hgyguj
DESCRIPTION
5tfgygvkjn;;lk;lnghvbgc n juykjncfcfhkblkjnkbhgfhjbnbgbjhjkhjnnn,mnghbhvggggggggggggghbhbcghvhhgnbghghgfghghgjhbjhTRANSCRIPT
Chapter 4 - Optical sensors:Chapter 4 - Optical sensors:
Optical sensorsOptical sensors
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).
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” radiationSpectrum of “optical” radiation Nomenclature:
Visible light Infrared radiation (not infrared “light”) Ultraviolet radiation (not UV “light”)
Ranges shown are approximate and somewhat arbitrary
Nomenclature: Visible light Infrared radiation (not infrared “light”) Ultraviolet radiation (not UV “light”)
Ranges shown are approximate and somewhat arbitrary
Infrared radiationInfrared radiation
Approximate spectrum 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=109m 1GHz=109 Hz, 1THz=1015 Hz
Approximate spectrum 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=109m 1GHz=109 Hz, 1THz=1015 Hz
Visible lightVisible light
Approximate spectrum 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
Approximate spectrum 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) radiationUltraviolet (UV) radiation
Approximate spectrum 400nm (750 THz) to 400pm (300 PHz)
Meaning - above violetUnderstood as “penetrating” radiationOnly the lower end of the UV spectrum
is usually sensedExceptions: radiation sensors based on
ionization (chapter 9)
Approximate spectrum 400nm (750 THz) to 400pm (300 PHz)
Meaning - above violetUnderstood as “penetrating” radiationOnly the lower end of the UV spectrum
is usually sensedExceptions: radiation sensors based on
ionization (chapter 9)
A word on unitsA word on units
SI units include: meter, kg, second, ampere, candela, temperature kelvin and the mole
All other units are derived unitsCandela “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”
SI units include: meter, kg, second, ampere, candela, temperature kelvin and the mole
All other units are derived unitsCandela “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”
Units of luminosityUnits of luminosity
Units of illuminanceUnits of illuminance
MaterialsMaterials
Optical sensingOptical sensing
Based on two principles 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.
Based on two principles 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.
The photoelectric effectThe photoelectric effect
Planck’s equation:e=hf [ev]
h = 6.6262x10 [joule.second] (Planck’s constant)
f = frequencye = energy of a photon at radiation frequency f.
This is called the quantum of energy 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
Planck’s equation:e=hf [ev]
h = 6.6262x10 [joule.second] (Planck’s constant)
f = frequencye = energy of a photon at radiation frequency f.
This is called the quantum of energy 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 effectThe photoelectric effect
Photons collide with electrons at the surface of a material
The electrons acquire energy and this energy allows the electron to: Release themtselves from the surface of
the material by overcoming the work function of the substance.
Excess energy imparts the electrons kinetic energy.
Photons collide with electrons at the surface of a material
The electrons acquire energy and this energy allows the electron to: 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 effectThe photoelectric effect
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”
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 effectThe photoelectric effect
For electrons to be released, the photon energy must be higher than the work function of the material. 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 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.
For electrons to be released, the photon energy must be higher than the work function of the material. 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 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 tableWork function tableTable 4.1. Work functions for selected materials given in [eV]Material Work FunctionAluminum 3.38Bismuth 4.17Cadmium 4.0Cobalt 4.21Copper 4.46Germanium 4.5Gold 4.46Iron 4.4Nickel 4.96Platinum 5.56Potasium 1.6Silicon 4.2Silver 4.44Tungsten 4.38Zinc 3.78
Table 4.1. Work functions for selected materials given in [eV]Material Work FunctionAluminum 3.38Bismuth 4.17Cadmium 4.0Cobalt 4.21Copper 4.46Germanium 4.5Gold 4.46Iron 4.4Nickel 4.96Platinum 5.56Potasium 1.6Silicon 4.2Silver 4.44Tungsten 4.38Zinc 3.78
Some notes:Some notes:
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
Electrons may be emitted by thermionic emission - a totally different issue based on thermal effect
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
Electrons may be emitted by thermionic emission - a totally different issue based on thermal effect
The photoconducting effectThe photoconducting effect
A solid state (volume) effectMost notable in semiconductorsBased on displacement of valence
and/or covalence electronsValence electrons: bound to individual
atoms in outer layersCovalence electrons: bound but shared
between neighboring atoms in the crystal
A solid state (volume) effectMost notable in semiconductorsBased on displacement of valence
and/or covalence electronsValence electrons: bound to individual
atoms in outer layersCovalence electrons: bound but shared
between neighboring atoms in the crystal
Model: photoconducting effectModel: photoconducting effect
Photons collide with electrons Electrons must acquire sufficient energy to:
Leave the valence band Move into the conduction band Minimum energy required: band gap energy
Photons collide with electrons Electrons must acquire sufficient energy to:
Leave the valence band Move into the conduction band Minimum energy required: band gap energy
Model: photoconducting effectModel: photoconducting effect
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
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 effectModel: photoconducting effect
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] - conductivity of the medium
Conductivity is temperature dependent (mobility and concentrations are temperature dependent)
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] - conductivity of the medium
Conductivity is temperature dependent (mobility and concentrations are temperature dependent)
= e en + pp
photoconducting effectphotoconducting effect 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
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 effectPhotoconducting effect
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).
