This document summarizes principles and applications of infrared photodetectors. It discusses the history and development of IR detectors from the 1800s to present. There are two main types of IR detectors - photon detectors and thermal detectors. Photon detectors respond to infrared photons and require cryogenic cooling, while thermal detectors respond to changes in temperature. The document focuses on mercury cadmium telluride (HgCdTe) short-wave infrared sensors, which can be tuned to detect different infrared wavelengths depending on their composition. HgCdTe detectors are widely used due to their high electron mobility and ability to absorb infrared radiation.

1. Principles and Applications of Infrared Photodetectors
Yuxin Wang, Ye Wang, Emeka Ikpeazu
Department of Electrical and Computer Engineering
ECE 5501—Photonics
Professor Mool Gupta
University of Virginia

2. Abstract:
The paper presents progress in infrared (IR) detector technologies during their
development. Basic infrared radiation theory and classification of two types of IR detectors
(photon detectors and thermal detectors) are done. The overview of working principle and
physics of IR detectors are presented. Also recent application in IR technologies is described.
Discussion is focused mainly on one of the most rapidly developing detectors: HgCdTe short-
wave-infrared sensor.
1. Introduction to Infrared Detectors
1.1 History of the development of IR detectors
Looking back over the past 1000 years we notice that infrared (IR) radiation itself was
unknown until 200 years ago when Sir Frederick William Herschel’s experiment with
thermometer was first reported. He built a crude monochromator that used a
thermometer as a detector so that he could measure the distribution of energy in
sunlight. In April 1800 he wrote [1]:
“Thermometer No. 1 rose 7 degrees in 10 minutes by an exposure to the full red
coloured rays. I drew back the stand... thermometerNo. 1 rose, in 16 minutes, 8 degrees
when its centre was 1/2 inch out of the visible rays.”
The early history of IR was reviewed about 40 years ago in two well-known
monographs [2,3]. The most important steps in development of IR detectors are the
following:
• In 1829 Leopoldo Nobili constructed the first thermopile by connecting a number of
thermocouples in series.
• In 1921 Thomas Johann Seebeck discovered the thermoelectric effect and soon
thereafter demonstrated the first thermocouple.
• In 1933 Macedonio Melloni modified thermocouple design and used bismuth and
antimony for it. Samuel Langley’s bolometer appeared in 1880.
Langley used two thin ribbons of platinum foil, connected so as to form two
arms of a Wheatstone bridge. Langley continued to develop his bolometer for the next
twenty years (400 times more sensitive than that of his first efforts). His latest
bolometer could detect a cow’s heat from a distance of a quarter of mile. Thus, the
initial development of IR detectors was connected with that of thermal detectors.
Photon detectors were developed in the 20th century. The first IR
photoconductor was developed by T.W. Case in 1917 [4]. In 1933 E.W. Kutzscher at
University of Berlin discovered that lead sulfide (from natural galena found in Sardinia)
was photoconductive and had response of about 3 lm.
Many materials have been investigated in the IR field. Observing a history of
the development of the IR detector technology, a simple theorem named after E.L.
Norton [5], can be stated: All physical phenomena in the range of about 0.1–1 eV can
be proposed for IR detectors. Among these effects are: thermoelectric power
(thermocouples), change in electrical conductivity (bolometers), gas expansion (Golay
cell), pyroelectricity (pyroelectric detectors), photon drag, Josephson effect
(Josephson junctions, SQUIDs), internal emission (PtSi Schottky barriers),
fundamental absorption (intrinsic photodetectors), impurity absorption (extrinsic
photodetectors), low-dimensional solids (superlattice (SL) and quantum well (QW)
detectors), different type of phase transitions, etc.

