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Active Pixel SensorNilay Kant Singh

Dept. of Electronics & communication

D.I.T. School of Engineering; Greater Noida

[email protected]

ABSTRACT-An active-pixel sensor (APS) is an

image sensor consisting of an integrated

circuit containing an array of pixel sensors,

each pixel containing a photo detector andan active amplifier. The adoption of active

pixel sensors (APS), based on an active

read-out scheme implemented at the pixel

level, has been recently proposed for

charged-particle detection purposes [1, 2].

Very good performances have been obtained,

in particular by exploiting some peculiar

features of the actual fabrication technology

(i.e., the presence of a relatively deep and

low-doped epitaxial layer).


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I. INTRODUCTIONAn 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 and in some DSLRs. Such an image

sensor is produced by a CMOS process (and is

hence also known as a (CMOS sensor) and hasemerged as an alternative to charge-coupled device

(CCD) imager sensors.

fig.cmos sensor

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 issometimes called an active pixel sensor imager,
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active-pixel image sensor, or active-pixel-sensor

(APS) imager.

An approach toward advanced VLSI CMOStechnologies with the aim of increasing spatial

resolution and optimizing the pixel-level response by

exploiting state-of-the art microelectronic devices.

To this purpose, accurate analyses have been

performed investigating performance dependency on

actual technology features in order to assessguidelines for the integration of physical sensors and

effective signal-processing circuitry on the same

chip.

II. HISTORYThe term active pixel sensor was coined by Tsutomu

Nakamura who worked on the Charge ModulationDevice 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
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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 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 ofscalability. The addition of anamplifier 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 transistorshad characteristics that changed over time (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, wasdemonstrated in 1981.

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
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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 inwidespread 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 simulationand 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 broadcastcameras, 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-chipcircuits 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. Thoughprimitive, these devices demonstrated good image
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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.

III. COMPARISON TO CCDSAPS 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
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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 imageacquisition, 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

IV. ARCHITECTUREPixel

Three-transistor active pixel sensor.
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The standard CMOS APS pixel today consists

of a photodetector (a pinned photodiode), afloating 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 chargetransfer can offer lower noise by enabling the

use ofcorrelated 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 4Tpixel 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,
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VRST, clearing all integrated charge. Since the

reset transistor is n-type, the pixel operates in

soft reset. The read-out transistor, Msf, acts asa 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 commonrolling 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 sensorinvented 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
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light in successive layers. By deconvolving the

response of each layered detector, red, green,

and blue signals can be reconstructed.

1.APS using TFTs

A two-transistor active/passive pixel sensor
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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 using

amorphous silicon TFTs. In the two-transistor

APS architecture on the right, TAMP is used as

a switched-amplifer 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.
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2.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.

V. ADVANTAGESActive Pixel Sensors (APS) for the detection of ionizing particles

A radiation sensor is a dispositive able to

detect incoming ionizing particles (such us

for example photons, , gamma oror alfa particles) and gives to the user

information like the energy, the intensity or

the position of the radiation. Among the

different approaches to the radiation detection,

one of the most studied in the last decades is
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the usage of semiconductor solid state

detector thanks to the continuous evolution of

the semiconductor industries.There are different families of detectors based

on semiconductor substrates (Charge Coupled

Devices CCD, Silicon Drift Detector SDD,

PIN photodiode, etc.) but the wide diffusion of

the CMOS technologies in the consumerelectronics has made very attractive all the

implementations of such kind of technology in

the field of radiation detectors; the growing

diffusion of the CMOS imaging sensors is, for

example, the most evident consequence of this

fact.

This entire page is focused on the most

common implementation of a radiation sensor

in CMOS technologies the Active Pixel

Sensor.The main element of this class of detectors is

an inverse biased photodiode (see Figure 1 and

Figure 2). When an incoming radiation with

an energy near or greater than the silicon band

gap crosses the depleted region of the p-n
http://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.htmlhttp://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.htmlhttp://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.htmlhttp://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.htmlhttp://meroli.web.cern.ch/meroli/Lecture_StragglingFunction.htmlhttp://meroli.web.cern.ch/meroli/Lecture_StragglingFunction.htmlhttp://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.htmlhttp://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.htmlhttp://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.htmlhttp://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.htmlhttp://meroli.web.cern.ch/meroli/CharacterizationofCMOSActivePixelSensors.html


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junction, some electrons can jump from the

valence to the conduction band with the

creation of electron hole pairs; the generatedelectron hole pairs are separated by the electric

field of the junction and collected at the

electrodes of the photodiode.

