensors with a film speed of ISO 4 million exist.[33]
Disadvantages of CMOS compared with CCD
Since a CMOS sensor typically captures a row at a time within approximately 1/60 or 1/50 of a second (depending on refresh rate) it may result in a "rolling shutter" effect, where the image is skewed (tilted to the left or right, depending on the direction of camera or subject movement). For example, when tracking a car moving at high speed, the car will not be distorted but the background will appear to be tilted. A frame-transfer CCD sensor or "global shutter" CMOS sensor does not have this problem; instead it captures the entire image at once into a frame store.
A long-standing advantage of CCD sensors has been their capability for capturing images with lower noise.[34] With improvements in CMOS technology, this advantage has closed as of 2020, with modern CMOS sensors available capable of outperforming CCD sensors.[35]
The active circuitry in CMOS pixels takes some area on the surface which is not light-sensitive, reducing the photon-detection efficiency of the device (microlenses and back-illuminated sensors can mitigate this problem). But the frame-transfer CCD also has about half the non-sensitive area for the frame store nodes, so the relative advantages depend on which types of sensors are being compared. [citation needed]
Architecture
This section needs additional citations for verification. (September 2007) |
Pixel
The standard
Array
A typical two-dimensional array of pixels is organized into rows and columns. Pixels in a given row share reset lines, so that a whole row is reset at a time. The row select lines of each pixel in a row are tied together as well. The outputs of each pixel in any given column are tied together. Since only one row is selected at a given time, no competition for the output line occurs. Further amplifier circuitry is typically on a column basis.[citation needed]
Size
The size of the pixel sensor is often given in height and width, but also in the optical format.[citation needed]
Lateral and vertical structures
There are two types of active-pixel sensor (APS) structures, the lateral APS and vertical APS.[1] Eric Fossum defines the lateral APS as follows:
A lateral APS structure is defined as one that has part of the pixel area used for photodetection and signal storage, and the other part is used for the active transistor(s). The advantage of this approach, compared to a vertically integrated APS, is that the fabrication process is simpler, and is highly compatible with state-of-the-art CMOS and CCD device processes.[1]
Fossum defines the vertical APS as follows:
A vertical APS structure increases fill-factor (or reduces pixel size) by storing the signal charge under the output transistor.[1]
Thin-film transistors
For applications such as large-area digital X-ray imaging, thin-film transistors (TFTs) can also be used in APS architecture. However, because of the larger size and lower transconductance gain of TFTs compared with CMOS transistors, it is necessary to have fewer on-pixel TFTs to maintain image resolution and quality at an acceptable level. A two-transistor APS/PPS architecture has been shown to be promising for APS using amorphous silicon TFTs. In the two-transistor APS architecture on the right, TAMP is used as a switched-amplifier integrating functions of both Msf and Msel in the three-transistor APS. This results in reduced transistor counts per pixel, as well as increased pixel transconductance gain.[37] Here, Cpix is the pixel storage capacitance, and it is also used to capacitively couple the addressing pulse of the "Read" to the gate of TAMP for ON-OFF switching. Such pixel readout circuits work best with low capacitance photoconductor detectors such as amorphous selenium.
Design variants
Many different pixel designs have been proposed and fabricated. The standard pixel uses the fewest wires and the fewest, most tightly packed transistors possible for an active pixel. It is important that the active circuitry in a pixel take up as little space as possible to allow more room for the photodetector. High transistor count hurts fill factor, that is, the percentage of the pixel area that is sensitive to light. Pixel size can be traded for desirable qualities such as noise reduction or reduced image lag. Noise is a measure of the accuracy with which the incident light can be measured. Lag occurs when traces of a previous frame remain in future frames, i.e. the pixel is not fully reset. The voltage noise variance in a soft-reset (gate-voltage regulated) pixel is , but image lag and fixed pattern noise may be problematic. In rms electrons, the noise is .
Combinations of hard and soft reset
Techniques such as flushed reset, pseudo-flash reset, and hard-to-soft reset combine soft and hard reset. The details of these methods differ, but the basic idea is the same. First, a hard reset is done, eliminating image lag. Next, a soft reset is done, causing a low noise reset without adding any lag.[38] Pseudo-flash reset requires separating VRST from VDD, while the other two techniques add more complicated column circuitry. Specifically, pseudo-flash reset and hard-to-soft reset both add transistors between the pixel power supplies and the actual VDD. The result is lower headroom, without affecting fill factor.[citation needed]
Active reset
A more radical pixel design is the active-reset pixel. Active reset can result in much lower noise levels. The tradeoff is a complicated reset scheme, as well as either a much larger pixel or extra column-level circuitry.[citation needed]
See also
- Angle-sensitive pixel
- Back-illuminated sensor
- Charge-coupled device
- Planar Fourier capture array
- Oversampled binary image sensor
- Category:Digital cameras with CMOS image sensor
References
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- ^ Fossum, Eric R. (1993). "Active Pixel Sensors vs Charge-Coupled Devices" (PDF). Imaging Systems Section, Jet Populsion Laboratory, California Institute of Technology.
