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TND6116/D
Image Sensor Terminology
INTRODUCTION

This technical note has been written to clarify some of theterminology used to describe the operation and performance of solidstate image sensors. It is intended for use by anyone considering using
these sensors in a systems design, and particularly for first time users.
This note provides only brief explanations of the common terms
encountered in image sensor specifications. A listing of suggested
readings on solid state image sensors and applications is located at theend of this document.

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TECHNICAL NOTECONTENTS
Accumulation Mode . . . . . . . . . . . . . . . . . . . . . . . . .
Active Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Blooming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Buried Channel CCD . . . . . . . . . . . . . . . . . . . . . . . .
CCD Clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Charge Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Charged Coupled Device . . . . . . . . . . . . . . . . . . . . .
Charge Transfer Efficiency . . . . . . . . . . . . . . . . . . . .
Charge Transfer Inefficiency . . . . . . . . . . . . . . . . . . .
Color Filter Array (CFA) . . . . . . . . . . . . . . . . . . . . .
Correlated Double Sampling (CDS) . . . . . . . . . . . . .
Dark Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dark Reference Pixels . . . . . . . . . . . . . . . . . . . . . . . .
Data Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Defective Pixel . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dynamic Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electronic Shutter . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fill Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fixed Pattern Noise . . . . . . . . . . . . . . . . . . . . . . . . . .
Floating Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . .
Four Phase CCD . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frame Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frame Transfer Image Sensors . . . . . . . . . . . . . . . . .
Full Frame Image Sensor . . . . . . . . . . . . . . . . . . . . .
Horizontal CCD . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Image Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Integration Period . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interlaced Image Sensor . . . . . . . . . . . . . . . . . . . . . .
Interline Image Sensor . . . . . . . . . . . . . . . . . . . . . . .
Lateral Overflow Drain . . . . . . . . . . . . . . . . . . . . . . .
Lenticular Array (Microlenses/Lenslets) . . . . . . . . .
Light Shield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Linear Image Sensor . . . . . . . . . . . . . . . . . . . . . . . . .
Modulation Transfer Function (MTF) . . . . . . . . . . .

© Semiconductor Components Industries, LLC, 2014

July, 2014 − Rev. 2

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Multiple Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Non−Interlaced . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . .
Photodiode Lag . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Photoresponse Nonuniformity . . . . . . . . . . . . . . . . .
Pixel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pixel−to−Pixel Crosstalk . . . . . . . . . . . . . . . . . . . . . .
Potential Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Progressive Scan . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pseudo Two Phase CCD . . . . . . . . . . . . . . . . . . . . . .
Quantum Efficiency . . . . . . . . . . . . . . . . . . . . . . . . .
Reset Clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Responsivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Schottky Barrier Diodes . . . . . . . . . . . . . . . . . . . . . .
Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Smear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spectral Response . . . . . . . . . . . . . . . . . . . . . . . . . . .
Surface Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Three Phase CCD . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transfer Gate Clock . . . . . . . . . . . . . . . . . . . . . . . . .
True Two Phase CCD . . . . . . . . . . . . . . . . . . . . . . . .
UV Enhancement Coating . . . . . . . . . . . . . . . . . . . .
Vertical CCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vertical Overflow Drain (VOD) . . . . . . . . . . . . . . . .
Wafer Thinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
For Information on Solid State Physics . . . . . . . . . .
For Information on CCD Image Sensors . . . . . . . . .
For Information on Electronic Imaging Systems . . .

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Publication Order Number:
TND6116/D

TND6116/D
Accumulation Mode

ESD protection circuitry at the inputs to protect the sensor,
and these circuits often limit the negative swing of theapplied voltages to greater than −1.0 Volts.

Accumulation mode, also referred to as MPP mode
(Multi−Pinned Phase) is the state in a semiconductor wherethemajority
carrier
concentration
attheoxide−semiconductor interface is greater than the substrateor bulk carrier concentration. When applied to solid state
image sensors, the accumulation mode of operation can
result in greatly reduced dark current and dark pattern noise.
Accumulation is achieved by applying the appropriate
voltage levels to the CCD and transfer gates. For an n−typeburied channel CCD, the majority carriers are holes. To
attract holes to the SiO2−Si interface, a voltage sufficiently
less than the substrate potential must be applied.
Image sensors which gain the most benefit from runningin accumulation mode are those which operate under longintegration times. Not all image sensors can support theaccumulation mode of operation. Many image sensors have

Active Area

The surface area of an image sensor which is lightsensitive is called the active area. In the case of interline andlinear sensors, this area is usually made up of only thephotodiode active area, since all other regions on the imager
are typically covered with a metal layer which prevents
incident light from being absorbed within the silicon
substrate. The area of the light sensitive CCD or photodiode(Lp x Wp) may be greater than the active area (La x Wa), in
which case the metal light shield is used to define the smaller
active area. In full−frame sensors, the active area is definedby this light shield.

Wa

Metal Light Shield

PhotodiodeLa

Photodiodee

Silicon Substrate

Figure 1. Photodiode with Aperture Light Shield
Incident
Photon

W

p

Wa
Lp

La
Figure 2. Array of Photodiodes

Blooming

blooming is a corrupted image near the blooming site. The
extent of the image degradation is dependent on the level ofexcess charge and on the architecture of the imager being
used. The effects of blooming can be minimized byincorporating an antiblooming structure near the charge

When the maximum charge capacity of the CCD orphoto−diode is exceeded, the excess charge will overflow
into adjacent CCD cells or photosites. This overflow ofphotogenerated charge is termed Blooming. The result of

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TND6116/D
whereas, lateral antiblooming structures reside adjacent tothe charge collection site and allow excess charge tooverflow into a reversed biased diode. Clocking schemes
may be used to reduce blooming; however, these are lesseffective at higher frame rates.

collection site. Antiblooming structures are constructed so
as to provide a safe path for the excess photogenerated
charge (i.e. blooming charge). Vertical antiblooming
structures reside below the charge collection site and allow
excess charge to overflow directly into the substrate;
Light

Overflow
Drain

Overflow
Gate

Transfer
Gate

CCD
Phase 1ÌÌÌÌ ÑÑÑÑ
ÌÌÌÌÌ
ÌÌÌÌÌ
ÏÏÌÌÌÌ
ÏÏÏÏÏÑÑÑÑ

Light Shield

Field SiO2−−−−−−−−−−−−−−−−−−−−−−−−−−−−

Field SiO2BLOOMING!

Electrostatic Potential

e

e

e

e ee e e ee ee e e e e
e e e
e

h
External Bias ”OFF”
External Bias ”ON”

Figure 3. Lateral Overflow Structure

Transfer
Gate
Light

ÏÏ
ÏÏ
ÏÏ
ÏÏ
ÏÏ
ÏÏ

BLOOMING!

