Download Thermal detectors

9. Light detectors
1.
2.
3.
4.
5.
Classification and parameters of detectors
Photodiode
Phototransistor
Photoresistor
Thermal detectors
5.1. Thermopiles
5.2. Pyroelectric detectors
5.3. Bolometers
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Detectors classification
From the point of view of interaction of electromagnetic radiation with matter, the light
detectors can be divided into two major groups:
-
quantum
(interaction of photons
with electrons)
thermal
(variation of detectors parameters with
temperature due to absorption of radiation)
photovoltaic
photoconductive
photomagnetoelectric
photoemission
- thermoelectric
- bolometric
- pyroelectric
Quantum detectors operate from the ultraviolet (UV) to mid infrared (MIR) spectral
range. Thermal detectors are used generally in MIR and far infrared (FIR) spectral ranges,
where their efficiencies at room temp. are higher than these for quantum detectors.
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Detectors parameters
Sensitivity (responsivity)
It is a ratio of output quantity Y (r.m.s. value of current, voltage) vs. input quantity X
(r.m.s. value of radiation power)
S = Y/X
and accordingly one gets voltage sensitivity SV [V/W],
current sensitivity SI [A/W]
Sensitivity is a function of wavelength, depending on the detector type. For quantum
detectors S initially increases with a wavelength, reaches maximum and then drops to zero
at a certain wavelength λC. Sensitivities of quantum detectors are higher than these for
thermal detectors and response times are shorter. The advantage of thermal detectors are
lower costs and possiblity of work at
room temperature. Arrays of thermal
detectors are essential part of cheap
thermovision cameras.
Relative spectral sensitivity
of thermal and photonic (quantum)
detectors.
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Detectors parameters
Noise equivalent power (NEP)
This is the r.m.s. value of radiation power, which at detectors output gives a signal equal to
the noise level, normalised to the unit bandwidth.
NEP( f , ) 
Vn ( f )
( f )1 / 2 SV ( f , )
[ W / Hz 1 / 2 ]
Commonly one uses the reciprocal quantity – the specific detectivity.
Specific detectivity D*(f, λ) (D-star)
This is a ratio of signal to noise at unit radiation power, unit sensitive area and unit noise
bandwidth.
A1 / 2 SV ( Af )1 / 2 Vs ( Af )1 / 2
D ( f , ) 


NEP
Vn
Vn
Pe

[ cmHz1 / 2 / W ]
Multiplication by A1/2 comes from the observation that the ratio A1/2/NEP is independent of
the detectors sensitive area.
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Comparison of detectivities
Detectivities in the infrared region of manufactured different
detector types (FD - photodiode, FR – photoresistor, TR – thermal
detector).
5
Photodiode
A small photocell polarized in a reverse direction is a photodiode. After illumination of
the junction the photocarriers generated in a depletion region drift under the influence of
junction field enhanced by the external voltage. The carriers generated outside junction
within the diffusion length, diffuse in the direction of a junction increasing the current.
Band
structure
model of
a junction
Photodiode current consists of a dark
current depending on the voltage and
a photocurrent SΦ depending on
illumination
I  Is (exp[
qU
]  1)  S
k T
S – sensitivity (A/W)
Φ – illumination power (W)
for U< 0
I= - Is- SΦ  - SΦ
Ln, Lp – diffusion lengths, Wp, Wn – depletion region
dp, dn – thickness of p+/n region
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Photodiode characteristics
Current-voltage characteristics of a
photodiode before and after illumination
An equivalent circuit of a photodiode:
Ip – photoelectric current
Id – dark current
Rp – leakage (shunt) resistance
Rs – series resistance
Cp – junction capacitance of a photodiode
The electrical bandwidth is determined by Δf = 1/(2πRsCp) and may be
improved by reducing Cp with reverse-biased voltage.
For a p-i-n photodiode with small Cp, Δf is of order tens GHz.
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Structure and working circuits of a photodiode
Schottky photodiode
On the n- semiconductor (absorption layer) one deposits a
thin semitransparent metal layer (Au 10 – 30 nm) and an
antireflection coating increasing the transmittance especially
in the short-wavelength region. One gets the bandwidth of
order 100 GHz. The following semiconductors are often
used: Si, GaAs, GaP, GaAsP. The advantage of these
photodiodes is their relatively high blue and UV sensitivity.

