1. 8. Gas sensors Introduction Market production of gas sensors
Development of resistance gas sensors technology
Influence of micromachining technology
Non-resistance type gas sensors
Measurement modules

2. Introduction Main fields of gas sensors applications :
safety control (combustible and toxic gases)
air quality (buildings, vehicles)
process control in industry
laboratory analytics
burning control in vehicle engines
Only in analytics and in industry one uses expensive gas analysers.
In other cases there is a need of cheap, small size and easy in application
gas sensors.

3. Market production
Gas sensors manufactured by Figaro Eng. Inc.

4. Market production
Lambda sensors used in car industry (fuel combustion control)

5. Gas sensors – historical review
First reports: gas sensing properties of Ge:
W.H.Brattain, J.Bardeen, Bell Syst. Tech. J. (1952)
First reports on gas sensing properties of metal oxides:
G. Heiland, Z. Physik (1954)
A. Bielański, J. Dereń, J. Haber, Nature (1957)
T. Seiyama et al., Anal. Chem. (1962)
N. Taguchi – gas sensors as a commercial product (TGS sensors),
production at the end of 1968.
The dominating technology at present: deposition of thick films onto ceramic substrates.
One observes increasing role of micromachining technology combined with technology of gas sensitive thin films.
The most frequently used gas sensitive oxide layers: SnO2, ZnO, TiO2, WO3, In2O3.

6. Understanding resistance gas sensor
Jonosorption reaction with
a capture of electron:
Reduction of oxygen ion
e.g. ½ O2 + CO CO2
Change in conductance:
(a), (c)
where: W-width, H-height, L – length
of a sample, x0 = LD
Vs – barrier height
NS – conc. of chemisorbed oxygen
(b), (d)

7. Development of gas sensors technology,
monocrystals
(a) – upper view
(b) – bottom view
Structure of a monocrystalline sensor:
1 – alumina substrate,
2 – platinum electrodes,
3 – conducting paste,
4 – crystalline ZnO,
5 – heater (screen printing),
6 – leads

8. Development of gas sensors technology,
sintered structures
TGS 203 sensor
for CO detection
RH = 1.9 Ω
VH = 0.8 V
VC = 12V
PH = 2 x 370 mW
Sintered semiconductor powder with selected catalytic admixtures and a binder

9. Development of gas sensors technology,
microsensors
FIS microsensor SB series: (a) structure, (b) basic measurement circuit.
Termination „1” is common for the heater voltage VH and the sensor voltage VC.
Due to miniaturisation the consumed power does not exceed 130 mW.
The sensor is used for detection of methane, propane and other inflammable and toxic gases.

10. Development of gas sensors technology
Figaro TGS 800 series
Basic measurement circuit
Structure of the sensor:
layer of SnO2 with a thickness of ca. 50 μm,
alumina tube (inner diam. 1 mm, length ca. 4.2 mm),
Pt wire leads (Φ = 0,08 mm),
platinum heater (30 Ω),
required working temp. ca. 300o C,
power of the heater ca. 830 mW.

11. FIS sensor SP series working in a pulse modulated
Thick films
FIS sensor SP series working in a pulse modulated
temperature regime.
Advanced thick film technology, Au contacts,
RuO or Pt heater, power consumption 300 – 400 mW.

12. Thick films Figaro sensor TGS 2400 series for detection of toxic gaes
One-side configuration, power consumption 14 mW (avg value)
Pulse control, power supply VC=VH= 5V

13. Influence of micromechining technology
(a) closed membrane
(b) suspended membrane
( hotplate type)

14. Influence of micromechining technology
Motorola MGS 1100 sensor
for CO detection
Sensor in a package
Sensors layout

15. Micromachined sensors manufactured in cooperation ITE Warsaw - AGH
Schematic of the resitance – type gas sensor
on the silicon membrane
Power consumption below 100 mW

