Transcript chapter4

Chapter 4 - Optical sensors:
Optical sensors
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Optical sensors are those sensors that detect
electromagnetic radiation in the broad optical range –
from far infrared to ultraviolet
Approximate range of wavelengths from 1mm
(3x1011 Hz or far infrared) to 1 nm (3x1017 Hz or
upper range of the ultraviolet range).
Direct methods of transduction from light to electrical
quantities (photovoltaic or photoconducting sensors)
Indirect methods such as conversion first into
temperature variation and then into electrical
quantities (PIR sensors).
Spectrum of “optical” radiation
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Nomenclature:
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Visible light
Infrared radiation (not infrared “light”)
Ultraviolet radiation (not UV “light”)
Ranges shown are approximate and somewhat
arbitrary
Infrared radiation
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Approximate spectrum
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1mm (300 GHz) to 700nm (430 THz)
Meaning: below red
Near infrared (closer to visible light)
Far infrared (closer to microwaves)
Invisible radiation, usually understood as
“thermal” radiation
1nm=10-9m 1GHz=109 Hz, 1THz=1015 Hz
Visible light
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Approximate spectrum
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700nm (430 THz) to 400nm (750 THz)
Based on our eye’s response
From red (low frequency, long wavelength)
To violet (high frequency, short wavelength)
Our eye is most sensitive in the middle (green
to yellow)
Optical sensors may cover the whole range,
may extend beyond it or may be narrower
Ultraviolet (UV) radiation
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Approximate spectrum
 400nm
(750 THz) to 400pm (300 PHz)
Meaning - above violet
 Understood as “penetrating” radiation
 Only the lower end of the UV spectrum
is usually sensed
 Exceptions: radiation sensors based on
ionization (chapter 9)
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A word on units
SI units include: meter, kg, second,
ampere, candela, temperature kelvin
and the mole
 All other units are derived units
 Candela “is the luminous intensity, in a
given direction, of a source that emits
monochromatic radiation of frequency
540x1012 Hz and that has a radiation
intensity of 1/683 watt per steradian”
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Units of luminosity
Units of illuminance
Materials
Optical sensing
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Based on two principles
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Thermal effects of radiation
Quantum effects of radiation
Thermal effects: absorption of radiation
of the medium through increased
motion in atoms. This may release
electrons (heating)
 Quantum effects: photon interaction
with the atoms and the resulting effects,
including release of electrons.
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The photoelectric effect
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Planck’s equation:
e=hf
[ev]
h = 6.6262x10- [joule.second] (Planck’s constant)
f = frequency
e = energy of a photon at radiation frequency f.
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This is called the quantum of energy
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Higher for higher frequency
Can be imparted to electrons as kinetic energy
Note: this energy is also called ionization energy and is used
to distinguish between “dangerous” and “benign” radiation
The photoelectric effect
Photons collide with electrons at the
surface of a material
 The electrons acquire energy and this
energy allows the electron to:
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 Release
themtselves from the surface of
the material by overcoming the work
function of the substance.
 Excess energy imparts the electrons
kinetic energy.
The photoelectric effect
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This theory was first postulated by Einstein in
his photon theory (photoelectric effect) in
1905 (for which he received the Nobel Prize):
hf - e0 = k
e0 is called the work function (energy required to leave the
surface of the material)
k represents the maximum kinetic energy the electron may
have outside the material. Energy is “quantized”
The photoelectric effect
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For electrons to be released, the photon
energy must be higher than the work function
of the material.
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Frequency must be sufficiently high or:
Work function must be low
Frequency at which the photon energy equals
the work function is called a cutoff
frequency
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Below it no quantum effects may be observed (only thermal
effects)
Above it, thermal and quantum effects are present.
At higher frequencies (UV radiation) quantum effects
dominate.
Work function table
Table 4.1. Wor k functions for selected materials given in [eV]
Material
Work Func tion
Alumi num
3.38
Bism uth
4.17
Cadmium
4.0
Cobalt
4.21
Copper
4.46
Germanium
4.5
Gold
4.46
Iron
4.4
Nickel
4.96
Platinum
5.56
Potasium
1.6
Silicon
4.2
Silver
4.44
Tungs ten
4.38
Zinc
3.78
Some notes:
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Thermoelectric effect is a surface effect
Most notable in conductors
Group 1 (Alkalis) has lowest work function
values - often used in thermoelectric cells
(later)
 The amount of electrons released becomes a
measure of radiation intensity
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Electrons may be emitted by thermionic
emission - a totally different issue based on
thermal effect
The photoconducting effect
A solid state (volume) effect
 Most notable in semiconductors
 Based on displacement of valence
and/or covalence electrons
 Valence electrons: bound to individual
atoms in outer layers
 Covalence electrons: bound but shared
between neighboring atoms in the
crystal
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Model: photoconducting effect
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Photons collide with electrons
Electrons must acquire sufficient energy to:
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Leave the valence band
Move into the conduction band
Minimum energy required: band gap energy
Model: photoconducting effect
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In the conduction band, electrons are mobile
and free to move as a current.
When electrons leave their sites, they leave
behind a “hole” which is simply a positive
charge carrier.
This hole may be taken by a neighboring
electron with little additional energy
(recombination)
Net current is due to electrons and holes.
Manifested as a change in concentration of
carriers (electrons and holes) in the
conduction band and therefore in conductivity
of the medium
Model: photoconducting effect
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Conductivity of the medium is:
e - charge of electron
 e - mobility of electrons [m2/Vs]
 p - mobility of holes [m2/Vs]
 n - concentration (density) of electrons
[/m3]
 p - concentration (density) of holes [/m3]
 s - conductivity of the medium
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Conductivity is temperature
dependent (mobility and
concentrations are temperature
dependent)
s = e en + p p
photoconducting effect
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This change in conductivity or the resulting change in current is
the a direct measure of radiation intensity.
The photoconducting effect is most common in semiconductors
because the band gaps are relatively small.
It exists in insulators as well but there the band gaps are very
high and therefore it is difficult to release electrons except at
very high energies.
In conductors, most electrons are free to move (they are in the
conduction band and hence far above the band gap in energy)
which indicates that photons will have minimal or no effect on
the conductivity of the medium.
Semiconductors are the obvious choice for sensors based on
the photoconducting effect while conductors will most often be
used in sensors based on the photoelectric effect
Photoconducting effect
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Conductivity results from the charge, mobilities of
electrons and holes and the concentrations of
electrons, n and p from whatever source.
 In the absence of light, the material exhibits what is
called dark conductivity, which in turn results in a
dark current.
 Depending on construction and materials, the
resistance of the device may be very high (a few
MegaOhms (M) or a few k.
 When the sensor is illuminated, its conductivity
changes depending on the change in carrier
concentrations (excess carrier concentrations).
Photoconducting effect
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s = e en + p p
This change in conductivity is
Carriers are generated at a certain
generation rate
They also recombine at a recombination
rate typical for the material, wavelength,
carrier lifetime, etc.
Generation and recombination exist
simultaneously
Under a given illumination a steady
state is obtained when these are equal.
Under this condition, the change in
conductivity is (tp,tn - lifetimes, f - # of
carriers generated per second per
volume
s = ef n tn + p tp
Photoconducting effect
If p - type carriers dominate - p-type
photoconductor
 If n - type carriers dominate - n type
photoconductor
 Opposite type carrier concentrations are
negligible
 A particular type is obtained by doping
(see chapter 3)
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Photoconducting effect sensitivity
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Sensitivity to radiation (efficiency)
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L is the length of the sensor (distance
between electrodes) and V the voltage
across the sensor.
Sensitivity: the number of carriers
generated per photon of the input
radiation.
To increase sensitivity
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materials with high carrier lifetimes
keep the length of the photoresistor small
the latter is typically achieved through the
meander construction shown below
G = V n tn + p tp
L2
Photoconductor - structure
photoconducting effect
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Properties vary among semiconductors
The lower the band gap the more effective
the semiconductor will be at detection at low
frequencies (long wavelengths).
 The longest wavelength specified for the
material is called the maximum useful
wavelength, above which the effect is
negligible.
 Availability of electrons is temperature
dependent - each semiconductor has a
maximum useful temperature (see table)
Photoconductive properties of
semiconductors
Table 4.2. Band gap energies, longest wavelength and wor king temperat ures for
selected semicondu ctors
Material
Band gap [eV]
Longes t wave leng th
Working temperature
max [m]
[K]
3.6
0.35
300
ZnS
CdS
2.41
0.52
300
CdSe
1.8
0.69
300
CdTe
1.5
0.83
300
Si
1.2
1.2
300
Ge
0.67
1.8
300
PbS
0.37
3.35
InAs
0.35
3.5
77
PbTe
0.3
4.13
PbSe
0.27
4.58
InSb
0.18
6.5
77
Ge:C u
30
18
Hg/CdTe
8-14
77
Pb/SnTe
8-14
77
InP
1.35
0.95
300
GaP
2.26
0.55
300
Note: properties of semi conduc tors vary w it h dop ing and o ther impuriti es. The values shown
shou ld be viewed as representative on ly.
Photoconductive properties
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Example: InSb (Indium Antimony):
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maximum wavelength of 5.5 m
sensitive in the near infrared range
band gap is very low - very sensitive.
but electrons can be easily released by thermal
sources
totally useless for sensing at room temperatures
(300K) (most electrons are in the conduction
band)
These carriers serve as a thermal background
noise for the photon generated carriers.
it is often necessary to cool these long
wavelength sensors to make them useful by
reducing the thermal noise.
Semiconductors
Various photoconductors
(photoresistors)
Photodiodes
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Semiconducting diode exposed to radiation
Excess carriers due to photons add to the
existing charges in the conduction band
exactly in the same fashion as for a pure
semiconductor.
 The diode itself may be reverse biased,
forward biased or unbiased
 Forward biased mode is not useful as a
photosensor
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Number of carrier in conducting mode is large
Number of carrier added by radiation small
Sensitivity is very low
Biasing of a diode
I-V charactersitsics of a diode
Photodiode - two modes
Two modes of operation as photodiode
 1. Photoconductive mode
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 Diode
is in reverse bias
 Operates similarly to a photoconductor
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2. Photovoltaic mode
 Diode
is not biased
 Operates as a source (solar cell for
example)
Photoconducting mode
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In dark mode there are very few carriers
flowing
 Photons release electrons from the valence
band either on the p on n side of the junction.
 These electrons and the resulting holes flow
towards the respective polarities (electrons
towards the positive pole, holes towards the
negative pole)
 A photocurrent, which in the absence of a
current in the diode constitute the only current
(a small leakage current exists - see
equivalent circuit).
Photoconductive mode additional effect
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The large inverse bias accelerates the
electrons
 Electrons can collide with other electrons and
release them across the band gap,
 This is called an avalanche effect
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it results in multiplication of the carriers available.
Sensors that operate in this mode are called
photomultiplier sensors
Photoconductive mode equivalent circuit
 It