The basic working circuit of a photodiode:
uout = RLIp = RLSΦ
UB
+
-
RL
uout
0< uout< UB then one obtains a much higher dynamics in
comparison to a photocell.
A circuit with a photocurrent gain:
for R1 = R2
uout = R1I = R1(Ip + Id) hence the dark current is also
amplified.
This configuration gives however a high bandwidth and
dynamics.
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Phototransistor
It is a light controlled bipolar transistor with internal
amplification. In most cases no external connection is made to
the base and the base current is supplied by the photogenerated
current. Reverse-biased base/collector junction plays the role of a
junction photodiode. The light is absobed in a base region.
Generated optically current is amplified due to the transistor
effect (as explained below).
Generated in a base region minority carriers, eg. electrons in a p-type
base, diffuse to the collector region increasing its current. Majority
carriers in a base region (holes) decrease the potential barrier of the
emitter/base junction, increasing at the same time the electron current
flowing from the emitter to the base. Electrons in a base are the
minority carriers and then are transferred to the collector increasing
greatly its current.
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Phototransistor
IB – optically generated current in a base
IE – emitter current
αIE – part of emitter current transferred to
collector (α<1)
α – common base current gain
After illumination of the base
region one obtains the current
(η – quantum efficieny):
Photocurrent amplification:
IB  

Pincq
h
IC
I
 C  e
q  Pinc / h I B / 
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Phototransistor
Phototransistors based on Si are readily available at low cost (figure below).
fg ~ a few hundred kHz
Poor frequency bandwidth arises from the
high capacitance of the base-collector
junction (τ  βRLC) and the long carrier
transit times across the base region.
However, the high internal gain can greatly simlify detection circuits
where the small bandwidth is not a problem (remote control in TV).
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Phototransistor
The frequency bandwidth can be increased by minimizing the phototransistor
dimensions.
Heterojunction n-p-n structure
of MESA type
InP with a higher energy gap plays the
role of a window layer and the light is
absorbed in a base region.
By attaching a lead to the base one removes the carriers from that region what
increases the cut-off frequency fg. One reduces at the same time the gain.
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Photoresistor
Photoresistor with interdigitated
electrodes
Thickness of the photosensitive layer should be:
• high enough to absorb the incident radiation
• low to decrease the dark conductance (low noise).
Thermal Johnson noise is generated by the dark current:
i y2 
4k BTf
R
Δf – bandwidth, R – resistance, iy – noise current
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Photoresistor
For photoresistors one can define a quantity being the number of electrons taking part in
conductivity, generated by one photon. This is the photoconductivity gain


t tr
ttr – transit time of electrons between electrodes
τ – carriers lifetime
The slow motion of carriers, the holes, will cause the draw of faster electrons, to maintain the
charge neutrality condition. This gives the gain of a photocurrent. With the help of mobility μ
one can write

V
L2
V- applied voltage, L – distance between electrodes
Inserting numerical data
τ = 3 ms, µ = 300 cm2/Vs, L = 0,4 mm, V = 2,4V
one obtains Γ = 450, what makes a photoresistor similar to a photomultiplier.
In this way it is beneficial to have small L and at the same time to maintain a high
photocurent, the large width of a sample. That is why one uses the interdigitated (comb-like)
structure of electrodes. The signal transmission rate is dependent on the element response
time bandwidth ~ 1 ,

