An active-pixel sensor 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, and has emerged as an alternative
to charge-coupled device image sensors. The term active pixel sensor is also
used to refer to the individual pixel sensor itself, as opposed to the image
sensor; in that case the image sensor is sometimes called an active pixel sensor
imager, or active-pixel image sensor. History
The term active pixel sensor was coined in 1985 by Tsutomu Nakamura who worked
on the Charge Modulation Device active pixel sensor at Olympus, and more
broadly defined by Eric Fossum in a 1993 paper.
Image sensor elements with in-pixel amplifiers were described by Noble in
1968, by Chamberlain in 1969, and by Weimer et al. in 1969, 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. Pixels were arrayed in a
two-dimensional structure, with an 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, 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.
Another type of active pixel sensor is the hybrid infrared focal plane array
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 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, and active-pixel sensors for low-resolution high-function
applications such as retina simulation 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 and low temporal noise
operation, and on-chip circuits for fixed-pattern noise reduction, and
published the first extensive article 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 cameras, DSLRs and 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. Most recent, 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.
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 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, 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=
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=
Since a CMOS video sensor typically captures a row at a time within
approximately 1/60th or 1/50th of a second it may result in a "rolling
shutter" effect, where the image is skewed. 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.
The active circuitry also takes some area on the surface which is not
light-sensitive, reducing the quantum efficiency of the device. Thus, CCDs are
preferred in astronomical applications. Architecture
= Pixel= The standard CMOS APS pixel today
consists of a photodetector, a floating diffusion, a transfer gate, reset gate,
selection gate and source-follower readout transistor—the 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
photodiode to the floating diffusion eliminating lag. The use of intrapixel
charge transfer can offer lower noise by enabling the use of correlated double
sampling. The Noble 3T pixel is still sometimes used since the fabrication
requirements are easier. The 3T pixel comprises the same elements as the 4T
pixel except the transfer gate and the photodiode. The reset transistor, Mrst,
acts as a switch to reset the floating diffusion which acts in this case as the
photodiode. 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, 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= For applications such as large-area
digital X-ray imaging, thin-film transistors 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 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. 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= 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=
The size of the pixel sensor is often given in height and width, but also in
the optical format. Design variants
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
pixel is , but image lag and fixed pattern noise may be problematic. In rms
electrons, the noise is . = Hard reset=
Operating the pixel via hard reset results in a Johnson–Nyquist 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=
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.
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=
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.
See also Angle-sensitive pixel
Back-illuminated sensor Charge-coupled device
Planar Fourier capture array Oversampled binary image sensor
References Further reading
John L. Vampola. "Chapter 5 - Readout electronics for infrared sensors". In
David L. Shumaker. The Infrared and Electro-Optical Systems Handbook, Volume
3 - Electro-Optical Components. The International Society for Optical
Engineering. ISBN 0-8194-1072-1. — one of the first books on CMOS imager array
design Mary J. Hewitt; John L. Vampola; Stephen
H. Black; Carolyn J. Nielsen. Eric R. Fossum, ed. "Infrared readout
electronics: a historical perspective". Proceedings of SPIE 2226: pages 108–119.
doi:10.1117/12.178474. CS1 maint: Extra text
Mark D. Nelson; Jerris F. Johnson; Terrence S. Lomheim. "General noise
processes in hybrid infrared focal plane arrays". Optical Engineering 30:
1682–1700. Bibcode:1991OptEn..30.1682N. doi:10.1117/12.55996.
Stefano Meroli; Leonello Servoli; Daniele Passeri. "Use of a standard CMOS
imager as position detector for charged particles". Nuclear Physics B -
Proceedings Supplements 215: 228–231. Bibcode:2011NuPhS.215..228S.
doi:10.1016/j.nuclphysbps.2011.04.016. Martin Vasey. "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