is the total current in the load
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Due to photons plus other sources
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Thermal
Leakage
Capacitances, etc.
Photoconductive diode operation
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Current in reverse biased mode is:
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I0 is the leakage current,
Vd is the voltage across the junction,
k=8.62x10-5 eV/K (Boltzman’s const.)
T is the absolute temperature
Current due to photons is:
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Id = I0 e eV d/KT - 1
P is the radiation power density (W/m2)
f is frequency
 is called the quantum absorption
efficiency
A is the area of the diode exposed (PA =
power absorbed by the junction)
h is Planck’s constant
Ip =
PAe
hf
Photoconductive diode operation (cont.)
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Total external current is
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I0 is typically small (negligible)
10 nA or less
Neglecting I0, the total
external current is
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This current gives a direct
reading of the power absorbed
by the diode
It is not constant since the
relation depends on frequency
and the power absorbed itself is
frequency dependent.
Il = Id - Ip = I0 e eV d/KT - 1 -
Il 
PAe
hf
PAe
hf
Photoconductive diode operation (cont.)
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As the input power
increases the
characteristic curve
of the diode changes
as shown, resulting in
an increase in
reverse current
 This current
represents the
sensed quantity
Photodiode - construction
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Any diode can serve as a photodiode if:
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n region, p region or pn junction are exposed to
radiation
Usually exposure is through a transparent window
or a lens
Sometimes opaque materials are used (IR, UV)
Specific structures have been developed to
improve one or more of the characteristics
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The most important improvement is in the dark
current
Structures of planar
photodiodes
Photodiodes - construction
A - Oxide layer increases resistivity reduced dark current
 B. - PIN diode
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 Addition
of the intrinsic p layer increases
resistance
 Reduces dark current
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C. - pnn+ diode - a layer of conducting
n+ added
 Reduces resistance
 Improves response to
low wavelengths
Photodiodes - construction
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D - A combination of B and C
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Addition of the intrinsic p layer increases resistance
 Reduces dark current and improves low wavelength
response
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E. - Schotky diode (metal-semiconductor junction)
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Improved infrared (high wavelength) response
Metal layer (hold) must be transparent (very thin layer
F. - npp+ diode - as in B
Photodiodes - construction
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Available in various packages and for
various applications
 Individual
diodes in cans with lenses
 Surface mount diodes used in infrared
remote controls
 Arrays (linear) of various sizes for scanners
 Infrared and UV diodes for sensing and
control
Photodiodes
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Photodiode as used in
in a CD player
Photodiode array used
a scanner
Photovoltaic diodes
The diode is not biased
 Serves as a generator
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 Carriers
generated by radiation create a
potential difference across the junction
 Any photodiode can operate in this mode
 Solar cells are especially large-surface
photodiodes
Photovoltaic mode
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Equivalent circuit of photodiode in photovoltaic mode
Capacitance is usually large
Leakage current is small
Response of solar cells is slow due to very large
capacitance
Solar cells
The phototransistor
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Two junctions
 One forward, one reverse biased
The phototransistors
With the bias shown, the upper diode
(the collector-base junction) is reverse
biased while the lower (base-emitter)
junction is forward biased.
 In a regular transistor, a current IB
injected into the base is amplified by the
amplification factor of the transistor
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The phototransistor
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In a regular transistor:
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b = amplification
Ib = base current
Ic = collector current
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IC = b Ib
Emitter current:
IE = Ib b + 1
 In phototransistor, the base is
eliminated. A dark current
exists:
IC = I0 b,
IE = I0 b + 1
 I0
= leakage current
The phototransistor (cont.)
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When the junction is illuminated:
Collector current:
Emitter current:
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(leakage current is neglected)
Operation of the phototransistor is
identical to that of the photodiode
except for the amplification b
provided by the transistor structure.
IB = Ip =
PAe
IC = Ip b = b
hf
PAe
hf
PAe
IE = b + 1
hf
Phototransistor (cont.)
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b for even the simplest
transistors is of the
order of 100 (and can
be much higher),
Amplification is linear in
most of the operation
range
The phototransistor is a
very useful device and
commonly used for
detection and sensing
Phototransistor - general
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The high amplification allows phototransistors
to operate at low illumination levels
 They are typically much smaller than
photodiodes.
 Thermal noise can be a bigger problem.
 In many cases, a simple lens is also provided
to concentrate the light on the junction, which
for transistors is very small.
A typical phototransistor
Photoelectric sensors,
Photomultipliers
Based on the photoelectric effect
 Metal electrodes
 Evacuated tubes
 Some of the oldest optical sensors
 Uses:
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 Presence detection, counting, security
 Sensing very weak sources, night vision
(photomultipliers)
Photoelectric sensors
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Sometimes called photoelectric cells
Made of a photocathode, photoanode in an
evacuated tube
 Photocathode - made of a low work function
material (usually alkali coated)
 Electrons are accelerated towards the
photoanode
 Current through the device is a measure of
radiation intensity
The alkali column
The photoelectric sensor
“light” represents radiation
 The voltage is usually a few hundred volts
 The photoanode and photocathode are
usually shaped for best prformance