3. Fig. 1.1 gives approximate dates of significant development efforts for the materials
mentioned. The years during World War II saw the origins of modern IR detector
technology. Interest has centered mainly on the wavelengths of the two atmospheric
windows 3–5 lm and 8–14 lm, though in recent years there has been increasing interest
in longer wavelengths stimulated by space applications [6].
Figure 1.1. History of the development of IR detectors
1.2 Infrared Radiation Theory
Infrared radiation has the following characteristics:
•Invisible to human eyes
•Small energy
•Long wavelength
•Emitted from all kinds of objects
In the electromagnetic spectrum, infrared radiation can be found between the visible
and microwave regions. The infrared waves typically have wavelengths between 0.7
and 1000 m.
The wavelength region which ranges from 0.7 to 3 m is known as the near infrared
regions. The region between 3 and 6 m is known as the mid-infrared and infrared
radiation which has a wavelength greater higher than 6 m is known as far infrared.
Infrared technology finds applications in many everyday products. Televisions use an
infrared detector to interpret the signals sent from a remote control. The key benefits
of infrared sensors include their low power requirements, their simple circuitry and
their portable features.

4. Table 1.1 Definition and relationship to the electromagnetic spectrum
Name Wavelength Frequency (Hz) Photon Energy (eV)
Gamma ray less than 0.01 nm more than 30 EHz 124 keV – 300+ GeV
X-ray 0.01 nm – 10 nm 30 EHz – 30 PHz 124 eV – 124 keV
Ultraviolet 10 nm – 400 nm 30 PHz – 790 THz 3.3 eV – 124 eV
Visible 400 nm–700 nm 790 THz – 430 THz 1.7 eV – 3.3 eV
Infrared 700 nm – 1 mm 430 THz – 300 GHz 1.24 meV – 1.7 eV
Microwave 1 mm – 1 meter 300 GHz – 300 MHz 1.24 µeV – 1.24 meV
Radio 1 meter – 100,000 km 300 MHz – 3 Hz 12.4 feV – 1.24 meV
1.3 Classification of IR detectors
1.3.1 Photon detector
In photon detectors the radiation is absorbed within the material by interaction with
electrons either bound to lattice atoms or to impurity atoms or with free electrons.
The observed electrical output signal results from the changed electronic energy
distribution. The photon detectors show a selective wavelength dependence of
response per unit incident radiation power (see Fig. 8). They exhibit both a good
signal−to−noise performance and a very fast response. But to achieve this, the photon
IR detectors require cryogenic cooling. This is necessary to prevent the thermal
generation of charge carriers. The thermal transitions compete with the optical ones,
making uncooled devices very noisy.
The spectral current responsivity of photon detectors is equal to:
where  is the wavelength, h is the Planck’s constant, c is the velocity of light, q is the
electron charge, and g is the photoelectric current gain. The current that flows through
the contacts of the device is noisy due to the statistical nature of the generation and
recombination processes, the fluctuation of both optical and thermal generation, and
the radiative and nonradiative recombination rates. Assuming that the current gain for
the photocurrent and the noise current are the same, the noise current is:
where Gop is the optical generation rate, Gth is the thermal generation rate, R is the
resulting recombination rate, and f is the frequency band.

5. It was found by F.E. Jones [2], that for many detectors the noise equivalent power
(NEP) is proportional to the square root of the detector signal that is proportional to
the detector area, Ad. The normalized detectivity D* suggested by Jones is defined as:
Detectivity, D*, is the main parameter to characterize normalized signal−to−noise
performance of detectors and can be also defined as:
The importance of D* is that this figure of merit permits comparison of detectors of
the same type, but having different areas. Either a spectral or blackbody D* can be
defined in terms of corresponding type of NEP.
At equilibrium, the generation and recombination rates are equal. In this case:
Background radiation frequently is the main source of noise in a IR detector.
Assuming no contribution due to recombination,
where 𝛷𝐵 is the background photon flux density. Therefore, at the background limited
performance conditions (BLIP conditions), once BLIP is reached, quantum efficiency
is the only detector parameter that can influence a detector’s performance.
Figure 1.2. Comparison of the D* of various available detectors when operated at the
indicated temperature. Chopping frequency is 1000 Hz for all detectors except the