then: bandwidth * gain = const
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Photoresistor
Dependence of optical absorption
coefficient α and photoconductivity σph
on the wavelength of absorbed light
Decrease of σph in the short wavelength region is
caused by absorption in a thin, low conducting
layer placed close to the surface.
In the long wavelength region photoconductivity
disappears due to the decrease of α. This is seen in
the variation of sensitivity S(λ) with wavelength.
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Photoresistor
Semiconductors used for photoresistors manufacturing
Eg(eV)
AlxGa1-xN
3,4 – 6,2 depending on x
CdS
2,4
CdSe
1,8
CdxZn1-xTe
dep. on x
Si
1,12
Ge
0,67
PbS
0,42
PbSe
0,23
InSb
0,23
Hg1-xCdxTe
dep. on x
Photoresistor of mercury cadmium telluride
working in IR range (2 – 30 μm).
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Thermal detectors
Thermal detectors are of low manufacturing costs in comparison to photon detectors.The
quality of these detectors was greatly improved after introduction of micromachining
technology. Recently the uncooled arrays of these detectors working in the IR region
(thermovision cameras) are commercially available.
Infrared radiation from a
scene segment, which is
focused on the pixel by a
lens, has one to one
correspondence with a pixel
on the IRFPA
Infrared imaging with focal plane array
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Thermal detectors
The change of detectors temperature can be calculated from the heat balance
equation (heat consumed and lost in a unit of time are equal to the absobed
power)
dT
C th
 G thT  
dt
ε – absorption coefficient
Cth – heat capacitance
Gth – heat conductivity
Φ – illumination power
If the incident light varies periodically with time as Φ=Φoexp[jωt], then above
equation for the stationary case has the solution:
T 
 o
G th 1  2 2th
where τth = Cth/Gth is a thermal time constant.
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Thermal detectors – cont.
One can also calculate ∆T from analogy to electrical equivalent circuit and with
the help of complex numbers calculus for ac excitation with frequency ω.
V0 
I0

G
I0
1
 2 C 2
2
R

I0
1
1  2 R 2 C 2
R
V0  T0
I 0    0 absorbed illu min ation power
Electrical equivalent circuit
for a thermal detector
From above analogy one obtains
for the thermal detector
R  R th thermal resis tan ce
C  C th thermal capaci tan ce
T0 
 o
1
1   2 R 2th C 2th
R th

 o
G th 1   2 th2
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Thermocouples
The voltage signal of a thermocouple is proportional to the temperature difference of the
junctions
V   T
Therefore voltage sensitivity of a thermocouple, taking into account the calculated ΔT for a
thermal detector, is equal
SV 
V
R th

o
1  2 2th
The sensitivity increases N times for a series connected N thermocouples, called a
thermopile. Taking into account that the basic kind of noise for thermocouples is a Johnson
noise, one can write expression for a detectivity as:
D*
SV Af
N A

Vn
G th 4kTR 1  2 2th
To obtain a high D* the junction must have small Cth (small time constant), high absorption
coef. ε and Seebeck coef. α, small heat conductivity Gth , small electrical resistance R.
Spectral sensitivity depends essentially on the transmission of encapsulation window.
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Thermopiles
A thermopile deposited on the
membrane.
Two pixels of an array with
thermopile structures.
SiN absorber, heating the
appropriate junction, covers
nearly the whole pixel.
Thermoelectrodes are made from
materials of very high figure of
merit.
Read-out circuitry is
manufactured in a silicon
substrate.
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Pyroelectric detectors
One uses the dependence of spontaneous polarization on temperature for the pyroelectric
material and also the dependence of elecrical permittivity on temperature (dielectric
bolometer).
Spontaneous polarization PS and pyroelectric
coefficient p as a function of temperature for a
pyroelectric material in vicinity of Curie
temperature TC .
Electrical permittivity as a function of
temperature in vicinity of TC .
If the temperature of a dielectric of ferroelectric capacitor changes by ΔT and then
polarization by ΔP, the surface charge will be induced with density ΔQ/A and in the
external circuit the following current will flow
I = dQ/dt = d(Ps A)/dt = pA d(ΔT)/dt
A – dielectric area
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Pyroelectric detectors
Knowing ΔT from the heat balance solution for periodically changing light intensity, one
can determine the current generated by the pyroelectric detector
I
pA o 
G th 1  2 2th
According to the equivalent circuit shown, the
voltage generated by the detector is equal
V
I
G 2  2C2
and accordingly the voltage sensitivity
SV 
V
pA