The photoelectric sensor
The number of emitted electrons per
photon is the quantum efficiency of the
sensor or Gain (or sensitivity) and
depends to a large extent on the
material used for the photocathode (its
work function)
 Photocathodes are made of the alkali
group and their alloys
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The photoelectric sensor
Photocathodes are made of the alkali group and
their alloys - cesium based materials are
most common:
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Low work function
 Spectral response from IR (1000nm) to UV
 Evacuated tube or argon filled (to increase
electron production)
 Older devices used metal cathodes, coated with
alkali compounds (Lithium, Potasium, Sodium or
Cesium or a combination of these)
Photoelectric sensors
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Typical gain about 10
 Newer photoelectric sensors:
 NEA (negative electron affinity) surfaces
 Constructed by evaporation of cesium or
cesium oxide onto a semiconductor’s
surface
 Operate the same as the older devices but
have lower work functions and require
lower anode voltages
Photomultipliers
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A development of photoelectric sensors
 The
output (number of electrons) is
multiplied by a large factor
 Has
a photocathode and a photoanode
 Additional intermediate cathodes, called
dynodes are added between the
photocathode and photoanode
Photomultiplier - principle
Photomultiplier - biasing
Photomultipliers - operation
Cathode and dynodes are made of low
work function materials such as
Beryllium-Copper (BeCu)
 Dynodes are at increasing potentials