6. thermopile (10 Hz), thermocouple (10 Hz), thermistor bolometer (10 Hz), Golay cell
(10 Hz) and pyroelectric detector (10 Hz). Each detector is assumed to view a
hemispherical surrounding at a temperature of 300 K. Theoretical curves for the
back− ground−limited D* (dashed lines) for ideal photovoltaic and photoconductive
detectors and thermal detectors are also shown. PC—photoconductive detector, PV—
photovoltaic detector, PEM—photoelectromagnetic detector, and HEB—hot electron
bolometer.
Depending on the nature of the interaction, the class of photon detectors is further
subdivided into different types. The most important are: intrinsic detectors, extrinsic
detectors, photoemissive (Schottky barriers). Different types of detectors are
described in details in monograph Infrared Detectors [9]. Figure 1.2 shows spectral
detectivity curves for a number of commercially available IR detectors.
1.3.2 Thermal detector
The second class of detectors is composed of thermal detectors. In a thermal detector
shown schematically in Fig 1.3, the incident radiation is absorbed to change the
material temperature and the resultant change in some physical property is used to
generate an electrical output. The detector is suspended on legs which are connected
to the heat sink. The signal does not depend upon the photonic nature of the incident
radiation. Thus, thermal effects are generally wavelength-independent (see Fig. 1.3);
the signal depends upon the radiant power (or its rate of change) but not upon its
spectral content.
Fig 1.3. Schematic diagram of thermal detector.
Since the radiation can be absorbed in a black surface coating, the spectral
response can be very broad. Attention is directed toward three approaches which have
found the greatest utility in infrared technology, namely, bolometers, pyroelectric and
thermoelectric effects. The thermopile is one of the oldest IR detectors, and is a
collection of thermocouples connected in series in order to achieve better temperature
sensitivity. In pyroelectric detectors a change in the internal electrical polarization is
measured, whereas in the case of thermistor bolometers a change in the electrical
resistance is measured. For a long time, thermal detectors were slow, insensitive,
bulky and costly devices. But with developments of the semiconductor technology,
they can be optimized for specific applications. Recently, thanks to conventional

7. CMOS processes and development of MEMS, the detector’s on-chip circuitry
technology has opened the door to a mass production.
Up until the nineties, thermal detectors have been considerably less exploited
in commercial and military systems in comparison with photon detectors. The reason
for this disparity is that thermal detectors are popularly believed to be rather slow and
insensitive in comparison with photon detectors. As a result, the worldwide effort to
develop thermal detectors was extremely small relative to that of photon detector. In
the last decade however, it has been shown that very high-quality imagery can be
obtained from large thermal detector arrays operating uncooled at TV frame rates.
The speed of thermal detectors is quite adequate for non-scanned imagers with two-
dimensional (2D) detectors. The moderate sensitivity of thermal detectors can be
compensated by a large number of elements in 2D electronically scanned arrays. With
large arrays of thermal detectors, the best values of NEDT below 0.1 K, could be
reached because effective noise bandwidths less than 100 Hz can be achieved.
Uncooled monolithic focal plane arrays (FPAs) fabricated from thermal
detectors may revolutionize development of thermal imagers. Recently, very
encouraging results have been obtained with micromachined silicon bolometers [2]
and pyroelectric detector arrays [9].
Table 1.2 Comparison of IR detectors

8. 2. Physical Mechanisms of IR Sensors
2.1 The Physics of Thermal (Mid-wave and long-wave) IR Sensors
2.1.1 Energy bandgap and wavelength absorptivity
Mercury cadmium telluride (HgCdTe or MCT) functions as an optimal
material for thermal IR sensing as it is a ternary alloy of HgTe and CdTe with a
bandgap that makes it highly absorptive of IR radiation spanning from the shortwave
IR and longwave IR. CdTe, a semiconductor, has a bandgap of approximately 1.5 eV
whereas HgTe, a semimetal, has a bandgap of 0 eV. One can tune the bandgap of
HgCdTe by changing relative quantities HgTe and CdTe in the material. In this way
the bandgap energy of Hg1-xCdxTe is expressed [10]:
𝐸𝑔 ( 𝑥, 𝑇) = 0.832𝑥3
− 0.81𝑥2
+ (5.35 ∙ 10−4) 𝑇(1 − 2𝑥) + 1.93𝑥 − 0.302 [eV]
where x is the Cd concentration and T is the temperature from 4.2 K to 300 K.
Accordingly, the cutoff (maximum) wavelength can be expressed using the relation
𝜆 𝑔 = max
1.24
𝐸 𝑔 [eV]
= {0.671𝑥3 − 0.65𝑥2 + (4.31 ∙ 10−4) 𝑇(1 − 2𝑥) + 1.556𝑥 − 0.244}−1 m
Figure 2.1. The energy bandgap is a function on the lattice constant. In this case it is
the distance between the Γ𝑠 and Γ6 points where Γ = 0.
2.1.2 Electron mobility
HgCdTe is known for its small effective masses; this leads to a very high
electron mobility tensor. The effects of holes in thermionic processes in HgCdTe
is studied in other work [11] but electron mobility effects dominate in Hg1-xCdxTe.
In the context of IR photodetectors, the ratio the electron mobility to the hole
mobility is given as a constant ratio 𝑏 = 𝜇 𝑒/𝜇ℎ = 100.
2.1.3 Optical properties
Much has been written about the optical properties of HgCdTe [12,13] but
what is still under ongoing study is absorption coefficients at various wavelengths.
Ascertaining these values at various wavelengths is problematized by the
mechanical stress induced in HgCdTe, residual microfabrication impurities,
variations in thickness, and non-uniform doping. However, absorption
coefficients have largely been ascertained at shortwave IR wavelengths (1.4-3m).