Electrostatic Potential
Distance in Silicon

Figure 4. Vertical Overflow Structure
Buried Channel CCD

A buried channel CCD is one in which the channel islocated some distance below the surface of the silicon. Thatis, below the silicon − silicon dioxide interface (Si−SiO2
interface), which is known to contain a higher density ofelectron traps and a higher dark current. Transferring charge
at or near the Si−SiO2 surface can degrade the chargetransfer efficiency (especially at higher CCD clocking
speeds) and cause an increase in dark noise.

The Charge Coupled Device (CCD) structure can be usedin image sensors to transport and collect photogenerated
charge.
The physical location of charge contained within a CCD
stage measured with respect to the surface of the silicon
substrate is called the channel.

SiO 2 − SiInterface

Electrostatic Potential

Electrostatic Potential

SiO 2 − SiInterface

ChannelZc

ChannelDistance Into SiliconDistance Into Silicon

Figure 5. Surface Channel / Buried Channel

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TND6116/D
CCD Clock

For a two phase CCD, two input timing signals are
required for operation. For charge to move from phase 1(F1) to phase 2 (F2), it is necessary that the phase 1 signal
turn ”OFF” (external bias = 0.0 V) and phase 2 signal turn“ON” (external bias ≥ VCCD V). Similarly, the phase 2
signal should be “OFF” and the phase 1 signal “ON” totransport charge from phase 2 to phase 1. This results in two
input timing signals which are complements of each other,as shown below. The “ON” / “OFF” duty cycle of each clock
is typically 50%, but may vary as long as the “ON” / “OFF”
times meet the specification requirements and the signals
remain complements of each other.

Charge Coupled Devices (CCDs) use input timing signals
to setup the electrostatic potentials necessary to transportcharge. A two phase CCD will require two input signals, a
three phase will require three signals, and a four phase CCD
will require four input signals. The amplitude of the CCD
input signals, combined with the built in channel potentialof each phase, will determine the magnitude of theelectrostatic potential under each phase, and the phaserelationships between the input clocks will permit thetransportation of charge.

One CCD Stage (cell)

One CCD Stage (cell)ö2ö1ÏÏÏÏÏÏ ÏÏÏÏÏ
ÏÏÏÏÏÏ
ÓÓÓÓÓÓÓÓÓÓÏÏÏÏÏ
++++++++++++++++++N Type Implant
P Type Implant

Electrostatic Potential++++++

External Bias ”OFF”
External Bias ”ON”

Figure 6. Cross Sectional View of True Two Phase CCD

Tp
ö1
ö2
Figure 7. Input Timing for Two Phase CCD

smear. If an anti−blooming structure is adjacent to the chargecollection site, the excess charge will be prevented fromoverflowing into adjacent charge collection and transport
structures; thus, prevent blooming from occurring.
Multiplying the charge capacity (Nsat) by the
charge−to−voltage conversion factor yields that maximum
output voltage, or saturation voltage.

The point at which the complementary clock signals cross(e.g. 50% of amplitude) is very important for optimum
operation. The phase 1 and phase 2 signals are controlled
differently during the photodiode to CCD charge transfer.
Once the charge transfer is complete, the CCD signals again
resume the complementary pattern.
Charge Capacity

V sat + N sat

The maximum amount of charge that an imager cancollect and transfer while maintaining all performance
specifications is termed the saturation charge level anddefines charge capacity. Charge capacity may be limited byeither the photosite or CCD capacity. If the charge capacity
is exceeded, the excess charge will overflow into adjacentstructures and produce artifacts known as blooming anddV [Volts]dN(eq. 1)

Charged Coupled Device

A Charge Coupled Device (CCD) is an integrated circuit
which allows individual charge packets to be transferredover a physical distance while maintaining the original

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TND6116/D
charge packet integrity. Charge coupled devices are ideallysuited for use in solid state imagers as a means oftransferring integrated photogenerated charge. The CCD
may be used to collect the photogenerated charge, or it may
be placed adjacent to a array of photodiodes orphotocapacitors. A CCD used to directly collectphotogenerated charge will have reduced photoresponse atshorter optical wavelengths due to the presence ofpolysilicon electrodes. Several of the more common CCD
structures are described in more detail in other sections of
this reference document.

CTI +
+

(eq. 2)

Some manufacturers define CTE as the charge transferredper CCD stage, so care should be taken when comparing
different manufacturer’s specifications for CTI and CTE toensure that both use the same definition. The total charge
remaining in a CCD stage after being clocked through theentire CCD is termed the CTE per line for linear imagers or
CTE per frame for area array image sensors, and is equal to:

V lost
CCD_Transfers(eq. 5)

Correlated Double Sampling (CDS)

A schematic diagram of a typical image sensor output
stage and the corresponding image sensor timing are shown
below. The output stage functions as follows. The reset gate
signal (FR) is turned “ON” to reset the floating diffusion
node. Then reset is turned “OFF” and the output signal isallowed to settle. Next the phase 2 CCD clock (F2) is turned“OFF” and the phase 1 CCD clock (F1) turned “ON”. As thephase 2 CCD clock turns “OFF”, the charge in the last phase2 stage is dumped onto the floating diffusion node and theoutput signal is allowed to settle at its new value. Finally, thereset gate signal is again turned “ON” and the cycle repeats.
The voltage level of the output signal after the reset gate
has turned “OFF” and before phase 2 is turned “OFF” iscalled the reset level. This level is typical in the range of 7
to 9 volts. The reset level will have a noise component due
to variations in the effective “ON” resistance of the resettransistor (Q1). This noise is very small, but detectable invery high dynamic range systems.

(eq. 3)(for Linear Image Sensors)
CTE Frame + (CTE X) X_CCD_Transfers

V infected

For color imaging applications, it is necessary to separate
the optical spectrum of the incident image into three color
bands. In most applications, it is desirable to perform thecolor separation on the imager. Color separation is typicallyaccomplished by depositing organic dyes on the imager
surface. The color dyes, or color filters, can be configured towork in an additive (RGB) or subtractive (YMC) process.
That is, the deposited layers may act as transmission filters
or as absorbing filters. The deposition of three color filters
yields three bandpass filters, which can be designed to occur
in any pattern across an imager.
On tricolor linear imagers, a blue bandpass filter isdeposited on one whole channel, a green bandpass filter isdeposited one another channel, and a red bandpass filter isdeposited on the remaining channel. Thus, a single pass scanof an object obtains all color information. Color filter
patterns on area arrays can also occur in varying
arrangements.