 Creates
potential difference to previous
dynode
 Accelerates the electrons towards the next
dynode
Photomultipliers - operation
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Cathode:
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Each photon releases n electrons
Electrons are accelerated towards 1st dynode
Dynodes:
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Each incoming electrons releases n electrons
Electrons are then accelerated towards the next
dynode
Number of dynodes can be large (10 or more)
Photomultipliers - Gain

Multiplication:
 Given k dynodes:
 Each dynode releases
n
secondary electrons:
 Gain of the photomultiplier is:
Net effect: a very low light
intensity can generate a very
large current
 Gain can exceed 106.
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G = nk
Photomultipliers - Gain
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Current gain depends on:
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Construction:
Number of dynodes:
Inter-dynode voltages:
Additional considerations:
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electrons must be “forced” to transit between
electrodes at about the same time to avoid
distortions in the signal.
To do so, the dynodes are often shaped as curved
surfaces which also guides the electrons towards
the next dynode
Grids and slats are added – to decrease transit
time and improve quality of the signal, (for imaging
applications)
Photomultipliers - noise
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Noise:
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Noise is critical because of the multiplying effect
Dark current due to thermal emission is both
potential and temperature dependent
I0 = aAT 2 e -E0/kT
a is a constant depending on materials
A area of the emitting cathode
T absolute temperature
Photomultipliers - noise
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Other sources of noise:
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Shot noise: due to fluctuations of the current of
discrete electrons
multiplication noise due to the statistical spread of
electrons
Susceptibility to magnetic fields. Since magnetic
fields apply a force on moving electrons, they can
force electrons out of their normal paths reducing
their gain and more distorting the signal in
imaging applications.
Photomultipliers - applications

Used for very low light applications such as
in night vision systems.
 Photomultiplier sensor are placed at the focal
point of a telescope to view extremely faint
objects in space.
 Photomultipliers are part of a broader class
of devices called image intensifiers which
use various methods (including electrostatic
and magnetic lenses) to increase the current.
 Have been largely replaced by CCD devices
CCD sensors and detectors
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CCD - Coupled Charge Device
Very common in optical devices
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Cameras
Video cameras
Have many of the properties of
photomultipliers - but simpler, cheaper and
higher quality images
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Low voltage, low radiation intensity
Color images, semiconductor construction
Very small and fully integrable devices
CCD - structure
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Made of a conducting
substrate
A p or n type semiconductor
layer is deposited on top.
Above it a thin insulating
layer made of Silicon Oxide
A transparent conducting
layer above the SiO2 (gate):
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Allows penetration of photons
Can be set at a desired
potential with respect to the
substrate
This structure is called a
Metal Oxide Semiconductor
(MOS)
CCD - operation
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The gate and the substrate form a capacitor.
Gate is biased positively with respect to the
substrate. A depletion region in the
semiconductor makes this device a very high
resistance device.
 Optical radiation impinges on the device,
penetrates through the gate and oxide layer
to release electrons into the depletion layer
 Charge density is proportional to radiation
intensity. These are attracted to the gate but
cannot flow through the oxide layer and are
trapped there.
CCD operation (cont.)