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 effectPhotoconducting effect
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 (p,n - lifetimes, f - # of carriers generated per second per volume
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 (p,n - lifetimes, f - # of carriers generated per second per volume
= e en + pp
= ef nn + pp
Photoconducting effectPhotoconducting 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)
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)
Photoconducting effect - sensitivity
Photoconducting effect - sensitivity
Sensitivity to radiation (efficiency) 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 materials with high carrier lifetimes keep the length of the photoresistor small the latter is typically achieved through the
meander construction shown below
Sensitivity to radiation (efficiency) 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 materials with high carrier lifetimes keep the length of the photoresistor small the latter is typically achieved through the
meander construction shown below
G = VL 2
nn + ppG = VL 2
nn + pp
Photoconductor - structurePhotoconductor - structure
photoconducting effectphotoconducting effect
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)
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
Photoconductive properties of semiconductors
Table 4.2. Band gap energies, longest wavelength and working temperatures forselected semiconductorsMaterial Band gap [eV] Longest wavelength
max [m]Working temperature[K]
ZnS 3.6 0.35 300CdS 2.41 0.52 300CdSe 1.8 0.69 300CdTe 1.5 0.83 300Si 1.2 1.2 300Ge 0.67 1.8 300PbS 0.37 3.35InAs 0.35 3.5 77PbTe 0.3 4.13PbSe 0.27 4.58InSb 0.18 6.5 77Ge:Cu 30 18Hg/CdTe 8-14 77Pb/SnTe 8-14 77InP 1.35 0.95 300GaP 2.26 0.55 300Note: properties of semiconductors vary with doping and other impurities. The values shownshould be viewed as representative only.
Table 4.2. Band gap energies, longest wavelength and working temperatures forselected semiconductorsMaterial Band gap [eV] Longest wavelength
max [m]Working temperature[K]
ZnS 3.6 0.35 300CdS 2.41 0.52 300CdSe 1.8 0.69 300CdTe 1.5 0.83 300Si 1.2 1.2 300Ge 0.67 1.8 300PbS 0.37 3.35InAs 0.35 3.5 77PbTe 0.3 4.13PbSe 0.27 4.58InSb 0.18 6.5 77Ge:Cu 30 18Hg/CdTe 8-14 77Pb/SnTe 8-14 77InP 1.35 0.95 300GaP 2.26 0.55 300Note: properties of semiconductors vary with doping and other impurities. The values shownshould be viewed as representative only.
Photoconductive propertiesPhotoconductive properties Example: InSb (Indium Antimony):
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
(300K) (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.
Example: InSb (Indium Antimony): 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
(300K) (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.
SemiconductorsSemiconductors
Various photoconductors (photoresistors)
Various photoconductors (photoresistors)
PhotodiodesPhotodiodes
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 Number of carrier in conducting mode is large Number of carrier added by radiation small Sensitivity is very low
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 Number of carrier in conducting mode is large Number of carrier added by radiation small Sensitivity is very low
Biasing of a diodeBiasing of a diode
I-V charactersitsics of a diodeI-V charactersitsics of a diode
Photodiode - two modesPhotodiode - two modes
Two modes of operation as photodiode1. Photoconductive mode
Diode is in reverse bias Operates similarly to a photoconductor
2. Photovoltaic mode Diode is not biased Operates as a source (solar cell for
example)
Two modes of operation as photodiode1. Photoconductive mode
Diode is in reverse bias Operates similarly to a photoconductor
2. Photovoltaic mode Diode is not biased Operates as a source (solar cell for
example)
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).