Charge Transfer Efficiency (CTE) is the fraction of charge
which is successfully transferred during one CCD transfer
cycle (note that a phase CCD will have two transfer cycles
per CCD stage). CTE is equal to one minus the Charge
Transfer Inefficiency (CTI), or:

CTE Line + (CTE) CCD_Transfers

N lost+
CCD_Transfers

Color Filter Array (CFA)

Charge Transfer Efficiency

CTE + 1 * CTI

N infected(CTE Y) Y_CCD_Transfers(eq. 4)(for Area Array Image Sensors)
Charge Transfer Inefficiency

Charge Transfer Inefficiency (CTI) is the fraction of
charge left behind during a CCD transfer.
Care should be taken when comparing different
manufacturer’s specifications for CTI or CTE to ensure thatboth use the same definition.
Charge Transfer Inefficiency is measured by injecting asequence of charge packets of known size into a CCD andthen monitoring the resultant imager output waveform. Notethat a two phase CCD will have two transfers per CCD stage.
The injected signal amplitude and the signal lost from theinjected signal are then used to calculate CTI as follows:

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TND6116/DReset
Dr ai n

Reset

ÏÏÏÏÏ
ÏÏÏÏÏ
ÓÓÓ
ö2

VDD

Q1

OG

e

Q2
Floating
Diffusion

Q4

e e

Q3

e e e
e e ee ee+−

Out put

Figure 8. Output Circuitry

ö1

ö2
öR

OutputBrightInput Light
Intensity
Dark

Figure 9. Signal Timingon each photosite (also known as Correlated Double
Sampling, or CDS), and goal number 2 is achieved byconverting the output signal to a ground referenced, positive
going signal.
The timing required to perform the CDS signal processing
is shown below. There are several common circuits used toperform the CDS function; however, all make a differential
measurement.

Typical saturation voltages are in the range of 1 to 3 volts.
This makes the lowest level of the output signal about 4 volts(7 − 3). Most analog−to−digital converters will not accept
inputs signals in this range, so some signal processing mustbe performed on the output signal.
The goals of processing the output signal are to (1) remove
the reset level noise, and (2) translate the output signal to alevel acceptable by analog to digital converters. Goal
number 1 is met by performing a differential measurement

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TND6116/D
OutputöC(clamp)öS(sample)Figure 10. CDS Timing Signals

dark signal is dependent on the image sensor architecture,mode of operation (see “Accumulation Mode”), and on theimage sensor operating temperature. Due to the present oflocalized defects in the silicon substrate, the dark signal
collected in each pixel will vary from pixel to pixel. Thisvariation in dark signal is called the dark signal noise. The
average current associated with the readout of a complete
dark image is referred to as the dark current. The dark current
will double for approximately every 9°C increase in image
sensor temperature.

One method is to AC couple the output signal and thenclamp it to ground during the flat reset portion of the signal.
Variations in the output which occur after the clamp is
complete will be with respect to ground. Then the groundreferenced signal is passed through an inverting amplifier,resulting in a positive going ground referenced signal. Only
one additional sample pulse is required to complete theprocessing by sampling the signal during the active portionof the signal (i.e. during the time when the phase 2 CCD
input is “OFF”).
The clamped and sampled signal is then ready for direct
input into an analog−to−digital converter.
Another method uses two sample pulses to charge one
capacitor to the reset level and another to the active portionof the output. The two signals are then fed into an inverting
differential amplifier where a difference measurement isperformed and the signal is converted to a positive going,ground referenced signal.

Dark Reference Pixels

Dark reference pixels are groups of photo−sensitive pixels
covered by a metal light shield. These pixels are used as a
black level reference for the image sensor output. Since theincident light is blocked from entering these pixels, thesignal contained in these pixels is due only to dark current.
It is assumed that each photo−sensitive pixel (active anddark reference) will have approximately the same dark
signal; thus, subtracting the average dark reference signal
from each active pixel signal will remove the backgrounddark signal level. Dark reference pixels are typically locatedat one or both ends of the arrays, as shown below for a linear
image sensor.

Dark Current

Dark signal is a termed used to refer to the backgroundsignal present in the image sensor readout when no light isincident upon the image sensor. This background signal is a
result of thermally emitted charge being collected in thephotosites transfer gates, and CCDs. The magnitude of theLS
LOG
Photodiodes

SG
IG
ID
ö2
ö1

Ï
Ï

TG1
TG2

ÌÌÌÌÌ ÌÌÌÌÌ
ÔÔÔÔÔÑÑÑÑÑÑÑÑÑÑÑ
ÔÔÔÑÑÑÑÑÑÑÑÑÑÑ
ÖÖÖÖÖÖÖÖÖÖÖöR

RD VDDFDÔÔÔÔ

Dark Reference PhotodiodeVID

OG SUB VSS

Active PhotodiodeFigure 11. Single Channel of Linear Image Sensor
Data Ratehorizontal CCD clocking rate. An imager with two outputs
will have a data rate equal to two times the horizontal CCD
clocking rate, assuming both outputs are clocked out inparallel.

The data rate is the total number of pixels being clocked
off an imager over a period of time. If the imager has only
one output, then the data rate is typically equal to the

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TND6116/D
Defective Pixel(rms) variation in dark signal voltage. The dark signal
includes components from dark current within the photosite
and CCD regions, reset transistor and output amplifier noise,
and input clocking noise. An input referred noise signal in
the charge domain can be calculated by dividing the darknoise voltage by the imager charge−to−voltage conversion
factor. The dynamic range is typically expressed in units ofdecibels as:

A defective pixel is one whose response to illumination
variations differs significantly from the mean response of allother pixels. The maximum deviation from the mean
response permitted is imager as well as application
dependent. The number and type of defects acceptable isalso application dependent, and can range from zero to as
many as 1000 defects in some cases. It is sometimes possible
to remove the effect of the defective pixel by applying oneof several signal processing defect correction algorithms.
One of the simplest such algorithms is to replace thedefective pixel with the average response of the two nearest
neighboring pixels, i.e.

DR + 20 log

V sat
N sat
ƪdBƫ
+ 20 log
V Dark,rms
N Dark,rms(eq. 6)

Electronic Shutter

An electronic shutter is used to vary the effective
integration time (Teff) of a group of pixels. The circuitry
used to perform the shuttering drains all charge out of thephotosensitive pixel for a fraction of the total integration
time (Tint). When used on tri−linear image sensor arrays, theelectronic shutter can be used to balance the color responseof the red, green and blue channels.

P d + 1 (P d*1 ) P d)1)
2
Dynamic Range

Dynamic Range (DR) is the ratio of the maximum output
signal, or saturation level, of an image sensor to the darknoise level of the imager. The dark noise level, or noise floorof an imager, is typically expressed as the root mean square

Tint

Transfer
Gate

T eff

Exposure
Control Gate
(LOG)

Figure 12. Typical Electronic Shutter Timing
LS
LOG
Photodiodes

SG
IG
ID

ÏÏ
ÏÏ

TG1
TG2

ÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌ
ÌÌÔÔÌÌÔÔÌÌÌÌÌÌÌÌÌÌ
ÔÌ
ÔÔ
ÔÔÔÔ
ÔÔÔ
ÖÖÖÖÖÖÖÖÖÖÖÖÖ
ÑÑÑÑÑÑÑÑÑÑÑÑÑ
ÖÖÖÖÖÖÖÖÖÖÖÖÖ

öR

ö2
ö1

RD

VDD

FD

VID

OG SUB VSS
Figure 13. Single Channel of Linear Image Sensor

Incident
Scanvenger

Exposure

Diode

Control Gate

Gate #1

Transfer

CCD

Metal

Gate #2

Phase 1

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PhotonPhotodiode++++++++++++++++++++

External Bias ”OFF”
External Bias ”ON”

Figure 14. Cross Sectional View of Lateral Electronic Shutter
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TND6116/D
Fill Factor

to have Light pattern noise or Photosensitivity pattern noise.
In addition to the reasons mentioned above, light patternnoise can be caused by the imager entering saturation, thenon−uniform clipping effect of the antiblooming circuit, andby non−uniform, photosensitive pixel areas often caused bydebris covering portions of some pixels.