To measure this
charge:
 Reverse bias the
MOS device to
discharge the
electrons through a
resistor
 The current through
the resistor is a
direct measure of
light intensity
CCD - method of sensing
charge
CCD - 2-D arrays
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Multiple rows in the two dimensional array.
A new image is obtained at the end of each scan.
Signal obtained is typically amplified and digitized
and used to produce the image
Image can then be displayed on a display array such
as a TV screen or a liquid crystal display.
There are many variation of this basic process:
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To sense color, filters may be used to separate colors into
their basic components (RGB – Red-Green-Blue).
Each color is sensed separately and forms part of the signal.
Thus, a color CCD will contain three cells per “pixel” each
reacting to one color.
CCD - applications

CCD devices are the core of most types of
electronic cameras and video recorders
 Also used in scanners (where linear arrays
are used).
 Used for very low light application by cooling
the CCDs to low temperatures.
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Sensitivity is much higher primarily due to reduced
thermal noise.
In this mode CCD have successfully displaced
photomultipliers.
A CCD array for a video camera
(500lines,625pixels,3colors)
Thermal-Based Optical
Sensors

Based on thermal effects of radiation

Most pronounced at lower frequencies (longer
wavelengths)
 Most useful in the infrared and microwave portions
of the spectrum.
 What is measured is the temperature associated
with radiation.
 A large variety of sensors exist
 In many cases, the only option available (such as
direct measurement of power at microwave and IR
frequencies)
Thermal-Based Optical
Sensors (cont.)
•
The sensors based on these principles
carry different names, some traditional,
some descriptive.




Early sensors were known as pyroelectric sensors (from
the greek pur for fire).
Bolometers are thermal radiation sensors, which are
essentially thermistors and the name refers mostly to its
application in microwave and mm wave measurements.
Others names like the PIR (Passive Infra Red) or AFIR
(Active Far Infra Red) are more descriptive but are
broader and encompass many types of sensors.
Almost any temperature sensor may be used to measure
radiation as long as a mechanism can be found to
transform radiation into heat.
Types of thermal radiation
sensors

Thermal radiation sensors are divided into
two classes – Passive Infrared (PIR) and
Active Infrared (AFIR) sensors.
 PIR: radiation is absorbed and converted to
heat.


Temperature rise is measured by a sensing
element to yield an indication of the radiative
power.
AFIR the device is heated from a power
source

Variations of this power due to radiation (for
example the current or voltage needed to keep the
temperature device constant) give an indication of
radiation.
PIR sensors - structure

PIR sensor has two basic components;
 An
absorption section that converts
radiation into heat
 A proper temperature sensor that converts
heat into an electrical signal.

Absorption section must be able to
 Absorb
as much of the incoming radiated
power at the sensor’s surface as possible
 Respond to changes in radiated power
density quickly.
PIR sensors - structure (cont.)
•
•
•
•
•
•
Absorber is made of a metal of good heat conductivity (gold
is a common choice in high quality sensors)
Often blackened to increase absorption.
Volume of the absorber is kept small to allow good response
(quick cooling) to changes in radiation
Absorber and the sensor will be encapsulated or placed in a
gas filled or evacuated hermetic chamber to avoid variations
in sensing signals due to air motion
A transparent (to infrared radiation) window typically made of
Silicon but other materials may be used (Germanium, Zinc
Selenide, etc.)
The choice of the sensor dictates to a large extent the
sensitivity, spectral response and physical construction of
the device.
Thermopile PIR sensor

In this device, sensing is done by a
thermopile.
 A thermopile is made of a number of
thermocouple connected in series electrically
but in parallel thermally (that is they are
exposed to identical thermal conditions).
 The thermopile generates a potential
proportional to radiation
 The thermopile is connected thermally to the
absorber but insulated electrically
Thermopile PIR sensor (cont.)
Any two materials can be used but
some material combinations produce
higher potential differences.
 Thermopiles can only measure
temperature differences hence the
thermopile is made of alternating cold
(reference) and hot (sensing) junctions

Thermopile PIR (cont.)