The generation of the electron-hole pairs is

governed by the approximated relation: n = E /Ebg where n is the number of generated pairs,

E is the energy released by the particle and

Ebg is the band-gap energy of the silicon. In

other words, the radiation generates a photo-

current through the photodiode which can be

measured directly or as a voltage drop at the

ends of the junction. However, the photo-

current discharges the photodiode reducing the

depleted region and consequentially the

sensibility of the detector; there are two

techniques to avoid this phenomenon:
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VI. APPLICATIONS1. IMAGERS FOR SPACE APPLICATIONS.

For any space application there will be a trade-

off concerning several key performance

parameters when selecting an image. The main

criteria used for this are:

Quantum efficiency* fill factor

Reliability


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Power consumption

Radiation tolerance

Although CCDs still perform significantlybetter on the first criterion (mainly due to the

higher fill factor), APS imagers

in the meantime seem to outperform CCDs on

all the other ones. Reliability of the sensors

system and power consumption are

interrelated in the sense that CCDs generally

require several highly accurate voltages to

operate properly and timing circuits need to be

integrated on a different chip because of

process incompatibilities.


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2. THE MICRO DIGITAL SUN-SENSOR.

The micro digital sun-sensor is one of theseproducts under development at TNO in co-

operation with the Delft University of

Technology. It is developed in frame of the

Dutch Microned program. This program is

intended to increase the knowledge and

stimulate the application of micro system

technology in the Netherlands.

A sunsensor is an attitude control sensor that

senses the position of the sun with respect to

its mounting plane. This type of sensor

typically operates as a photo-diode of which


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the current produced is proportional to the

angel of incidence (coarse sun-sensors) or a

photo diode array with a suspended membrane(fine sun-sensors). In case the photodiode

array is a real 2D array, the sensors are called

digital sun-sensors. TNOs digital sunsensors

currently use a standard active pixel sensor

array and a standard FPGA for signal

processing. The APS sensors are selected

because the amount of light

available is much more than the signal

required for proper operation and their ability

to do windowing (create a window of interest

on the sensor which is read out at a higherspeed while discarding the information from

the other part of the sensor).


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VIII. REFERENCES

1.Alexander G. Dickinson et al., "Active

pixel sensor and imaging system having

differential mode", US 56317042. Zimmermann, Horst (2000).Integrated

Silicon Optoelectronics.

Springer. ISBN3540666621.

3. Lawrence T. Clark, Mark A. Beiley, Eric

J. Hoffman, "Sensor cell having a softsaturation circuit"US 6133563 [1]

4. Kazuya Matsumoto et al., "A new MOS

phototransistor operating in a non-

destructive readout mode" Jpn. J. Appl.

Phys. 24 (1985) L323
http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=US5631704http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/3540666621http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=US6133563http://www.google.com/patents?id=u-QFAAAAEBAJ&pg=PA11&vq=active-pixel-image-sensor&dq=active-pixel-image-sensorhttp://jjap.ipap.jp/link?JJAP/24/L323/http://jjap.ipap.jp/link?JJAP/24/L323/http://jjap.ipap.jp/link?JJAP/24/L323/http://jjap.ipap.jp/link?JJAP/24/L323/http://www.google.com/patents?id=u-QFAAAAEBAJ&pg=PA11&vq=active-pixel-image-sensor&dq=active-pixel-image-sensorhttp://v3.espacenet.com/textdoc?DB=EPODOC&IDX=US6133563http://en.wikipedia.org/wiki/Special:BookSources/3540666621http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://v3.espacenet.com/textdoc?DB=EPODOC&IDX=US5631704


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5.Eric R. Fossum (1993), "Active Pixel

Sensors: Are CCD's Dinosaurs?" Proc.

SPIE Vol. 1900, p. 214, Charge-CoupledDevices and Solid State Optical Sensors

III, Morley M. Blouke; Ed.

6.Peter J. W. Noble (Apr. 1968). Self-

Scanned Silicon Image Detector

Arrays. ED-15. IEEE. pp. 202209.

7.Savvas G. Chamberlain (December 1969).

"Photosensitivity and Scanning of Silicon

Image Detector Arrays".IEEE Journal of

Solid-State CircuitsSC-4 (6): 333342.

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