The fill factor is the ratio of the light sensitive area to thetotal photosite area. Fill factor on some types of area arrays
can be improved using lenlets (see “Lenticular Array(Microlenses/Lenslets)”).
Fixed Pattern Noise

If the output of an image sensor under no illumination isviewed at high gain a distinct non−uniform pattern, or fixedpattern noise, can be seen. This fixed pattern can be removed
from the video by subtracting the dark value of each pixel
from the pixel values read out in all subsequent frames. Dark
fixed pattern noise is usually caused by variations in darkcurrent across an imager, but can also be caused by input
clocking signals abruptly starting or stopping or by having
the CCD clocks not being close compliments of each other.
Mismatched CCD clocks can result in high instantaneous
substrate currents, which when combined with the fact thatthe silicon substrate has some non−zero resistance can resultin the substrate potential bouncing. The pattern noise canalso be seen when the imager is under uniform illumination.
An imager which exhibits a fixed pattern noise underuniform illumination and shows no pattern in the dark is said

Floating Diffusion

The floating diffusion is the charge sensing node used toconvert the charge packets carried by the CCD into a voltagechange which can be detected at the imager output. The term
floating diffusion describes the charge sensing node
structure, which is typically formed by implanting anddiffusing a N−type dopant into a P−type substrate. During
operation, the N side of the diode (the diffusion) is reset toa positive potential by the reset transistor (Q1) and thenallowed to float. When charge is subsequently dumped onto
the floating diffusion a proportional change in voltage
occurs. The change in voltage due to a charge packet of size
N on the charge sensing node will be V = q N / C, where C
is the effective node capacitance and q is the elementarycharge.

Reset
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Figure 15. Typical Image Sensor Output Amplifier

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Out put

TND6116/D
Four Phase CCD

A four phase CCD is one which requires four polysilicon electrodes to make up one CCD cell. Four phase CCDs require
four input clocks to properly transport charge.
One CCD Stage (cell)

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External Bias ”OFF”
External Bias ”ON”

Figure 16. Four Phase CCD
Frame Ratestorage region. These devices operate by first turning “OFF”
the vertical CCDs and opening the external shutter. At theend of the integration time, the image in region A is quickly
transferred into region B, which is not light sensitive. The
vertical CCDs are again turned “OFF” and the external
shutter opened to acquire the next image. At the same timethe image in region B is clocked out of the imager one line
at a time.

The frame period (Tp) of an imager is the time elapsed
between successive image readouts, and the frame rate is thenumber of images which can be read out during one second(Fp = 1/Tp).
Frame Transfer Image Sensors

A frame transfer image sensor is similar to the full frame
imager with the addition of an optically isolated frame

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TND6116/D
Verical CCD(Photosites)

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Region B

Horizontal CCD

OutputAmplifierFigure 17. Full Frame Transfer Image Sensor
Full Frame Image Sensor

most portions of the image sensor are photo−sensitive, anexternal shutter is required to remove incident light before
any charge transferring begins.

An area array which uses the CCDs to both collectphoto−generated charge and transport the charge into ahorizontal CCD is termed a Full−Frame Image Sensor. Since
Verical CCD(Photosites)

Transfer
Gate

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Figure 18. Full Frame Area Image Sensor

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OutputAmplifierTND6116/D
Horizontal CCD

the photosite charge packets are transferred directly into theadjacent horizontal CCD. Since linear image sensors use
only one orientation of CCD, the horizontal prefix is
dropped.

Horizontal CCDs are used to transfer photosite charge
packets to the output amplifier. In area image sensors, thecharge packets are first transferred into the vertical CCDs
and then into the horizontal CCD. In linear image sensors,ÓÓÓÓÏÏÓ
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Horizontal CCD

OutputAmplifierFigure 19. Orientation of CCDs on Area Image Sensor
LS
LOG
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Figure 20. Orientation of CCD on Linear Image Sensor
Image Sensor

Interlaced Image Sensor

A device capable of converting an incident optical pattern(i.e. image) into an electronic signal which contains all
spatial and intensity relationships of the original pattern.
The term is usually used to refer to solid state semiconductor
image sensors.

An interlaced image sensor is one which transfers out aportion of the image being integrated during one frame andthe remaining section of the image during the next frame.
NTSC compatible interlaced imagers output the odd field of
the image and then the even field of the image. An interlaced
display, like the typical NTSC compatible television, writes
every odd line of the image (e.g. lines 1, 3, 5, ... etc.) to thedisplay, and then writes all even lines of the image to display.
If the two images are written quickly enough(< 1/30 second), then the viewer will be unable to detect theindividual fields.

Integration Period

The integration period is the total time the image sensor
collects photons from the incident light pattern. Image
sensors with electronic shutters can have effective
integration times less than the actual integration period.

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12

TND6116/DInterline Image Sensor

the aluminum light shield, so while one image is being
integrated the previous image can be safely transferred outof the image sensor. Interline imager sensors, unlike
full−frame devices, do not require an external shutter.

An interline image sensor has a light shielded CCD
adjacent to each photosite array. An area array interline
image sensor and a linear interline image sensor are depicted
below. Note that only the photosite arrays are not covered byÓÓÏÏÓÓÏÏÓ
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OutputAmplifierFigure 21. Interline Area Array Image Sensor
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Lateral Overflow Drain

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factor; that is, the active area of a pixel is significantly lessthan the total pixel area. One way to increase the effective
active area on such devices is to manufacture a tiny opticallens, or lenslet on each photosite. Increases in effective areaof 2 to 3 times can be achieved. This in turn increases thephotosite responsivity by an equal factor.

See “Blooming” for a complete description.
Lenticular Array (Microlenses/Lenslets)

Interline array imager sensors and array imagers withlateral overflow drains suffer from reduced optical fill

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TND6116/DÏÏÏÏÏÏÏ
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Figure 23. Lenslets are Fabricated over Photodiodes

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Light Shieldarray is at least one CCD and one charge sensing amplifier.
Linear imager sensors are ideally suited for scanningapplications such as paper copiers, fax machines, and film
scanners.
A tri−linear image sensor is one which has three parallel
linear photosites arrays, each with its own CCD and output
amplifier. It is common practice to sequentially fabricate
red, green, and blue bandpass filters such that each one of thearrays is uniquely covered by one of the primary colors toyield an image sensor which can obtain full color
information with a single scan over the document.