- Lucent Technologies Inc.
- ISBN 978-3-540-66662-2.[page needed]
- Intel Corp.
- ISBN 978-3-319-49088-5.
- ISBN 978-0-470-53794-7. Retrieved 6 October 2019.
- ^ ISBN 9781420019155.
- S2CID 51669416.
- S2CID 60722329.
- ^ U.S. Patent 4,484,210: Solid-state imaging device having a reduced image lag
- S2CID 44669969.
- ^ S2CID 123351913.
- ^ . (Noble was later presented with an award for 'Seminal contributions to the early years of image sensors' by the International Image sensor Society in 2015.)
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- ^ NASA Spinoff. NASA. Retrieved 6 November 2019.
- ISBN 978-90-440-0111-2. Archived from the original(PDF) on 2020-12-06. Retrieved 2019-11-19.
- ^ a b "Imaging and Sensing Technology". Sony Semiconductor Solutions Group. Sony. Archived from the original on 18 May 2020. Retrieved 13 November 2019.
- ^ "CMOS Image Sensor Sales Stay on Record-Breaking Pace". IC Insights. May 8, 2018. Retrieved 6 October 2019.
- ^ "Advanced image sensors and camera systems | Thayer School of Engineering at Dartmouth". engineering.dartmouth.edu. Archived from the original on 6 June 2019.
- ^ US 7655918, Liu, Xinqiao & Fowler, Boyd, "CMOS image sensors adapted for dental applications", published 2010-02-02, assigned to Fairchild Imaging Inc.
- ^ "Sensors Expo 2019: Who's Who In Sensor Tech". Fierce Electronics. 18 June 2019. Retrieved 2020-06-25.
- ^ a b c Muenstermann, Daniel (2014). Overview of HV-CMOS devices (PDF). The 23rd International Workshop on Vertex Detectors – via CERN Indico.
- ^ Stefano, Meroli. "CMOS vs CCD sensor. Who is the clear winner?". meroli.web.cern.ch. Retrieved 28 March 2020.
- ^ "Canon : Technology | CMOS sensor". www.canon.com.
- ^ Group, Techbriefs Media (July 2014). "CCD and CMOS Sensors". www.techbriefs.com. Retrieved 28 March 2020.
- ^ "The difference between CCD and CMOS image sensing". www.testandmeasurementtips.com. Retrieved 28 March 2020.
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- ^ IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 1, JANUARY 2003[title missing][page needed]
Further reading
- John L. Vampola (January 1993). "Readout electronics for infrared sensors". In David L. Shumaker (ed.). The Infrared and Electro-Optical Systems Handbook, Volume 3 – Electro-Optical Components. The International Society for Optical Engineering. DTIC ADA364023. — one of the first books on CMOS imager array design
- Mary J. Hewitt; John L. Vampola; Stephen H. Black; Carolyn J. Nielsen (June 1994). Eric R. Fossum (ed.). "Infrared readout electronics: a historical perspective". Proceedings of SPIE. 2226 (Infrared Readout Electronics II). The International Society for Optical Engineering: 108–119. S2CID 109585056.
- Mark D. Nelson; Jerris F. Johnson; Terrence S. Lomheim (November 1991). "General noise processes in hybrid infrared focal plane arrays". Optical Engineering. 30 (11). The International Society for Optical Engineering: 1682–1700. doi:10.1117/12.55996.
- Stefano Meroli; Leonello Servoli; Daniele Passeri (June 2011). "Use of a standard CMOS imager as position detector for charged particles". Nuclear Physics B – Proceedings Supplements. 215 (1). Elsevier: 228–231. .
- Martin Vasey (September 2009). "CMOS Image Sensor Testing: An Integrated Approach". Jova Solutions. San Francisco, CA.
External links
- CMOS camera as a sensor Tutorial showing how low cost CMOS camera can replace sensors in robotics applications
- CMOS APS vs CCD CMOS Active Pixel Sensor Vs CCD. Performance comparison
- Image sensor inventor Peter J. W. Noble's web page with papers and video of 2015 presentation
- Image showing FSI and BSI sensor topology