9. The difficulty now primarily comes from understanding them at longwave IR
wavelengths (8-15m).
Figure 2.2. In the
dashed lines the
absorption is plotted and
the in the solid line the
response is plotted. As
thickness decreases, the
microfabrication errors
and mechanical stresses
and strains become more
significant [14].
Also studied in depth is the effect of temperature on the emission spectrum of
HgCdTe. The optical properties of HgCdTe at the nanostructural level exhibit a red-
shift at low temperatures [15]. Low temperatures induce “alloy disorder” in HgCdTe
and post-growth annealing can combat this vice in the material.
2.1.4 Optoelectronic applications
Like silicon and germanium, HgCdTe is very sensitive to lattice mismatches
and thus needs sturdy substrate for epitaxial growth. Cadmium zinc telluride is an
optimal substrate but it is also very expensive to many often settle for gallium arsenide
(GaAs).
Figure 2.3. The figure [15] shows that lower
temperatures redshift the photoluminescence
spectrum. That is to say low temperatures make
HgCdTe more responsive to longer wavelengths.
Post-growth annealing—especially at higher
temperatures—appears to reverse this process.
Furthermore, it was measured that the peak energy of photoluminescent responsivity
approached the bandgap energy 𝐸𝑔 for 𝑇 > 100 K. Alloy disorder makes the equation
for 𝐸𝑔( 𝑥, 𝑇) described in the first section a poor estimate for the peak energy for
photoluminescent responsivity. Still, at low temperatures the spectral response of
HgCdTe is still in the IR region as the unannealed sample in the figure had a peak
response at a corresponding wavelength of 𝜆 = 3.32 μm at 5 K.
Figure 2.4. The above figure shows the way in which IR detectors and other similar

10. optoelectronic photovoltaic devices work. The incident radiation induces electron-
hole generation and recombination insofar as its energy is above 𝐸𝑔. Here, (a)
represents the p-n junction structure, (b) diagrams the energy band, (c) profiles the
electric field, and (d) shows the I-V characteristics of the signal current and the noise
(dark) currents.
In the context of radiation detection, MCT photodiodes can function at any bias
voltage. However, higher reverse bias voltage does reduce both the diffusion
capacitance and the junction or depletion capacitance, leading to a faster response
time. Photodiodes are typically operated at zero bias to minimize the heat load and
the pink noise (1/f noise). At zero bias, the detectivity, the minimum power required
for the signal to exceed the noise, is expressed
𝐷∗
=
𝑞𝜆𝜂
ℎ𝑐
[
4𝑘 𝐵 𝑇
𝑅0 𝐴
+ 2𝑞2
𝜂Φ 𝑏]
−1/2
where:
𝑅0 𝐴 is the resistance-area product;
𝜂 is the quantum efficiency;
and Φ 𝑏 is the photon flux density.
Figure 2.5. The above diagram shows the
detectivity at various IR ranges and the
apparent increase in detectivity as the
wavelength decreases. Conversely, the
detectivity decreases as the temperature
increase, namely because an increase in
temperature multiplies the thermal noise in
the device.
2.2 The Physics of Photonic (Short-wave) IR Detectors
Indium compounds such as InAs (indium arsenide) and InSb (indium
antimonide)—both of which are III-V semiconductors—are useful for higher
frequency—shorter wavelength—detection of IR radiation. The focus of this section
will be on the more useful of the two in the photonic regime, InAs. InAs has a
wavelength range of 1m (80300THz) whereas InSb has a broader range of
15m (60z). InAs detectors have their primary application in photovoltaic
photodetectors.
2.2.1 Energy bandgap properties
Unlike MCT, InAs is a semiconductor compound whereas MCT is a ternary
semiconductor alloy with a tunable bandgap. InAs can acquire this property of
tenability via the addition of gallium to form indium gallium arsenide, or InGaAs.
InGaAs has a narrower wavelength range of 1m [21].