A light shield is a layer of non−translucent material
deposited on all areas of the image sensor except thephotosensitive portion of the pixels. The light shieldmaterial is usually aluminum and will reflect a portion of theincident light and absorb the rest. The light shield is often
electrically connected to scavenger diodes located about theimager perimeter and biased to a positive voltage.
Linear Image Sensor

A linear image sensor is one which contains one or more
linear arrays of photosites. Associated with each photodiode

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TND6116/DLS
LOGÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌ
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Figure 25. Single Channel of a Typical Linear Image Sensor
Modulation Transfer Function (MTF)

transistor noise can be removed using correlated double
sampling signal processing. The photon shot noise cannot be
eliminated; however, by acquiring and averaging several
frames it, and all temporal noise sources, can be reduced.
The variation in dark current from pixel to pixel leads toa dark noise pattern across an imager. The effects of this dark
pattern noise can be minimized by averaging several frames
and then using the pixel−referenced, dark frame data as thezero reference level for each pixel.

MTF is the magnitude of the spatial frequency response ofa solid state imager. The three main components of imager
MTF are termed the aperture MTF, diffusion MTF, andcharge transfer efficiency MTF. The aperture MTF resultsfrom the discrete sampling nature of solid state imagers,
with smaller pixel pitches yielding a higher MTF response.
The diffusion of photogenerated charge degrades the imager
response and is responsible for the second component. The
third component is due to inefficient charge transfer in theshift register. The maximum frequency an imager can detect
without aliasing occurring is defined as the Nyquist
frequency and is equal to one over two times the pixel pitch.
MTF is typically reported at the Nyquist frequency, 1/2
Nyquist, and 1/4 Nyquist. The aperture MTF limits the
maximum response at Nyquist to 0.637 (Note that the
maximum MTF response is 1.0). The diffusion component
will further degrade this value, especially at longer optical
wavelengths.

Non−Interlaced

See “Progressive Scan”.
Output Amplifier

The photogenerated charge collected by the photosites istransferred to the output amplifier using a CCD. The output
amplifier must then convert each charge packet into a
corresponding voltage change at the device output pin. The
circuit frequently used to accomplish this task is show
below. This circuit consists of a two stage source follower
amplifier with the biasing load for the last stage contained
off chip. It is noted that the typical operating voltages for
VDD and the reset drain are 10 to 15 volts.
The charge−to−voltage conversion begins by turning“ON” the reset transistor (Q1) at the same time the phase 2
CCD clock is turned “ON” (see region A in Figure 27). Thisaction clears all charge off the floating diffusion node andresets the node voltage to the reset drain voltage. Due to thegate−to−source capacitance of Q1, the reset gate clock will
feed through to the floating diffusion node, resulting in an
output voltage slightly higher than the reset drain voltage.
This feed through voltage disappears when the resettransistor is turned “OFF”, as depicted in region B. The last
step is to simultaneously turn phase 1 “ON” and phase 2“OFF.” This action causes the charge located in the phase 2
region of the last CCD stage to flow onto the floating
diffusion, resulting in a decreased node voltage (see region
C in Figure 27).

Multiple Outputs

An image sensor with multiple outputs is designed todivide up the photosite array(s) into sections. Each section
can be readout using CCDs to a unique output amplifier. The
advantage of an image sensor with multiple outputs is thatthe integrated image can be read out of the imager N times
as faster, where N is the number of outputs. The outputs may
be read out in parallel to increase the data rate, or may be
output in a particular sequence to ease the task of image
reconstruction.
Noise

Noise is any unwanted signal added to the imager output.
Temporal noise sources present in a typical imager include
the dark current, photon shot noise, reset transistor noise,
CCD clocking noise, and the output amplifier noise. Dark
current is dependent on the imager operating temperatureand can be reduced by cooling the imager. The reset

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15

TND6116/DReset
Dr ai n

Reset

VDD

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Figure 26. Typical Solid State Imager Output Amplifier

The magnitude of the voltage drop is determined by the capacitance of the floating diffusion node (i.e. Vout = Q / Cfd).

ö1
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öR
OutputRegions

A

B

C

A

B

C

A

B

C

Figure 27. Corresponding Output Waveform

process, the electron exposure structure nonlinearities (if itexists), the efficiency of charge transportation from thephotosite to the output amplifier, and the output amplifier
linearity.
The absorption of photons within the silicon substrate canbe considered an ideal linear function of incident
illumination level when averaged over a given period oftime. The existence of an electronic exposure control circuit
adjacent to the photosensitive sites can introduce anon−linearity into the overall response by allowing small
quantities of charge to remain isolated in unwanted potential
wells. Whether or not any potential wells exist depends onthe design and manufacturing of the particular image sensor.
The existence of such potential wells in the exposure
circuitry, also called exposure defects, will degrade thelinearity only at small signal levels and may be different
from one photosite to the next. An image sensor withexcessive exposure defects would be rejected during quality
assurance testing.
The loss of charge during the transportation of charge
packets from the photosite to the CCD, which is termed lag,tends to affect the linearity only at very small signal levels.

Transistor Q3 is a constant current bias for the first stage
source following transistor Q2. The second stage source
follower transistor (Q4) is designed to drive much higher
loads than Q2, which implies higher drive currents. To
minimize the power dissipation on the image sensor, thebiasing load for Q4 is often located off the device (i.e. on thedriver board). Depending on the expected application, somedevices are designed with on chip bias loads for Q4. Since
the dark current level (thermal noise level) doubles for every
9°C change in temperature, it is desirable to keep as much
power dissipation off the device as possible. If the output
amplifier dissipates significant power, the localized
temperature around the amplifier will increase. This in turnwill result in a temperature gradient across the device, whichleads to a dark current gradient across the device.
Output Linearity

The non−linearity of an image sensor is typically definedas the percent deviation from the ideal linear response,
which is defined by the line passing through Vsat and Vdark.
The percent linearity is then 100 minus the non−linearity.
The output linearity of a solid state image sensor isdetermined from the linearity of the photon collection

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TND6116/D
overall effect on linearity is generally not a concern. If
biased properly, the output amplifier will yield anonlinearity of typically less than 2%.
Non linearity at signal levels beyond the saturation levelis expected and can often vary significantly from pixel topixel.

