Sensors are needed convert real life quantities into
signal variations and hence has a very high importance. Or-ganics semiconductors have their own advantages, which can
be exploited to create sensors. One of the mostly used sensor
based on organic materials is the Organic Field-Effect Transistor
(OFET). The channel material made from the organic compound
interacts with the analyte and in turn causes variations in the
device parameters.
The major applications of OFET sensors are as bio-sensors,
chemical, and gas sensors. Bio-sensors helps in disease diagnostics
by detecting DNA, proteins, enzymes etc. Chemical sensors are
used to find out the presence of ions, humidity, and pH levels. To
get more information, furthur discussion is about a single OFET
sensor fabricated with P3HT and CuTPP used for detecting nitro-based explosive compounds. OFET sensors are very promising
and could be used in real applications in near future.
2. Introduction
Sensors
Transducer
Conversion into signals
Applications
[2]
Organic conducting polymers
Discovery [1]
Applications
Organic Thin Film Transistor (OTFT)
[1] H. Shirakawa et al. , “Synthesis of electrically conducting organic polymers: halogen derivatives of
polyacetylene, (ch),”, 1977
[2] P. Lin and F. Yan, “Organic thin-film transistors for chemical and biological sensing,” 2012
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3. More about OTFTs
Internal amplification and noise correction
Compatibility with existing VLSI Technology
Sensors could be
Biodegradable
Flexible
Cost effective
More information than any other sensor
OTFT
OFET Sensors : IIT Bombay
OECT
OFET
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4. Device Structure & Working
OECT
Source & Drain electrodes
OSC channel
Electrolytic layer on top
OFET
Similar to OECT
Direct interaction with analyte
Two configurations
[1]
[1] P. Lin and F. Yan, “Organic thin-film transistors for chemical and biological sensing,”, 2012
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5. OFETs in detail
Two configurations
Top contact
Bottom contact
Lower mobility in the channel
Follows standard MOSFET equations
[1]
[1] H. Ma, et al., “Multifunctional phosphoric acid self-assembled monolayers on metal oxides as
dielectrics, interface modification layers and semiconductors for low-voltage high-performance organic
field-effect transistors,”, 2012
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6. OFET Sensors in detail
OFET Sensors
Bio-Sensors
DNA
Proteins
Glucose
Others
OFET Sensors : IIT Bombay
Gas Sensors
Chemical
Sensors
Ions
Humidity
Other sensors
X-ray
Others
pH
Others
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7. Bio-sensors : DNA
[2]
Zhang et al. used pentacene as the OSC channel
DNA adsorption VT shift Electron extraction by DNA
Improvement suggestions
Reducing film thickness or
current
Increase substrate
temperature
[3]
Yan et al. proposed another device using P3HT
[2]
[1]
[1]http://www.intechopen.com/books/biosensors/design-and-fabrication-of-nanowire-basedconductance-biosensor-using-spacer-patterning-techniq
[2] Q. Zhang and V. Subramanian, 2012
[3] F. Yan, S. M. Mok, J. Yu, H. L. Chan, and M. Yang, 2009
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8. Bio-sensors : Glucose & Others
[1]
J.Liu et al. proposed PEDOT-PSS organic channel with GOx
entrapped
GOx entrapped during polymerization
Redox reaction channel and glucose with Gox
catalysis
[2]
Sensor based on Ta2O5 and P3HT , by Bartic et al.
GOx anchored on surface
Cyanopropyltrichlorosilane treatment
[3]
Roberts et al. OFET to detect glucose, cystein, and MPA by
DDFTTF as the OSC channel
[1] J. Liu, M. Agarwal, and K. Varahramyan, “Glucose sensor based on organic thin film transistor using glucose
oxidase and conducting polymer,” , 2008
[2] C. Bartic, A. Campitelli, and S. Borghs, “Field-effect detection of chemical species with hybrid
organic/inorganic transistors,” , 2003
[3] M. E. Roberts, S. C. B. Mannsfeld, N. Queralt, C. Reese, J. Locklin, W. Knoll, and Z. Bao, “Water-stable organic
transistors and their application in chemical and biological sensors,”
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10. Chemical Sensors : Ions & pH
[1]
Ji et al. P3HT as the OSC, with Ta2O5 and valinomycin
subsequently deposited on top
+ +
Detects K , H ions and pH levels
[2]
Scarpa et al. used P3HT as channel
+ +
2+
K , Na, Ca ions and pH levels even at 0.001%
[3]
Maddalena et al. had a sulfate receptor with incorporated
thiol group, coupled polystyrene layer
Detects sulphate ions with 1mM
[1] T. Ji, P. Rai, S. Jung, and V. K. Varadan, “In vitro evaluation of flexible ph and potassium ion-sensitive organic
field effect transistor sensors,” 2008
[2] G. Scarpa, A.-L. Idzko, A. Yadav, and S. Thalhammer, “Organic ISFET based on poly (3-hexylthiophene),” 2010
[3] F. Maddalena, M. J. Kuiper, B. Poolman, F. Brouwer, J. C. Hummelen, D. M. de Leeuw, B. De Boer, and
P. W. M. Blom, “Organic field-effect transistor-based biosensors functionalized with protein receptors,” , 2010
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11. Chemical Sensors : Ions & pH
[1]
Bartic et al. reported a P3HT OSC OFET
Indicates the pH values after in-situ amplification
Coating with arachidic acid improves the sensitivity
[2]
Pentacene OSC based OFET proposed by Loi et al.
