Active Pixel Camera App

[Dawnell Sechler]

Theterm 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]

At RCA Laboratories, a research team including Paul K. Weimer, W.S. Pike and G. Sadasiv in 1969 proposed a solid-state image sensor with scanning circuits using thin-film transistors (TFTs), with photoconductive film used for the photodetector.[10][11] A low-resolution "mostly digital" N-channel MOSFET (NMOS) imager with intra-pixel amplification, for an optical mouse application, was demonstrated by Richard F. Lyon in 1981.[12] Another type of image sensor technology that is related to the APS is the hybrid infrared focal plane array (IRFPA),[1] designed to operate at cryogenic temperatures in the infrared spectrum. The devices are two chips that are put together like a sandwich: one chip contains detector elements made in InGaAs or HgCdTe, and the other chip is typically made of silicon and is used to read out the photodetectors. The exact date of origin of these devices is classified, but they were in use by the mid-1980s.[citation needed]

A key element of the modern CMOS sensor is the pinned photodiode (PPD).[2] It was invented by Nobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara at NEC in 1980,[2][13] and then publicly reported by Teranishi and Ishihara with A. Kohono, E. Oda and K. Arai in 1982, with the addition of an anti-blooming structure.[2][14] The pinned photodiode is a photodetector structure with low lag, low noise, high quantum efficiency and low dark current.[2] The new photodetector structure invented at NEC was given the name "pinned photodiode" (PPD) by B.C. Burkey at Kodak in 1984. In 1987, the PPD began to be incorporated into most CCD sensors, becoming a fixture in consumer electronic video cameras and then digital still cameras. Since then, the PPD has been used in nearly all CCD sensors and then CMOS sensors.[2]

The precursor to the APS was the passive-pixel sensor (PPS), a type of photodiode array (PDA).[2] A passive-pixel sensor consists of passive pixels which are read out without amplification, with each pixel consisting of a photodiode and a MOSFET switch.[15] In a photodiode array, pixels contain a p-n junction, integrated capacitor, and MOSFETs as selection transistors. A photodiode array was proposed by G. Weckler in 1968, predating the CCD.[1] This was the basis for the PPS,[2] which had image sensor elements with in-pixel selection transistors, proposed by Peter J.W. Noble in 1968,[16][2][10] and by Savvas G. Chamberlain in 1969.[17]

The active-pixel sensor consists of active pixels, each containing one or more MOSFET amplifiers which convert the photo-generated charge to a voltage, amplify the signal voltage, and reduce noise.[15] The concept of an active-pixel device was proposed by Peter Noble in 1968. He created sensor arrays with active MOS readout amplifiers per pixel, in essentially the modern three-transistor configuration: the buried photodiode-structure, selection transistor and MOS amplifier.[19][16]

The MOS active-pixel concept was implemented as the charge modulation device (CMD) by Olympus in Japan during the mid-1980s. This was enabled by advances in MOSFET semiconductor device fabrication, with MOSFET scaling reaching smaller micron and then sub-micron levels during the 1980s to early 1990s.[1][20] The first MOS APS was fabricated by Tsutomu Nakamura's team at Olympus in 1985. The term active pixel sensor (APS) was coined by Nakamura while working on the CMD active-pixel sensor at Olympus.[21] The CMD imager had a vertical APS structure, which increases fill-factor (or reduces pixel size) by storing the signal charge under an output NMOS transistor. Other Japanese semiconductor companies soon followed with their own active pixel sensors during the late 1980s to early 1990s. Between 1988 and 1991, Toshiba developed the "double-gate floating surface transistor" sensor, which had a lateral APS structure, with each pixel containing a buried-channel MOS photogate and a PMOS output amplifier. Between 1989 and 1992, Canon developed the base-stored image sensor (BASIS), which used a vertical APS structure similar to the Olympus sensor, but with bipolar transistors rather than MOSFETs.[1]

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]

By the late 1980s to early 1990s, the CMOS process was well-established as a well-controlled stable semiconductor manufacturing process and was the baseline process for almost all logic and microprocessors. There was a resurgence in the use of passive-pixel sensors for low-end imaging applications,[22] while active-pixel sensors began being used for low-resolution high-function applications such as retina simulation[23] and high-energy particle detectors. However, CCDs continued to have much lower temporal noise and fixed-pattern noise and were the dominant technology for consumer applications such as camcorders as well as for broadcast cameras, where they were displacing video camera tubes.

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]

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 digital cameras and camera phones, which bolstered the rise of social media and selfie culture, and impacted social and political movements around the world.[24] By 2007, sales of CMOS active-pixel sensors had surpassed CCD sensors, with CMOS sensors accounting for 54% of the global image sensor market at the time. By 2012, CMOS sensors increased their share to 74% of the market. As of 2017, CMOS sensors account for 89% of global image sensor sales.[27] In recent years,[when?] the CMOS sensor technology has spread to medium-format photography with Phase One being the first to launch a medium format digital back with a Sony-built CMOS sensor.

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 OmniVision is known for his work in CMOS image sensor development. His contributions include the first digital-pixel CMOS image sensor in 1994; the first scientific linear CMOS image sensor with single-electron RMS read noise in 2003; the first multi-megapixel scientific area CMOS image sensor with simultaneous high dynamic range (86 dB), fast readout (100 frames/second) and ultra-low read noise (1.2e- RMS) (sCMOS) in 2010. He also patented the first CMOS image sensor for inter-oral dental X-rays with clipped corners for better patient comfort.[29][30]

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)

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 integrated circuit. APS sensors have found markets in many consumer applications, especially camera phones. They have also been used in other fields including digital radiography, military ultra high speed image acquisition, security cameras, and optical mice. Manufacturers include Aptina Imaging (independent spinout from Micron Technology, who purchased Photobit in 2001), Canon, Samsung, STMicroelectronics, Toshiba, OmniVision Technologies, Sony, and Foveon, among others. CMOS-type APS sensors are typically suited to applications in which packaging, power management, and on-chip processing are important. CMOS type sensors are widely used, from high-end digital photography down to mobile-phone cameras.[citation needed]

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