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).
Photoconducting modePhotoconducting mode
Photoconductive mode - additional effect
Photoconductive mode - additional effect
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 it results in multiplication of the carriers available. Sensors that operate in this mode are called
photomultiplier sensors
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 it results in multiplication of the carriers available. Sensors that operate in this mode are called
photomultiplier sensors
Photoconductive mode - equivalent circuit
Photoconductive mode - equivalent circuit
It is the total current in the load Due to photons plus other sources
ThermalLeakageCapacitances, etc.
It is the total current in the load Due to photons plus other sources
ThermalLeakageCapacitances, etc.
Photoconductive diode - operation
Photoconductive diode - operation
Current in reverse biased mode is: 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: 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
Current in reverse biased mode is: 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: 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
Id = I0 eeVd/KT 1
Ip = PAe
hf
Photoconductive diode - operation (cont.)
Photoconductive diode - operation (cont.)
Total external current is I0 is typically small (negligible) 10 nA or less
Neglecting I0, the total external current is 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.
Total external current is I0 is typically small (negligible) 10 nA or less
Neglecting I0, the total external current is 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 eeVd/KT 1 PAe
hfIl = Id Ip = I0 eeVd/KT 1
PAehf
Il PAe
hf
Photoconductive diode - operation (cont.)
Photoconductive diode - operation (cont.)
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
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 - constructionPhotodiode - construction
Any diode can serve as a photodiode if: 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 The most important improvement is in the dark
current
Any diode can serve as a photodiode if: 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 The most important improvement is in the dark
current
Structures of planar photodiodes
Structures of planar photodiodes
Photodiodes - constructionPhotodiodes - construction
A - Oxide layer increases resistivity - reduced dark current
B. - PIN diodeAddition of the intrinsic p layer increases
resistanceReduces dark current
C. - pnn+ diode - a layer of conducting n+ added Reduces resistance Improves response to low wavelengths
A - Oxide layer increases resistivity - reduced dark current
B. - PIN diodeAddition of the intrinsic p layer increases
resistanceReduces dark current
C. - pnn+ diode - a layer of conducting n+ added Reduces resistance Improves response to low wavelengths
Photodiodes - constructionPhotodiodes - construction
D - A combination of B and C Addition of the intrinsic p layer increases resistance Reduces dark current and improves low wavelength
response
E. - Schotky diode (metal-semiconductor junction) Improved infrared (high wavelength) response Metal layer (hold) must be transparent (very thin layer
F. - npp+ diode - as in B
D - A combination of B and C Addition of the intrinsic p layer increases resistance Reduces dark current and improves low wavelength
response
E. - Schotky diode (metal-semiconductor junction) Improved infrared (high wavelength) response Metal layer (hold) must be transparent (very thin layer
F. - npp+ diode - as in B
Photodiodes - constructionPhotodiodes - construction
Available in various packages and for various applications
Individual diodes in cans with lensesSurface mount diodes used in infrared
remote controlsArrays (linear) of various sizes for scanners Infrared and UV diodes for sensing and
control
Available in various packages and for various applications
Individual diodes in cans with lensesSurface mount diodes used in infrared
remote controlsArrays (linear) of various sizes for scanners Infrared and UV diodes for sensing and
control
Photodiodes Photodiodes
Photodiode as used in Photodiode array used in a CD player a scanner
Photodiode as used in Photodiode array used in a CD player a scanner
Photovoltaic diodesPhotovoltaic diodes
The diode is not biasedServes as a generator
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
The diode is not biasedServes as a generator
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 modePhotovoltaic mode
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
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 cellsSolar cells
The phototransistorThe phototransistor
Two junctions One forward, one reverse biased
Two junctions One forward, one reverse biased
The phototransistors 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
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
The phototransistorThe phototransistor
In a regular transistor: = amplification Ib = base current Ic = collector current
Emitter current: In phototransistor, the base is
eliminated. A dark current exists: I0 = leakage current
In a regular transistor: = amplification Ib = base current Ic = collector current
Emitter current: In phototransistor, the base is
eliminated. A dark current exists: I0 = leakage current
IC = Ib
IE = Ib + 1
IC = I0, IE = I0 + 1
The phototransistor (cont.)The phototransistor (cont.)
When the junction is illuminated: Collector current: Emitter current:
(leakage current is neglected) Operation of the phototransistor is
identical to that of the photodiode except for the amplification provided by the transistor structure.