Structure of a thermopile PIR with reference
temperature sensor
Thermopile PIR (cont.)
All “cold” junctions are held at a known lower
temperature
 All “hot” junctions are held at the sensing
temperature.
 Cold junctions are placed on a relatively large
frame that has high thermal capacity and
hence the temperature will fluctuate slowly
 Hot junctions are in contact with the absorber
which is small and has low heat capacity

Thermopile PIR (cont.)

The frame may be cooled (or a heat
exchanger may be used), or a reference
sensor may be used on the frame so that the
temperature difference can be properly
monitored and related to the radiated power
density at the sensor.
 In most PIRs a crystalline or polycrystaline
silicon and aluminum are used:



Silicon has a very high thermoelectric coefficient
Is compatible with other components of the sensor
aluminum has a low coefficient and can be easily
deposited on silicon surfaces.
Pyroelectric sensors

Pyroelectric effect: an electric charge
generated in response to heat flow through
the body of a crystal (a passive sensor)
 Charge is proportional to the change in
temperature

Heat-flow sensors
 Pyroelectric sensors are best viewed as sensing
changes in radiation.
 Used mostly in motion sensing
Pyroelectric sensors (cont.)
Pyroelectricity was discovered in the 18th
century in Tourmaline crystals.
 By the end of the 19th century, pyroelectric
sensors were made of Rochelle salt.
 Currently there are many materials used:







Barium Titanate Oxide (BaTiO3)
Lead Titanite Oxide (PbTiO3)
PZT materials (PbZrO3).
PVF (polyvinyl fluoride)
PVDF (polyvinylidene fluoride) are also used.
Many others
Pyroelectic sensors - theory

When a pyroelectric material is
exposed to temperature change T,
a charge Q is generated as:
Q = PQAT


A is the area of the sensor
PQ is the pyroelectric charge coefficient
defined as:
dPs
P
=
 Ps is the spontaneous polarization of
Q
dT
the material (a property of the
material, related to its electric
permittivity)
Pyroelectic sensors - theory

A potential difference V is
developed across the sensor as:




h is the thickness of the crystal
PV its pyroelectric voltage
coefficient:
E the electric field across the sensor
The two coefficients, (voltage and
charge coefficients) are related as
follows:
V = PVhT
PV = dE
dT
PQ dPs
=
= 0 r
PV dE
Pyroelectic sensors - theory





By definition, the sensor’s
capacitance is:
Or:
Change in voltage is proportional
to the change in temperature.
Depends strongly on permittivity
Thin samples provide larger
change (larger capacitance)
C=
Q
= 0rA
V
h


0
V = PQ rT
h
Table of pyroelectric materials
Table 4.9 . Pyroelectric materials ans s ome of their properties .
Material
PQ [C/m2K]
TGS
3.5x10-
(single crystal)
LiTaO 3
2.0x10-
(single crystal)
BaTiO 3
4.0x10-
(Ceramic)
PZT
4.2x10-
(Ceramic)
PVDF
0.4x10-
(polymer)
PbTiO 3
2.3x10-
(polycrystalline)
TGS = TriGlycine Sulfate
PZT = Pb(Zr,Ti)O3
PV [V/mK]
r
1.3x106
30
Curie Temp.
[C]
49
0.5x106
45
618
0.05x106
1000
120
0.03x106
1600
340
0.4x106
12
205
0.13x106
200
470
Pyroelectric sensors structure