“Pinned” photodiodes, or buried photodiodes, haveextremely small lag (< 0.5%), and can be considered to belag free. The CCD charge transfer inefficiency (CTI) will
reduce the amplitude of the charge packet as it is transported
towards the output amplifier, with the greatest effectrealized at very small signal levels. Modern CCD’s have
CTE in excess of 0.999999 per CCD transfer; thus, theOutput Voltage

Vsat

Actual Response
Vdark
0

Ideal Response0

Incident Light Intensity

Isat

Figure 28. Definition of Non−Linearity
Output Sensitivity

majority on the lighter doped side. As photogenerated
charge is collected, the space−charge region collapses. At
the end of the integration period, an adjacent Transfer gate
potential is increased by applying an external bias and thecollected charge is allowed to flow into the adjacent CCD.
As the charge flows out of the photodiode the space−charge
region begins to extend back to the starting boundaries. At
the tail end of this process, the photodiode potential
approaches the value of the adjacent transfer gate potential,
and the flow of charge is subject to sub−threshold effects.
That is, the driving potential between the photodiode and thetransfer gate becomes significantly small.
The result is that the last portion of charge will not betransferred as quickly as the first portion. The net effect isthat not all charge is transferred out of the photodiode. The
remaining charge will appear in the readout of the next lines
or frames where the photosite is not illuminated. “Pinned”
photodiodes have greatly improved lag performance due tothe construction of the space−charge region. A “pinned”
photodiode is created by making a P+NP type structure.
If the P+ implant is made sufficiently high, the N typeregion will be totally depleted and will have an associatedelectro−static potential independent of the adjacent transfer
gate potential during the charge transfer process. The net
effect is that a substantial potential difference can be
maintained between the photodiode and the transfer gate
during the whole transfer period, regardless of the amountof remaining charge in the photodiode.

The output sensitivity of an imager is defined as the ratioof the change in output voltage for a given quantity of chargetransferred onto the charge detection node (i.e. floating
diffusion). This is also referred to as the charge−to−voltage
conversion factor (dV/dN). Noting that the voltage at a given
node is equal to the charge on the node divided by the nodecapacitance (V = Q/C), the change in output voltage is seen
to vary linearly with the change in charge, or:
DV +

DQ
C

(eq. 7)

To maximize the output sensitivity, the floating diffusion
node capacitance (C) is made as small as possible. A typicalvalue for the node capacitance is 20 to 50 fF (20 − 50 × 1015).
Photodiode Lag

Photodiode lag is defined as the percentage of total chargetransferred from the photodiode to the adjacent CCD which
remains in the photodiode after the transfer period iscomplete. Ideally, the lag should be zero (i.e. all charge istransferred). Certain types of photodiodes have much higher
values of lag than others. For example, a standard PN
photodiode will have the highest lag while a “pinned”
photodiode will have the smallest. The origins of lag can beunderstood by picturing the reversed biased PN diode acting
as a photon collection site. A reversed biased PN junction
results in a space−charge region (also called depletion
region) extending into both semiconductor types, with the

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TND6116/D
Incident
PhotonScanvenger
Diode

Exposure
Control Gate
”OFF”

Transfer
Gate #1”OFF”

CCD
Phase 1

Metal
Light Shield”OFF”ÏÏÏÑÑÑÑÑÑÒÒÒÒÒÒÑÑÑÑÑÑ
ÌÌÌÌÌÑÑÑÑÑ
+++++++++++++++++++

Electrostatic Potential (y)Transfer
Gate #2”OFF”++++++

Photodiode+++++++++++ +++

e
e e
e e
ee e ee ee eee ee e eeee

Figure 29. (1) Lag−Integrating Bright Line

Scanvenger
Diode

Exposure
Control Gate
”OFF”

Transfer
Gate #1”ON”

CCD
Phase 1”ON”

Metal
Light ShieldÏÏÏÑÑÑÑÑÑÒÒÒÒÒÒÑÑÑÑÑÑ
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Electrostatic Potential (y)Transfer
Gate #2”ON”++++++

Photodiodee++++++++++++++

e
e
e
e e
e ee ee ee e ee e e e eee

Figure 30. (2) Lag−Light Removed, Transfer Begins

Scanvenger
Diode

Exposure
Control Gate
”OFF”

Transfer
Gate #1”OFF”

CCD
Phase 1”ON”

Metal
Light ShieldÌÌÌÌÏÏÏÑÑÑÑÑÑÒÒÒÒÒÒÑÑÑÑÑÑÌÌÌÌÑÑÑÑÑÑ
+++++++++++++++++++

Electrostatic Potential (y)Transfer
Gate #2”ON”

Photodiodee++++++e++++++++++++++eeee e ee e e
e e ee e e e e eee

Figure 31. (3) Lag−Transfer Continues

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TND6116/D
Scanvenger
Diode

Exposure
Control Gate
”OFF”

Transfer
Gate #1”OFF”

Transfer
Gate #2”OFF”

CCD
Phase 1”ON”

Metal
Light ShieldÏÏÏÑÑÑÑÑÑÒÒÒÒÒÒÑÑÑÑÑÑÌÌÌÌÑÑÑÑÑÑ
Electrostatic Potential (y)+++++++++++++++++++

Photodiodee++++++++++++++++++++

e

e e
e
ee e ee ee eee ee e e eee

Figure 32. (4) Lag−Transfer Complete
Photoresponse Nonuniformity

between the number and arrangement of pixels on an imager
and the display device (e.g. CRT) used to view the captured
image.

The peak−to−peak variation in output signal underuniform illumination of the image sensor is called the
Photoresponse Nonuniformity (PRNU). PRNU is verysmall for monochrome imagers (i.e. imagers with no color
filters), since the only varying factors are the photosite
quantum efficiency, the dark current, and the effective active
area. These factors typically are very uniform across animager. Color imagers have higher PRNU due to slightvariations in the external color filters. Variations in color
filter thickness will cause some photosites to have higher orlower photoresponse than others. These variations can be
minimized using proper fabrication techniques and careduring the design phase to minimize variation due totopology across the imager.

Pixel−to−Pixel Crosstalk

The absorption of a photon in silicon generates anelectron−hole pair. The depth at which the absorption
process occurs has a lot to do with whether or not thephotogenerated electron will be collected within thepotential well of the photosensitive pixel through which thephoton originally entered. If absorption occurs within thephotosensitive pixel space−charge region (1) (seeFigure 34), the photogenerated electron will be collected by
the built−in electric field. Since there are no electric fields
below the space−charge region, electrons generated in thisregion (2) (see Figure 34) are free to randomly diffuse until
they recombine in the substrate or are captured by aphotosite or CCD register structure. Electrons generated
below the space−charge region of a photodiode which arecaptured in a neighboring photosensitive pixel constitutepixel−to−pixel crosstalk. The effect of this type of crosstalk
is to reduce the spatial resolving power (MTF) of theimaging array.

Pixel

A pixel is an individual photosite which has the ability tocollect photogenerated charge and restrict the location of thecharge to a discrete volume of space within the silicon
substrate. Photodiodes and photocapacitors are two types ofpixels, since both can collect and hold photogenerated
charge. There is usually a one−to−one correspondence

Incident PhotonsÒÒÒÒÒÒÒÒÒÒÒÒÒÒ
PhotodiodePhotodiodePhotodiodeee e e e e e eeElectrostatic PotentialehFigure 33. Absorption of Photons Below the Space−Charge Region

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TND6116/D

(2)

(1)
e

h

Incident
PhotonshElectrostatic Potential

SiO 2− SiInterfaceeChannelZc

Distance Into Silicon

Figure 34. Space−Charge Regions 1 and 2

below as a function of wavelength and absorption depth. A
typical photosensitive pixel space−charge region depth isalso indicated on Figure 35. As an example, it can be seenthat 90% of all 520 nm photons will be absorbed within thespace−charge region, but only 42% of all 670 nm photons
will be absorbed.