 o G G th 1  2 2th 1  2 e2
Pyroelectric detector (shown in a form of an
electrical equivalent circuit) at the input of
transimpedance amplifier.
τe = C/G – electrical time constant
Typical values of τth are of order 10 ms, τe can vary in a range 10-12 – 100 s.
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Pyroelectric detectors
Voltage sensitivity of a pyroelectric
detector as a function of frequency. Both
electrical and thermal time constants are
important.
In order to obtain small thermal time constant,
pyroelectric detectors are manufactured onto suspended
membranes .
Bottom electrode and a mirror form a resonant cavity
for IR radiation (thickness λ/4). By splitting upper
electrode two capacitors are formed, which connected
in series enable reducing of noise caused by mechanical
vibrations.
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Bolometers
Bolometer is a resistance element with a high TCR and a small heat capacity. Absorbed
infrared radiation changes the temperature and then the resistance of a bolometer.
TCR coefficient is defined as:
a
1 dR
R dT
Change of temperature by ΔT causes a change of bolometers resistance by ΔR what gives
the output signal
V  IB R  IB R a T
Use of high values of R and IB is limited by noise, which in this case are Johnson noise;
VJ  4k T R f
and 1/f noise:
I 2BR 2 n
Vf 
f
n – 1/f noise parameter
Additionally IB is also limited by the maximum working temperature of the sensor.
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Metallic and thermistor bolometers
Voltage sensitivity of a detector at the sinusoidal variation of incident radiation power can be
determined from the dependence as for a thermocouple substituting α by IB a R :
SV 
I Ba R  R th
1  2 2th
Bolometers in a form of thin metallic films (Ni, Bi, Sb) with
moderate TCR (typ. 0.3%/K) but high stability and low
noise are still manufactured.
Thermistor bolometers (sinterings of Co, Mn, Ni, V oxides)
have one order of magnitude higher TCR but also higher
current noise.
Bolometer in a bridge circuit
with compensation of ambient
temperature variations
To increase the detectivity one uses sometimes immersion
lenses as radiation concentrators. In this case the signal to
noise ratio increases n2 times due to n-th increase of a
detector area
(n- refractive index of a lens material, eg. Si, Ge).
Bolometer with immersion26lens
Semiconductor and micromachined bolometers
High detectivities are obtained by using semiconductor bolometers (Ge, Si) at cryogenic
temperatures. At very low temperatures the relative changes of semicondutor resistance are
higher and absorbing samples are thicker (lower specific heat) what increases absorption.
In a far infrared region these detectors have detectivities comparable to those of photonic
detectors, hence their applications in astronomy, spectroscopy etc.
Application of micromachining enabled
manufacturing of microbolometers with
a high thermal resistance (1x 108 K/W) ,
close to the theoretical limit caused by
radiation.
Bipolar CMOS input amplifiers are
used in readout circuits.
Double-level 50 x 50 µm2 bolometer manufactured in
micromachining technology (Honeywell 1992). Active
VOx layer is deposited onto Si3N4 plate supported by
two narrow legs over the silicon substrate with27
integrated readout circuit.
Superconducting bolometers
Very high variation of resistance for a small change in
temperature is obtained for superconductors in the vicinity
of critical temperature Tc.
The development of this technology was possible due to the
discovery of high temperature semiconductors, HTSC.
The typical HTSC material is YbBa2Cu3O7-x (YBaCuO)
with transition temp. ca 90 K.
Using this material and micromachining technology it was
possible to obtain bolometers with detectivities close to
Variation of resistance in the
vicinity of critical temperature Tc.
1010 cmHz1/2/W (better than for photonic detectors at 77 K).
Superconducting microbolometer in
micromachining technology
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