Consists of a thin crystal of a pyroelectric
material between two electrodes (like a
capacitor)
 Some sensors use a dual element


The second element can be used as a reference
by, for example, shielding it from radiation and is
often used to compensate for common mode
effects such as vibrations or very rapid thermal
changes which can cause false effects
The two elements are connected in series or in
parallel.
Pyroelectric sensor - structure
PIR motion detector
PIR motion detector - data






RE200B dual IR sensor
designed for motion detection.
includes a differential FET amplifier
operates at 3-10 V
field of view of 138º horizontally (wide dimension
of window) and 125º vertically.
optical bandwidth (sensitivity region) between 7
and 14 m (in the near infrared region).
Pyroelectric sensors application
Motion detection, especially of the
human body (sometimes of animals)
 The change in temperature of infrared
radiation (between 4 and 20 m)
causes a change in the voltage across
the sensor which then is used to
activate a switch or some other type of
indication

Pyroelectric sensors application



TGS and Lithium tantalite crystals are most often
used for these sensors
Ceramic materials and now the polymeric materials
are also very commonly used
Decay time: time needed for the charge on the
electrodes to diffuse.



Of the order os 1-2 seconds because of the very high
resistance of the materials
It also depends on the external connection of the device.
This response time is very important in the ability of the
sensors to detect slow motion
Bolometers

Simple radiation power sensor (RMS) over
the whole spectrum of electromagnetic
radiation
 Most commonly used in microwave and far
infrared ranges.
 Consist of any temperature measuring device
but usually of a small RTD or a thermistor.
 Usually very small in size to allow local
measurements
Bolometers (cont.)

The operation is as follows:





Radiation is absorbed by the device directly
This causes a change in its temperature.
This temperature rise is proportional to the
radiated power density at the location of sensing.
This change causes a change in the resistance of
the sensing element which is then related to the
power or power density at the location being
sensed.
Background temperature must be known or
compensated for by a separate measurement.
Bolometers - sensitivity

Sensitivity is given as:







ZTR0 T
 = (dR/dT)/R is the
b = s
2
TCR of the bolometer,
1 +  0 T 1 + (t)2
s its surface emissivity,
ZT is the thermal
resistance of the
bolometer,
For best results, thermal
R0 its resistance at the
impedance should be high
background temperature,
(well insulated sensor) and
 the frequency,
t the thermal time
its resistance should be
constant
high as well
T the rise in
temperature
Bolometers - construction
Bolometers are fabricated as very small
thermistors or RTDs,
 Usually as individual components or as
integrated devices.
 It is important to insulate the sensing
element from the structure supporting it
so that its thermal impedance is high.
 This can be done by simple suspension
of the sensor by this wires.

Bolometers - notes
Bolometers are some of the oldest
devices used for radiation sensing
 Are beeing used for many applications
in the microwave region including:

 Mapping
of antenna radiation patterns,
 Detection of infrared radiation,
 Testing of microwave devices and much
more.
Active Far Infrared (AFIR)
Sensors





Principle (simplistic):
A power source heats the sensing element to
a temperature above ambient
Temperature is kept constant
Additional heat is provided to the sensors
through radiation
Power necessary to keep the temperature
constant is a measure of radiated power
AFIR - theory



Temperature of the sensing
element is constant
AFIR sensor can be viewed as
being time independent.
Power supplied to the sensor:



P is power supplied by an
external source
PL is power lost through
conduction
F is radiation power sensed
P = PL + F
AFIR - theory





PL = s Ts - Ta
Power loss is:
s is a loss coefficient or
thermal conductivity (which
depends on materials and
construction),
Ts the sensor’s temperature
Ta the ambient temperature
Temperature of the radiating
source is:




 is emissivity (total)
s is electric conductivity
Ta is ambient temperature
A is area of the sensor
Tm =
4
2
Ts4 - 1 V - s Ts - Ta
As R
AFIR - application and notes

AFIR are rather complex
 Require
stable power supply and
temperature control circuitry
Much more sensitive than PIRs
 Used for low contrast radiation source
 Rarely used for motion detection