It is known that photons of longer wavelength, such as
from red light, have a mean absorption depth greater thanphotons of shorter wavelength, such as from blue light. Thisphenomena means that red light will produce higher
pixel−to−pixel crosstalk than green light, and green lightwill produce higher crosstalk than blue light. The percent ofphotons absorbed within the silicon substrate is shown100Ï Ï Ï Ï Ï ÏÏ Ï Ï Ï ÏÏÏ ÏÏ ÏÏ Ï
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Ld = 2.2um

Distance in Silicon [um]

10

1

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.01

10%

350

400

450

500

550

600

650

700

750

800

850

900

Wavelength [nm]Figure 35. Percent of Photons Absorbed in Silicon versus Wavelength and Depth
Potential Well

application of an external force (e.g. external voltage). A
potential well can be created by the application of an
external bias to a MOS type structure (e.g. a CCD gate),
creating a dynamic deep depletion region, or by setting up
a variation if doping concentration, or both.
A CCD structure can be thought of as a MOS capacitor
with two different potentials built in near the silicon surface.
Dynamically applying an external voltage (potential) to the
MOS capacitor will cause both potentials to increase or
decrease, depending on the polarity of applied voltage.

A potential well is a localized region within asemiconductor substrate which has a higher electrostatic
potential then the surrounding area. A region of increased
electrostatic potential is equivalent to a region of reduced
potential energy, since P.E. = −q x V, where q = magnitudeof electronic charge and V is the electrostatic potential.
Electrons introduced into an area of a potential well will be
confined to that area until they recombine, are thermallyemitted out of well, or are forced from the well by the

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TND6116/D

ö1
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N Type Implant+−+−+−+−+− − − − − − − − − − −+−+−+−+−+− − − − − − − − − −+−+−+−+−+− − − − − P Type Implant

Electrostatic Potential

Built in
Potential Well

Built in
Potential Well

Potential Wellfrom External Bias

External Bias ”OFF”
External Bias ”ON”

Figure 36. Potential Wells in True Two−Phase CC D
Progressive Scan

displaying two fields of the image (i.e. the odd and evenfields).

An area array image sensor which transfers out the entire
image during one exposure cycle is called a Progressive
Scan Imager, or non−interlaced image sensor. Similarly, a
CRT display which displays the entire image during one
frame is called a progressive or non−interlaced display. The
other common type of readout encountered is calledinterlaced, which involves reading out a portion of theimager during one exposure cycle and the remainder in thenext cycle. Interlaced displays are used in standard NTSC
television, where the image on the screen is created by

Pseudo Two Phase CCD

A pseudo two phase CCD is similar to a true two phase
CCD in that only two external clocks are required to operate
the CCD. The difference is that the true two phase CCD
requires only two polysilicon electrodes per CCD stage;
whereas, the pseudo two phase CCD requires fourelectrodes, as shown in Figure 37. Generally the true twophase CCD architecture requires less area to implement.

One CCD Stage (cell)ö2ö1ÏÏÏÏÏÏÏ
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Electrostatic Potential (y)− − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −N Type Implant
P Type Implant++++++++++++++++++

External Bias ”OFF”
External Bias ”ON”

Figure 37. Pseudo Two Phase CC Dǒ

Quantum Efficiency

QE + 1 *The ratio of the number of photogenerated electrons
captured by a pixel to the number of photons incident upon
the pixel during a period of time is termed the Quantum
Efficiency (QE). Ideal photodiode quantum efficiency canbe expressed as:e *a(l) Ld
1 ) a(l) L n

Ǔ Ǹåå

si
0

ǒ Ǔ
|E )
|2
si
|E )
|2
in

(eq. 8)Where e is the permittivity, (λ) is the absorption
coefficient, Ln is the depth from the silicon surface to the endof the photodiode space−charge region, Ld is the electron

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TND6116/D
diffusion length, and E+ is the complex electric fieldstrength. Quantum efficiency will vary as a function of
optical wavelength due to the wavelength dependence of theabsorption coefficient. In addition, interference effects
caused by the thin films of silicon dioxide, silicon nitride andpolysilicon on the surface will result in a wavelength
dependence, which is described by the ratio of complex
electric field amplitudes in the above equation.

is generally applied to the gate of a FET, acting as a switch(Q1 in Figure 38). The drain of the FET is connected to a DC
source of approximately 11 volts. When the reset FET isturned “ON”, or allowed to conduct, the charge on thesensing node is swept off to the drain and the node is resetto the drain voltage. The reset clock is turned “OFF” to allow
the charge sensing node to stabilize and then the next charge
packet is dumped onto the charge sensing node.

Reset Clock

The reset clock is the signal used to clear all charge off ofthe charge sensing node (floating diffusion). The reset clock
Reset
Dr ai n

Reset

ö2

VDD

Q1

OG

Q2

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Floating
Diffusion

e

Q4

e e

Q3

e e e
e e ee ee+−

Out put

Figure 38. Typical Imager Output Amplifier
Resolution

resolution of 2540 DPI (1000 / (1000 x 0.01 mm x1”/25.4 mm)). If the sensor is used in an optical system toimage an 8” wide document, then the resolution in the
document plane would be 125 DPI (1000 pixels / 8”). This
example is slightly misleading in that it does not consider thefrequency response of the sensor or the optics. In reality, thesensor will have an MTF of between 0.2 and 0.7 at the
Nyquist spatial frequency and the optics are likely to havean MTF of 0.6 to 0.9 at the Nyquist frequency. It is important
to note that even though a sensor may have a high enoughsampling frequency for a particular application, the overallfrequency response of the sensor and optics may not be
sufficient for the application!

The resolution of solid state image sensor is the spatial
resolving power of that sensor. The spatial resolution of asensor is described in the spatial frequency domain byplotting the modulation transfer function (MTF) versus
spatial frequency. The discrete sampling nature of solid state
image sensors gives rise to a sampling frequency which will
determine the upper limit of the sensor’s frequency
response.
Resolution is frequently described in terms of the numberof dots or photosites per inch (DPI) in the imager or object
planes. For example, a linear image sensor with a single
array of 1000 photosites of pitch 10 mm would have a

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TND6116/D
Image Plane

Object Plane

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Linear Imager

Optics8” Wide Document

Figure 39. Linear Imager in a Simple Imager System

Image Plane

PhotodiodeArray inImager Plane

OpticsŠŠŠŠŠFigure 40. Object/Image Spatial Resolution Comparison
Responsivity

A

dV
dN

QE(l)

E(l)

+A

dV
dN

QE(l)

Saturationlhc

See “Charge Capacity”.
(eq. 9)Schottky Barrier Diodes

Schottky barrier diodes are used in platinum silicide
infrared image sensors as the photosites. Infrared radiation
with photon energy less than the bandgap of silicon istransmitted through the substrate and is absorbed by the PtSi
layer. The absorbed photons excite valence electrons in themetal silicide to states above the fermi level, leaving behindholes. Holes which have sufficiency energy to pass over the
Schottky barrier formed between the silicide and the silicon
are called hot holes and are injected into the substrate.