11. Figure 2.6. The quantum efficiency GaxIn1-xAs—( 𝑥 = 0.47)—is shown above as
function of the wavelength. Interestingly enough, 𝜂 𝑒𝑥𝑡( 𝜆) is also variable for cases of
front-illumination and back-illumination. Here, back-illumination is geared toward
higher wavelengths and front-illumination is geared towards lower wavelengths.
The energy bandgap for InGaAs is also a function of compositional ratios of its two
metals and is approximately expressed
𝐸𝑔( 𝑥) = 0.436𝑥2
− 1.501𝑥 + 1.425 eV
where x is the fraction of indium in the compound [22,23]. Likewise, the cutoff
wavelength is expressed
𝜆 𝑐 = {0.352𝑥2
− 1.21𝑥 + 1.149}−1
μm
(a) (b)
Figure 2.7. The energy bandgap decreases as the indium concentration increases as
shown in (a) and the cutoff wavelength increases as shown in (b).
2.2.2 Electronic properties
Carrier dynamics have an effect on the electronic properties of InGaAs. For
Ga0.47In0.53As at room temperature, the value of 𝜇 𝑒/𝜇ℎ hovers around 15 dB or 31.62
when the total impurity concentration is roughly between 2.51 ∙ 1017
cm−3
and
1019
cm−3
[24]. The high electron mobility allows high electric fields to be sustained
in InGaAs photodetectors, leading to shorter transit times. InGaAs, particularly
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0.5
1
1.5
2
2.5
3
3.5
g
[m]
Indium fraction x

12. Ga0.47In0.53As, has one of the fastest response times of any currently used
semiconductor.
2.2.3 Optical properties
The optical properties of InGaAs already been discussed but there is an
additional temperature dependence in regards to photoluminescent responsivity of
Ga0.47In0.53As [25].
Figure 2.8. The photoluminescent
responsivity is plotted as a function of
wavelength for both p-type and n-type
Ga0.47In0.53As at 𝑇 =
2 K, 77 K,and 295 K. At room
temperature (295 K), the differences in
the manner of doping become negligible
but become more pronounced at lower
temperatures as does the wavelength-
selectivity.
3. HgCdTe short-wave-infrared (SWIR) sensorin LADAR applications [16]
3.1 Introduction
Laser-based detection and ranging (LADAR) technique has tremendous
enabling potentials in wide application scenarios, including civilian and military
applications [17]. In agriculture, LADAR can be used to monitor crops growth and
map land topology. The advantage of LADAR over conventional GPS-based
navigation system in this setting is that LADAR can operate in locations where thick
overhead foliage is present, while GPS cannot, since foliage may have significant
impact on GPS signal. LADAR can also be deployed to monitor weather and terrain,
identify greenhouse gas emission sources and alert forest fires. Due to the properties
of high-precision 3D mapping capability and accurate ranging measurement from
LADAR, this technique is appealing to machine vision and artificial intelligence. In
military, LADAR can help troops see through obstacles in visible wavelength, and
have a clear image of targets in longer wavelength (SWIR). All these applications are
further boosted by the nature of eye safety and low attenuation propagation of SWIR.
3.2 Technical approach
Figure 3.1 shows the system diagram of LADAR. A FPGA is controlling the
entire system [18]. First, the FPGA synchronize the Erbium fiber laser to emit ultra-
short optical pulses at a rate of 200 kHz with 2 μJ within 2 ns pulse duration. The laser
size is 9 cm × 7 cm × 1.5 cm in L × W × H, respectively, and power consumption is
10 W. The output of the laser is coupled into a graded index (GRIN) fiber collimator
to converge the beam diameter, so that the beam spot can match to the size of the
MEMS tunable mirror. After the GRIN fiber collimator, the beam is 0.4 mm in
diameter and 2 milliradians in divergence. The fiber GRIN collimator is 3 mm
diameter and 13 mm long.