When the junction is illuminated: Collector current: Emitter current:
(leakage current is neglected) Operation of the phototransistor is
identical to that of the photodiode except for the amplification provided by the transistor structure.
IB = Ip = PAe
hfIC = Ip =
PAehf
IE = + 1PAe
hf
Phototransistor (cont.)Phototransistor (cont.)
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
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 - generalPhototransistor - general
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.
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 phototransistorA typical phototransistor
Photoelectric sensors, Photomultipliers
Photoelectric sensors, Photomultipliers
Based on the photoelectric effectMetal electrodesEvacuated tubesSome of the oldest optical sensorsUses:
Presence detection, counting, security Sensing very weak sources, night vision
(photomultipliers)
Based on the photoelectric effectMetal electrodesEvacuated tubesSome of the oldest optical sensorsUses:
Presence detection, counting, security Sensing very weak sources, night vision
(photomultipliers)
Photoelectric sensorsPhotoelectric sensors
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
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 columnThe alkali column
The photoelectric sensorThe photoelectric sensor
“light” represents radiation The voltage is usually a few hundred volts The photoanode and photocathode are
usually shaped for best prformance
“light” represents radiation The voltage is usually a few hundred volts The photoanode and photocathode are
usually shaped for best prformance
The photoelectric sensorThe 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
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
The photoelectric sensorThe photoelectric sensor
Photocathodes are made of the alkali group and their alloys - cesium based materials are most common: 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)
Photocathodes are made of the alkali group and their alloys - cesium based materials are most common: 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 sensorsPhotoelectric sensors
Typical gain about 10Newer 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
Typical gain about 10Newer 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
PhotomultipliersPhotomultipliers
A development of photoelectric sensorsThe 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
A development of photoelectric sensorsThe 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 - principlePhotomultiplier - principle
Photomultiplier - biasingPhotomultiplier - biasing
Photomultipliers - operationPhotomultipliers - 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
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 - operationPhotomultipliers - operation
Cathode: Each photon releases n electrons Electrons are accelerated towards 1st dynode
Dynodes: Each incoming electrons releases n electrons Electrons are then accelerated towards the next
dynode Number of dynodes can be large (10 or more)
Cathode: Each photon releases n electrons Electrons are accelerated towards 1st dynode
Dynodes: Each incoming electrons releases n electrons Electrons are then accelerated towards the next
dynode Number of dynodes can be large (10 or more)
Photomultipliers - GainPhotomultipliers - 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.
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.
G = nk
Photomultipliers - GainPhotomultipliers - Gain
Current gain depends on: Construction: Number of dynodes: Inter-dynode voltages:
Additional considerations: 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)
Current gain depends on: Construction: Number of dynodes: Inter-dynode voltages:
Additional considerations: 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 - noisePhotomultipliers - noise
Noise: Noise is critical because of the multiplying effect Dark current due to thermal emission is both
potential and temperature dependent
Noise: Noise is critical because of the multiplying effect Dark current due to thermal emission is both
potential and temperature dependent
a is a constant depending on materialsA area of the emitting cathode T absolute temperature
I0 = aAT2e E0/kT
Photomultipliers - noisePhotomultipliers - noise
Other sources of noise: 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.
Other sources of noise: 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 - applicationsPhotomultipliers - 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
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 detectorsCCD sensors and detectors
CCD - Coupled Charge Device Very common in optical devices
Cameras Video cameras
Have many of the properties of photomultipliers - but simpler, cheaper and higher quality images Low voltage, low radiation intensity Color images, semiconductor construction Very small and fully integrable devices
CCD - Coupled Charge Device Very common in optical devices
Cameras Video cameras
Have many of the properties of photomultipliers - but simpler, cheaper and higher quality images Low voltage, low radiation intensity Color images, semiconductor construction Very small and fully integrable devices
CCD - structureCCD - structure 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): 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)
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): 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 - operationCCD - operation
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.
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.)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
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 - method of sensing charge
CCD - 2-D arraysCCD - 2-D arrays 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:
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.
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:
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 - applicationsCCD - 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. Sensitivity is much higher primarily due to reduced
thermal noise. In this mode CCD have successfully displaced
photomultipliers.
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. 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)
A CCD array for a video camera (500lines,625pixels,3colors)
Thermal-Based Optical Sensors
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)
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.)
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.
• 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
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.
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 - structurePIR sensors - structurePIR 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 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.)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.