As both the quantum efficiency and the photon energy arefunctions of the optical wavelength (l), responsivity will bedependent on the wavelength. A curve of responsivity
versus wavelength is required to calculate the imager output
response to a given optical input. If the spectral irradiance(I(l)) is known at the imager plane, then the imager output
voltage can found from:
V out + T int

ŕ

l max

l min

R(l)

PhotodiodeArray inObject Plane

where Tint is the integration time in seconds, and I(l) is inunit of W/(m2−nm). The spectral irradiance can be measured
using a commercial spectral radiometer, or it can becalculated from the optical system components.
NOTE: See reference 3.1 in the bibliography section forconversion methods from photometric toradiation units and vice−versa.

The imager output voltage per incident optical energy
density is called the imager responsivity, and it is expressedin volts per micro−joule per centimeter squared, or
V/(mJ/cm2). Responsivity can be calculated from thephotodiode quantum efficiency (QE), charge−to−voltage
conversion factor (dV/dN), the energy per photon (E), andthe pixel active area (A) as:
R(l) +

Ú
Ú
Ú
Ú
Ú
Ú
Ú
Ú
Ú
Ú

Object Plane

I(l)dl(eq. 10)

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TND6116/D
Sensitivity

signal to each charge packet being transported by the CCD,
which reduces the dynamic range of the image sensor. Smear
is more likely to occur at longer optical wavelengths, since
the mean absorption depth within silicon increases forlonger wavelengths. Smear can be minimized by (1)increasing the photosite−to−CCD distance, (2) increasing
the depth of the photosite space−charge region, or by (3)collecting the smear charge in first transfer gate (i.e. leave
TG1 “ON” while integrating image signal). Smear fromphotogenerated electrons from outside the image sensor
active area can also be minimized by the introduction of ascavenger diode around the photo−sensitive array.

The sensitivity of an imager is determined by a
combination of the pixel quantum efficiency and thecharge−to−voltage conversion factor of the output amplifier
and is specified as the imager responsivity.
Smear

Figure 41 depicts the situation where photogenerated
charge originating below the space−charge region of aphotosite diffuses into the adjacent CCD. This diffusion
constitutes a photosite−to−CCD crosstalk signal, which isfrequently referred to as smear. The result of smear is to add

Scanvenger
Diode

Incident
PhotonExposure
Control Gate

Transfer
Gate #1

Transfer
Gate #2

Metal
Light Shield

CCD
Phase 1ÏÏÏÑÑÑÑÑÑÑÒÒÒÒÒÑÑÑÑÑÑ
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+++++++++++++++++++

Photodiode+++++++++++++++++ +++SMEAR!
External Bias ”OFF”
External Bias ”ON”

Figure 41. Photodiode−to−CCD Crosstalk (Smear)
Spectral Response

Three Phase CCD

See the “Quantum Efficiency” and/or “Responsivity” fordetails.

A three phase CCD is one which uses three polysiliconelectrodes per CCD stage. This type of CCD will require
three input clocks to properly transport charge packets.

Surface Channel

See “Buried Channel CCD”.
One CCD Stage (cell)

ö3
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Electrostatic Potential (y)−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

External Bias ”OFF”
External Bias ”ON”

Figure 42. Three Phase CCD
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N Type Implant

TND6116/D
Transfer Gate Clock

clocking is stopped and the transfer gate biases are applied,causing the photosite charge to transfer to the adjacent CCD
cell.

The transfer gate clocks are used to isolate thephotogenerated charge collected in the photosite from theadjacent CCD structure. At the appropriate time, the CCD
LS
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Incident
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Control Gate

VDD

Transfer
Gate #1

Photodiode++++++

Transfer
Gate #2++++++++++++++

External Bias ”OFF”
External Bias ”ON”

Figure 44. Cross Section of Linear CCD Imager

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25

VID

OG SUB VSS

CCD
Phase 1ÏÏÏÏÑÑÑÑÑÑÒÒÒÒÒÒÑÑÑÑÑÑÌÌÌÌÌ
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Electrostatic Potential (y)RDFD

Figure 43. Single Channel of Linear CCD Imager

Scanvenger
Diode

öR

Metal
Light ShieldTND6116/D
True Two Phase CCDvery dense CCD structures to be developed, which isnecessary in very high resolution image sensors.

A true two phase CCD is one which uses only two
polysilicon electrodes per CCD stage. This design permits
One CCD Stage (cell)

ö1
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P Type Implant

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External Bias ”OFF”
External Bias ”ON”

Figure 45. True Two Phase CCD StageUV Enhancement Coating

Vertical CCD

A UV enhancement coating is a material which can beplaced on an image sensor which will increase the quantum
efficiency in the UV spectrum. UV photons are absorbed in
the coating and re−emitted as lower energy, longer
wavelength photons which are more readily absorbed by thesilicon based photo−sensitive pixels. Lumogen is a common
UV enhancement material.

Vertical CCDs are used in area imagers to transfer
photo−collected charge packets into the horizontal CCD,
which then transports the charge packets to the output
amplifier.

Vertical CCD

Transfer
Gate

Photodiodes

Photodiodes

Photodiodes

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Photodiodes

Vertical“Transfer Gate

Horizontal CCD

OutputAmplifierFigure 46. Orientation of CCDs and Transfer Gates on Area Imager

http://onsemi.com
26

TND6116/D
Vertical Overflow Drain (VOD)(sensitivity) to visible and near infrared radiation. Afterthinning, the backside of the substrate is passivated. In atypical application, the backside of the thinned substrate
imager is positioned in the image focal plane so that theimage sensor is illuminated through the thinned substratefrom the backside of the device.

See “Lateral Overflow Drain (LOD)”.
Wafer Thinning

A process in which the silicon substrate thickness is
significantly reduced to enhance the quantum efficiency

BIBLIOGRAPHYFor Information on Solid State Physics

For Information on Electronic Imaging Systems

1. Sze, S. M., “Physics of Semiconductor Devices”,
John Wiley & Sons, New York 1981, ISBN
0−471−05661−8.
2. Jespers, et al, “Solid State Imaging”,
Noordhoff International Publishing, 1975, ISBN90−286−0046−9.

1. Schreiber, W. F., “Fundamentals of Electronic
Imaging Systems”, Springer−Verlag, Berlin,
Germany, 1991, ISBN 0−387−53272−2.
2. Howes, et al, “Charge−Coupled Devices andSystems”, John Wiley & Sons, New York, 1980,
ISBN 0−471−99665−3.

For Information on CCD Image Sensors

1. Beynon, et al, “Charge−Coupled Devices and their
Applications”, McGraw−Hill, Berkshire, England,
1980, ISBN 0−07−084522−0.
2. Howes, et al, “Charge−Coupled Devices andSystems”, John Wiley & Sons, New York, 1980,
ISBN 0−471−99665−3.

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TND6116/D