16. Micromachined sensors manufactured in
cooperation ITE Warsaw – AGH, cont.
Single sensor die,
upper view
3” Si wafer with manufactured
sensors

17. Micromachined sensors manufactured in
cooperation ITE Warsaw – AGH, cont.
Micromachined gas sensor in TO-5 encapsulation without a cap
Sensor with a cap and a steel protecting mesh

18. Pulse power supply of the sensors
Modulation of sensors working temperature
400 ppm NO2
1000 CO NO2
air
1000 ppm CO
Pulse power supply of the sensors
heater

19. Pellistor – type sensors
Pellistor with a catalyst surface layer (diam. ca.1 mm)
Pellistor fixed in an encapsulation
Platinum coil is surrounded by ceramics on which surface a catalytic layer of a noble metal (Pt, Pd, Rh) is deposited. At elevated temperature an inflammable detected gas is catalytically oxidized which produces additional heat and increase in resistance of a Pt coil. Platinum coil serves also as a heater for initial heating of a pellistor to reach the required working temperature.

20. Thermoconductometric TCD sensors
(catharometers)
λG - heat conductivity of a gas, M0 – molecular weight
d0 - molecule diameter
Gas with small molecular weight and small particles, e.g. H2, will have high heat conductivity.
μTAS with separation column and TCD detector for the analysis of chemical composition of a gas mixture.
Intensity of cooling of a given resistor depends on gas heat conductivity
SEM picture of the flow channel with suspended Pt thermoresistor
(magnification 212 x)

21. Electrochemical sensors
YSZ –solid state electrolyte Yttrium Stabilised Zirconia
Oxygen Lambda sensor characteristics at a working temp. ca. 600oC.
p- measured pressure, pR – reference presure
Oxygen Lambda sensor of the finger type installed in a car exhaust pipe.
When concentration p of exhaust oxygen increases, generated SEM tends to zero (according to the Nernst relation where pR is a partial pressure of oxygen in air).
λ – air excess coefficient
A – quantity of air
F – quantity of a fuel

22. Electrochemical amperometric sensors
Oxygen sensor with
a current limit caused by a slit.
Saturated current depends on gas concentration.
Thin film amperometric sensor with a current limit due to chemical reactions.

23. Optical sensors based on absorption
Methane absorption in a main
abs. band 3200 – 3400 nm;
1 – diode LED1,
2 - diode LED2,
3 - detector
Two-diode system for absorption
measurement in the NIR region

24. Optical sensors based on fluorescence
Absorption spectrum A
and fluorescence emission of DPB compound in nitrogen atmosphere B and oxygen atmosphere C .
Integrated fluorescence sensor coupled
with a μTAS microchannel structure.

25. Structues basd on the field effect
MISFET type transistor as a gas sensor.
Gate metal – mostly Pd
Change of the threshold voltage of a FET transistor as a consequence of interaction with a gas atmosphere.

26. Structues basd on the field effect
Mechanism of detection of hydrogen molecules and other hydrocarbons by a thick palladium gate.
Hydrogen molecules dissociate on Pd surface, diffuse as atoms through the metal and adsorb as dipoles on metal/insulator interface. This dipole layer is responsible for lowering of the threshold voltage and increase of the drain current.

27. MOSFET arrays Nordic Sensor Four MOSFET-s with cathalytic gate in a
micromachined structure:
1 – MOSFET
2 – Al
3 – PECVD nitride
4 – LPCVD nitride
Power consumption ca. 100 mW at 200oC.

28. Organic composites Cyrano Sciences Sensor Technology
Thin layer of a composite: conducting carbon in a polimer matrix
Swelling of a polymer after interaction with a gas causes breaking of carbon chains
and increase of composite resistance.

29. Measurement with the help of sensor array (MSc thesis)
Calculation block
(microprocessor
with software)
Sensor 1
Gas
mixture
Calculated
concentrations
Sensor 2
Meter for measurements of CO and C3H8 concentrations in a mixture.