Active pixel sensor From Wikipedia, the free encyclopedia

An active-pixel sensor (APS) is an image sensor consisting of an integrated circuit containing an array of pixel sensors, each

pixel containing a photodetector and an active amplifier. There are many types of active pixel sensors including the CMOS

APS used most commonly in cell phone cameras, web cameras, most digital pocket cameras since circa 2010, and in

most DSLRs. Such an image sensor is produced by a CMOS process (and is hence also known as a CMOS sensor), and has

emerged as an alternative to charge-coupled device (CCD) image sensors.

The term active pixel sensor is also used to refer to the individual pixel sensor itself, as opposed to the image sensor;[1] in that

case the image sensor is sometimes called an active pixel sensor imager,[2] active-pixel image sensor,[3] or active-pixel-sensor

(APS) imager.

History[edit]

The term active pixel sensor was coined by Tsutomu Nakamura who worked on the Charge Modulation Device active pixel

sensor at Olympus,[4] and more broadly defined by Eric Fossum in a 1993 paper.[5]

Image sensor elements with in-pixel amplifiers were described by Noble in 1968,[6] by Chamberlain in 1969,[7] and by Weimer et

al. in 1969,[8] at a time when passive-pixel sensors that is, pixel sensors without their own amplifiers were being investigated

as a solid-state alternative to vacuum-tube imaging devices. The MOS passive-pixel sensor used just a simple switch in the pixel

to read out the photodiode integrated charge.[9] Pixels were arrayed in a two-dimensional structure, with access enable wire

shared by pixels in the same row, and output wire shared by column. At the end of each column was an amplifier. Passive-pixel

sensors suffered from many limitations, such as high noise, slow readout, and lack of scalability. The addition of an amplifier to

each pixel addressed these problems, and resulted in the creation of the active-pixel sensor. Noble in 1968 and Chamberlain in

1969 created sensor arrays with active MOS readout amplifiers per pixel, in essentially the modern three-transistor

configuration. The CCD was invented in 1970 at Bell Labs. Because the MOS process was so variable and MOS transistors had

characteristics that changed over time (Vth instability), the CCD's charge-domain operation was more manufacturable and

quickly eclipsed MOS passive and active pixel sensors. A low-resolution "mostly digital" N-channel MOSFET imager with intra-

pixel amplification, for an optical mouse application, was demonstrated in 1981.[10]

Another type of active pixel sensor is the hybrid infrared focal plane array (IRFPA) 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 readout the

photodetectors. The exact date of origin of these devices is classified, but by the mid-1980s they were in widespread use.

By the late 1980s and early 1990s, the CMOS process was well established as a well controlled stable 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,[11] and active-pixel sensors for low-resolution high-function applications such as retina

simulation[12] and high energy particle detector. 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.

Eric Fossum, et al., invented the image sensor that used intra-pixel charge transfer along with an in-pixel amplifier to achieve

true correlated double sampling (CDS) and low temporal noise operation, and on-chip circuits for fixed-pattern noise reduction,

and published the first extensive article[5] predicting the emergence of APS imagers as the commercial successor of CCDs.

Between 1993 and 1995, the Jet Propulsion Laboratory developed a number of prototype devices, which validated the key

features of the technology. Though primitive, these devices demonstrated good image performance with high readout speed and

low power consumption.

In 1995, personnel from JPL founded Photobit Corp., who continued to develop and commercialize APS technology for a

number of applications, such as web cams, high speed and motion capture cameras, digital radiography, endoscopy (pill)

cameras, DSLRs and of course, camera-phones. Many other small image sensor companies also sprang to life shortly

thereafter due to the accessibility of the CMOS process and all quickly adopted the active pixel sensor approach.

Comparison to CCDs[edit]

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.

Advantages of CMOS compared to CCD[edit]

The biggest advantage of a CMOS sensor is that it is typically less expensive than a CCD sensor. A CMOS camera also has

weaker blooming effects if a light source has overloaded the sensitivity of the sensor, causing the sensor to bleed the light

source onto other pixels.

Disadvantages of CMOS compared to CCD[edit]

Since a CMOS video sensor typically captures a row at time within approximately 1/60th or 1/50th 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 does not have this problem, instead capturing the

entire image at once into a frame store.

Architecture[edit]

Pixel[edit]

A three-transistor active pixel sensor.

The standard CMOS APS pixel today consists of a photodetector (a

pinned photodiode), a floating diffusion, a transfer gate, reset gate, selection gate

and source-follower readout transistorthe so-called 4T cell. The pinned

photodiode was originally used in interline transfer CCDs due to its low dark current and good blue response, and when coupled

with the transfer gate, allows complete charge transfer from the pinned photo diode to the floating diffusion (which is further

connected to the gate of the read-out transistor) eliminating lag. The use of intrapixel charge transfer can offer lower noise by

enabling the use of correlated double sampling (CDS). The Noble 3T pixel is still often used since the fabrication requirements

are easier. The 3T pixel comprises the same elements as the 4T pixel except the transfer gate and the pinned photo diode. The

reset transistor, Mrst, acts as a switch to reset the floating diffusion which acts in this case as the photo diode. When the reset

transistor is turned on, the photodiode is effectively connected to the power supply, VRST, clearing all integrated charge. Since

the reset transistor is n-type, the pixel operates in soft reset. The read-out transistor, Msf, acts as a buffer (specifically, a source

follower), an amplifier which allows the pixel voltage to be observed without removing the accumulated charge. Its power supply,

VDD, is typically tied to the power supply of the reset transistor. The select transistor, Msel, allows a single row of the pixel array to

be read by the read-out electronics. Other innovations of the pixels such as 5T and 6T pixels also exist. By adding extra

transistors, functions such as global shutter, as opposed to the more common rolling shutter, are possible. In order to increase

the pixel densities, shared-row, four-ways and eight-ways shared read out, and other architectures can be employed. A variant

of the 3T active pixel is the Foveon X3 sensor invented by Dick Merrill. In this device, three photodiodes are stacked on top of

each other using planar fabrication techniques, each photodiode having its own 3T circuit. Each successive layer acts as a filter

for the layer below it shifting the spectrum of absorbed light in successive layers. By deconvolving the response of each layered

detector, red, green, and blue signals can be reconstructed.

APS using TFTs[edit]

A two-transistor active/passive pixel sensor

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 to 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 usingamorphous

silicon TFTs. In the two-transistor APS architecture on the right, TAMP is used as a

switched-amplifier integrating functions of both Msfand Msel in the three-transistor

APS. This results in reduced transistor counts per pixel, as well as increased pixel transconductance gain.[13]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.

Array[edit]

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.

Size[edit]

The size of the pixel sensor is often given in height and width, but also in the optical format.

Design variants[edit]

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Many different pixel designs have been proposed and fabricated. The standard pixel is the most common because it 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 .

Hard reset[edit]

Operating the pixel via hard reset results in a JohnsonNyquist noise on the photodiode of

or , but prevents image lag, sometimes a desirable tradeoff. One way to use hard reset is replace Mrst with a

p-type transistor and invert the polarity of the RST signal. The presence of the p-type device reduces fill factor, as extra space is

required between p- and n-devices; it also removes the possibility of using the reset transistor as an overflow anti-blooming

drain, which is a commonly exploited benefit of the n-type reset FET. Another way to achieve hard reset, with the n-type FET, is

to lower the voltage of VRST relative to the on-voltage of RST. This reduction may reduce headroom, or full-well charge capacity,

but does not affect fill factor, unless VDD is then routed on a separate wire with its original voltage.

Combinations of hard and soft reset[edit]

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.[14] 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.

Active reset[edit]

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