13. Figure 3.1. Diagram of MEMES mirror and HgCdTe sensor LADAR system
The MEMS mirror chip is 4.5 mm x 4.5 mm square, and the actual mirror is 1.2 mm in
diameter. The MEMS mirror can provide ±6 degrees beam scan range at 700 Hz, which
is sufficient to the refreshing rate requirement, 256 × 128 at 6 Hz, in this application.
The MEMS mirror is steered by high voltage signal, which is controlled by the FPGA
via a digital-to-analog converter and a high voltage amplifier. The mirror scan pattern
is defined by mirror memory connected to the FPGA. The telescope can increase the
scan angle range to ±30 degrees and also reduce the size of the system.
As the emitted light reflected by the target and propagates back to the
system, the light signal is detected by the HgCdTe SWIR sensor and converted
into electrical current signal. The current signal then is further proceeded and
converted to digital domain by and analog-to-digital converter, and finally,
conveyed to the central control unit, FPGA. At last, the constructed information
is encoded and visualized in a PC screen.
3.3 Results and discussion
The HgCdTe SWIR sensors are grown by molecular beam epitaxy (MBW) on
a silicon substrate with CdTe buffer layer. The epitaxial growth of HgCdTe is high
quality and low cost. The HgCdTe grown on silicon substrate was fabricated into 5 x 5
n+/p sensor arrays, each diode is 1 mm x 1 mm. Multiple 5 x 5 sensor arrays were
fabricated simultaneously on the same wafer and separated later by dicing, as shown
in Figure 3.2. Figure 3.3 shows a cross section of a diode unit. The diode is based on
the n+-on-p double layer planar heterostructure.
The main mechanisms that limits the frequency response of the diode are: slow
diffusion of photon-generated carriers (transit-time limit) and junction capacitance
(RC limit).

14. Figure 3.2. Picture of 5x5 SWIR HgCdTe
sensor arrow
Figure 3.3. HgCdTe device cross
section
Inset: equivalent circuit
The frequency response from the transit-time limit can be described by equation [19]
𝑓 =
2.43𝐷
2𝜋𝑑2
Where d is the carrier diffusion distance and D is the minority carrier diffusion
coefficient. Figure 3.4 shows the calculated 3-dB bandwidth for both n- and p-
type absorber. From this figure, we can see, device using p-type absorber is
preferred, as the electrons are minority carriers and have high mobility.
The frequency response from the RC limit can be described by equation
𝑓 =
1
2𝜋𝐶( 𝑅 𝑠 + 𝑅𝑙)
where C is the diode’s junction capacitance and 𝑅 𝑠 and 𝑅𝑙 are the parasitic shunt and
the load resistance, respectively. Given the typical load resistance is 50 ohms, if the
RC limit bandwidth needs to be ~GHz level, the C needs to be fairly low, ~pF. Figure
3.5 shows calculated RC-limited 3-dB bandwidth, under different load impedance.
Figure 3.4. Cutoff frequency as a
function of diffusion distance
Figure 3.5. Cutoff frequency as a
function of junction capacitance under
different transimpedance.
Inset: equivalent circuit