• 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 sensorThermopile 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
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.)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
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.)Thermopile PIR (cont.)
Structure of a thermopile PIR with reference temperature sensor
Structure of a thermopile PIR with reference temperature sensor
Thermopile PIR (cont.)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
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.)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.
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 sensorsPyroelectric 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 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.)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
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 - theoryPyroelectic sensors - theory
When a pyroelectric material is exposed to temperature change T, a charge Q is generated as: A is the area of the sensor PQ is the pyroelectric charge coefficient
defined as: Ps is the spontaneous polarization of
the material (a property of the material, related to its electric permittivity)
When a pyroelectric material is exposed to temperature change T, a charge Q is generated as: A is the area of the sensor PQ is the pyroelectric charge coefficient
defined as: Ps is the spontaneous polarization of
the material (a property of the material, related to its electric permittivity)
Q = PQATQ = PQAT
PQ = dPsdT
Pyroelectic sensors - theoryPyroelectic sensors - theory
V = PVhTV = PVhT
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:
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:
PV = dEdT
PQ
PV = dPs
dE = 0r
Pyroelectic sensors - theoryPyroelectic 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)
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 = QV
= 0rAh
C = QV
= 0rAh
V = PQ0rh
T
Table of pyroelectric materialsTable of pyroelectric materialsTable 4.9 . Pyroelectric materials ans some of their properties.
Material PQ [C/m2K] PV [V/mK] r Curie Temp.[ C]
TGS(single crystal)
3.5x10 1.3x106 30 49
LiTaO 3
(single crystal)2.0x10 0.5x106 45 618
BaTiO 3
(Ceramic)4.0x10 0.05x106 1000 120
PZT(Ceramic)
4.2x10 0.03x106 1600 340
PVDF(polymer)
0.4x10 0.4x106 12 205
PbTiO 3
(polycrystalline)2.3x10 0.13x106 200 470
TGS = TriGlycine SulfatePZT = Pb(Zr,Ti)O3
Table 4.9 . Pyroelectric materials ans some of their properties.
Material PQ [C/m2K] PV [V/mK] r Curie Temp.[ C]
TGS(single crystal)
3.5x10 1.3x106 30 49
LiTaO 3
(single crystal)2.0x10 0.5x106 45 618
BaTiO 3
(Ceramic)4.0x10 0.05x106 1000 120
PZT(Ceramic)
4.2x10 0.03x106 1600 340
PVDF(polymer)
0.4x10 0.4x106 12 205
PbTiO 3
(polycrystalline)2.3x10 0.13x106 200 470
TGS = TriGlycine SulfatePZT = Pb(Zr,Ti)O3
Pyroelectric sensors - structure
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.
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 - structurePyroelectric sensor - structure
PIR motion detectorPIR motion detector
PIR motion detector - dataPIR 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).
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
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
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
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
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 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
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.) 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.
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 - sensitivityBolometers - sensitivity Sensitivity is given as:
= (dR/dT)/R is the TCR of the bolometer,
s its surface emissivity, ZT is the thermal
resistance of the bolometer,
R0 its resistance at the background temperature,
the frequency, the thermal time
constant T the rise in
temperature
Sensitivity is given as: = (dR/dT)/R is the
TCR of the bolometer, s its surface emissivity, ZT is the thermal
resistance of the bolometer,
R0 its resistance at the background temperature,
the frequency, the thermal time
constant T the rise in
temperature
= s2
ZTR0T
1 + 0T 1 + ()2 = s
2ZTR0T
1 + 0T 1 + ()2
For best results, thermal impedance should be high (well insulated sensor) and its resistance should be high as well
Bolometers - constructionBolometers - 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 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 - notesBolometers - 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.
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
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
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 - theoryAFIR - 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 is radiation power sensed
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 is radiation power sensed
P = PL + P = PL +
AFIR - theoryAFIR - theory 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) is electric conductivity Ta is ambient temperature A is area of the sensor
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) is electric conductivity Ta is ambient temperature A is area of the sensor
PL = s Ts TaPL = s Ts Ta
Tm = Ts4 1
AV2
R s Ts Ta
4
AFIR - application and notesAFIR - application and notes
AFIR are rather complex Require stable power supply and
temperature control circuitryMuch more sensitive than PIRsUsed for low contrast radiation sourceRarely used for motion detection
AFIR are rather complex Require stable power supply and
temperature control circuitryMuch more sensitive than PIRsUsed for low contrast radiation sourceRarely used for motion detection