variations in charge at the gate-channel interface
coating the floating gate with thioaminic groups
Water stable OFET which can detect pH 3 to 11, using DFTTF
OSC channel by Roberts et al.[3]
[1] C. Bartic, A. Campitelli, and S. Borghs, “Field-effect detection of chemical species with hybrid
organic/inorganic transistors,” 2003
[2] A. Loi, I. Manunza, and A. Bonfiglio, “Flexible, organic, ion-sensitive field-effect transistor,” 2005
[3] M. E. Roberts, S. C. B. Mannsfeld, N. Queralt, C. Reese, J. Locklin, W. Knoll, and Z. Bao, “Water-stable organic
transistors and their application in chemical and biological sensors,” 2008
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13. Gas Sensors
[1]
Laurs et al. observed that oxygen, iodine and bromine could
vary the current through the OFET fabricated with
phthalocyanines (Pcs) as the OSC.
[2]
Torsi et al. fabricated NTCDA
based OFET, and found four
characteristic parameters to
be varying.
[3]
[4]
Someya et al. and Torsi et al.
Reported the dependancy of
sensitivity on grain size.
[1] H. Laurs and G. Heiland, “Electrical and optical properties of phthalocyanine films,” 1987
[2] L. Torsi, A. Dodabalapur, L. Sabbatini, and P. Zambonin, “Multi-parameter gas sensors based on organic thinfilm-transistors,” 2000
[3] T. Someya, A. Dodabalapur, A. Gelperin, H. E. Katz, and Z. Bao, “Integration and response of organic
electronics with aqueous microfluidics,” 2002
[4] L. Torsi, A. J. Lovinger, B. Crone, T. Someya, A. Dodabalapur, H. E.Katz, and A. Gelperin, “Correlation between
oligothiophene thin film transistor morphology and vapor responses,” 2002
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15. Explosive vapor sensor
A particular type of gas sensor
OFETs as explosive sensors
RDX
[1]
TNT
Materials proposed to be
used as OSC channel
Poly 3-hexylthiophene
(P3HT)
[2]
II
Cu tetraphenylpophyrin
(CuTPP)
[1] Ravishankar S. et al. “Explosive vapor sensor using poly 3-hexylthiophene and Cu tetraphenylporphyrin
composite based organic field effect transistors“ 2008
[2] http://www.aist.go.jp/aist_e/aist_laboratories/2information
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16. Device structure and fabrication
[1]
SiO2 layer grown on n silicon wafer
Au / Ti Source & Drains are patterned
HMDS Surface enhancement
CuTPP & P3HT dissolved in chloroform is spin coated
[1] Ravishankar S. et al. “Explosive vapor sensor using poly 3-hexylthiophene and Cu tetraphenylporphyrin
composite based organic field effect transistors“ 2008
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17. Device Characterization
Significant rise in drain current & conductance in the presence
of nitro compounds
Threshold voltage is found out by linear fit of Transfer Chara
Behavior can be modeled by using existing equations
Shift in FTIR peaks on sensor exposure to RDX
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18. OFET explosive vapor sensor :
Results & Conclusion
Selectivity of
the sensor for
various vapors
The OFET formed has high sensitivity to nitro based explosives
ION & S parameters can be evaluated to check the presence
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19. Conclusions & Future Scope
OFETs are effective sensors for detecting various types of
materials
Many materials being tried out to be used in sensors have a
promising performance
Also there is a need for new structures and modifications to
enhance the sensing abilities of sensors
Sensitivity, selectivity, stability all have to be improved before
using them in real life applications
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