15. Figure 3.6 shows the current-voltage (I-V) performance of diodes in a sensor array at
room-temperature. Each diode is 1 mm × 1 mm. We can see, I-V curves overlap pretty
well with each other, so good uniformity among diodes are achieved, thanks to the
mature HgCdTe epitaxy deposition technique.
Figure 3.6. I-V of 1 mm × 1 mm diode.
Inset: dynamic resistance
Figure 3.7 shows the measured responsivity and quantum efficiency (QE) as a
function of incident light wavelength at room-temperature. The diodes do not have
any anti-reflection coating (ARC). We can see from Figure 3.7, there is clear sharp
cutoff at ~1.8 μm. As for capacitance, 9 pF/mm2 is measured, which enables high
RC-limited bandwidth.
4. Pyroelectric short-wave-infrared (SWIR) sensorin spectroscopy application
[20]
4.1 Introduction
Gas detection and spectroscopy is one of the appealing research fields. Gas
spectroscopy has wide application potential in environment monitoring, agriculture
optimization and health care. Among different types of interesting gases, carbon
dioxide (CO2) is the most significant one, because it has strong absorption in infrared
radiation. Therefore, it is the main reason causing the greenhouse effect and global
Figure 3.7. Responsivity and QE of a 1mm × 1mm diode, without AR coating.

16. warming. Other than the environmental impact, CO2 also can be utilized as food
spoilage indicator.
4.2 Technical approach
Among Beer-Lambert Law-based CO2 detection systems, they can be divided
by number of wavelength used in the system: single wavelength systems, dual
wavelength systems and multiple wavelength systems. In the single wavelength
system, as shown in Figure 4.1 (a), the presence of a compound is measured by the
level of radiation absorption at certain wavelength, compared to the level where no
absorption is present. The main problem is that, it requires frequent calibration to
exclude the influence imposed by sample temperature, emission intensity fluctuation,
emitter and receiver misalignment and emitter lifetime.
Dual wavelength systems can overcome such disadvantages by employing one
more wavelength as reference. One detector with a working wavelength centered to
sample absorption peak, the other is tuned to a wavelength close to absorption peak as
normalization reference. The schematic is shown in Figure 4.1 (b).
The proposed system is shown in Figure 4.2. Two signals are obtained, one as
reference signal and the other as sample signal. Reference signal is used for
normalizing out system ‘common mode’ fluctuation. Halogen bulb was used to
provide emission radiation. The wall plug power of this lamp is 10 W and it is
amplitude modulated at 250 MHz by a lock-in amplifier. A chamber with a 1 cm3
volume was used as gas sample housing. The chamber was thermally insulated to
avoid measurement error caused by environment temperature variation. Pyroelectric
signals were filtered out and amplified by 250 MHz lock-in amplifier, then the analog
signal was converted to digital domain by an ADC at 100 Hz sampling rate, finally,
concentration information was processed and displayed.
Figure 4.1. (a) classical experimental setup for photopyroelectric measurement; (b) proposed
dual-wavelength detection scheme

17. 4.3 Results and discussions
Figure 4.3 shows pyroelectric signals as function of time under five different
CO2 concentrations. It is worthwhile to point out that the reading fluctuation was
caused by gas dynamics after sample gas was injected into the chamber. Therefore, it
is necessary to start recording data after 10 seconds of gas injection as stabilization
time.
In the case of 0% CO2, pyroelectric signal increases and then stays constant,
this phenomenon is believed to be caused by the fact that more IR radiation reach the
pyroelectric detector since there is not absorption at all, thus the amplitude increases.
Figure 4.4 shows the stabilized amplitude of different CO2 concentration. Solid
squares represent experimental results and the red line is the proposed fitting function.
Error bars were obtained from the standard deviation of averaged points under each of
the concentration measurements.
Figure 4.2. dual window scheme for CO2 concentration measurement, A is a narrow infrared
optical window with central wavelength of 4 m; B is a narrow infrared optical window with
central wavelength of 4.26 um
Figure 4.3. Pyroelectric signals as time function of different gas concentrations

18. Conclusion
In this paper, first we went through the history and classification of IR sensors.
Then physics behind IR sensors were discussed in details. Energy band theory, optical
properties and electron mobility of HgCdTe were covered. At last, we introduced a
HgCdTe-based LADAR system as an example of the application of HgCdTe sensors.
There is still more to be understood about the working of IR sensors in general but
have has been clearly demonstrated is their ability to detect, respond to, and absorb
infrared radiation to a degree that allows for their application in many electromagnetic
and photonic devices.
Figure 4.4. Stable state pyroelectric values for different CO2 concentrations; squares
represent experimental values and the solid line is the proposed behavior of signal amplitude.

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