Sensors & Transducers Magazine, Vol.39, Issue 1, 2004, pp.100-105
Sensors & Transducers
ISSN 1726- 5479
© 2004 by IFSA
http://www.sensorsportal.com
Gas-sensing Properties of a Field-Effect Transistor with a
Bis[phthalocyaninato] Samarium Complex/SiO2/Si Structure
Dan Xie1, Yadong Jiang2, Tianling Ren1, Litian Liu1
1
2
Institute of Microelectronics, Tsinghua University, Beijing 100084, P. R. China
School of Optoelectronic Information, University of Electronic Science and Technology of China,
Chengdu, 610054 P.R.China
Phone: ++86-10-62789147-304, e-mail: xiedan@mail.tsinghua.edu.cn
Received: 15 December 2004
/Accepted: 14 January 2004
/Published: 18 January 2004
Abstract: Based on the conventional metal-oxide-semiconductor-field-effect transistor (MOSFET),
a new chemical field-effect transistor (ChemFET) gas-sensing device was fabricated by depositing
organic gas-sensing material on the gate area of MOSFET replacing the gate metal. Sandwich-like
bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato]
samarium
complex
Sm[Pc*]2
*
(Pc =Pc(OC8H17)) was used as the gas-sensing material for detecting nitrogen dioxide (NO2). Using
Langmuir-Blodgett (LB) technology, Sm[Pc*]2 LB film was prepared and deposited on the gate area
forming the gas-sensing film/oxide/semiconductor structure with a sensitive gate area of
50m×50m. The gas-sensing property and response-recovery property of Sm[Pc*]2 LB film/SiO2/Si
structural ChemFET sensor to NO2 gas was studied by the change of drain current (IDS) during gas
exposure. The results show that ChemFET gas sensor with Sm[Pc*]2 LB film can detect NO2 gas
down to 2.5ppm. And the response and recovery time to 40ppm NO2 gas was about 15 s and 3 min.
The mechanism of sensitivity of Sm[Pc*]2 LB film ChemFET to NO2 was also discussed in this paper.
Keywords: Gas sensor, ChemFET, LB films, NO2 gas, Bis[phthalocyaninato] samarium
________________________________________________________________________________
1. Introduction
The Langmuir-Blodgett (LB) technique is a promising means to develop highly-ordered organic thin
films. Because such ultrathin films have high ratios of surface area to bulk volume, the use of organic
gas-sensitive substances and LB deposition technique have a great potential for improving the
performance of gas sensors. It can be expected to obtain an efficient and quick response gas sensor by
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using LB technique with good molecular packing and with gas-sensitive molecular groups aligned
near the surface of LB films [1-3]. In order to fabricate stable and applied gas sensors, it is very
important to select the gas sensitive materials and design the device structure. Generally, LB film
based gas sensors are fabricated by depositing LB film on planar interdigitated electrode pairs
measuring the change of current or conductance. Since most organic materials are highly resistive,
the current or conductance measurement is somewhat difficult. At the same time, complex detection
and good shielding are required to avoid excessive problems with noise. Therefore, it is very difficult
for such sensors to perform with high accuracy. To overcome this difficulty, the basic structure of
metal-oxide-semiconductor field-effect transistor (MOSFET) has been introduced into the device
design of sensors, in which the gate electrode is replaced by an organic sensitive film so as to form an
organic film/oxide/semiconductor field-effect transistor without gate metal. It is a kind of chemical
field effect transistor (ChemFET). As shown in Fig. 1, LB film/SiO2/Si structural FET device is
described. When being exposed to different gases, the interaction between LB film and adsorbed gas
can change the conductive state of drain-source channel, resulting in the variation of drain-source
current (IDS) of ChemFET. The major advantage of using such ChemFET in gas sensor applications is
that current levels down to several microamperes can be measured. It suggests that such ChemFET
device can be used as an effective gas sensor [4, 5].
Fig. 1: A schematic cross-section of ChemFET gas sensor with Sm[Pc*] 2 LB film as sensitive gate
It is known that phthalocyanines are excellent gas-sensing materials owing to their thermal and
chemical stability, especially to some oxidizing gases such as nitrogen dioxide (NO2) [6, 7]. In the
paper, substituted bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] samarium complex
Sm[Pc*]2 (Pc*=Pc(OC8H17)8) based LB film was deposited on the gate area of FET forming a new
ChemFET for detecting NO2 gas. The electrical and gas-sensing characteristics of such ChemFET
device were studied and it is found that the gas sensing properties and responsivity are well. Such
structural FET also shows great potential for application in molecular electronics, electrochromic and
molecular magnetic devices.
2. Experiments
Based on standard MOSFET, two kinds of FET structure were designed on the same monolithic
silicon in order to compare the relative characteristics, and conventional techniques of solid-state
device fabrication were used [8]. One is a conventional MOSFET with the metal gate electrode, the
other is an organic film/SiO2/Si FET structure with LB film replacing the gate metal. Fig. 1 shows a
schematic of the ChemFET structure. An insulating layer of thermal silicon dioxide (100nm SiO2) is
grown on top of a highly doped n++ silicon wafer ((100) p-type), which acts as the gate contact.
Source and drain gold contacts were fabricated on top of the insulator by standard photolithographic
techniques to form a n-channel. The metallization comprises of a sputtered thin film of chromium
(20nm), upon which a thicker film of gold (80nm) was sputtered.
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Sm[Pc*]2 LB film was deposited onto the FET substrate from the uniform Langmuir monolayer with
the concentration of 1.2~1.3mol/ml. The synthesis of Sm[Pc*]2 spreading solution and the
preparation of LB film were reported elsewhere [9, 10]. The thickness of LB film ranged from 0.1m
to 0.4m in the studies. Sm[Pc*]2 LB film/SiO2/Si structural ChemFET with the sensitive area of
50m×50m was fabricated by photolithographic and reactive ion etching (RIE) techniques.
The gas sensing properties were studied by placing the samples in a chamber through which gas could
be passed. NO2 gas was diluted with high purity level nitrogen (99.99%) passing through the test
chamber at a flow rate of 500ml/min, controlled by a National Standards Research Center MF-2
model gas blender. Gas entering the chamber passed directly over the sensor surface and desorption
cycle were performed in pure nitrogen. The concentration we used in the experiment was varied from
1ppm to 100ppm. The sensitivity and responsivity of Sm[Pc*]2 based FET device to NO2 were
obtained by measuring the change of drain current at room temperature. The electrical characteristics
were recorded using the Victor DT890D digital voltmeter linked to a computer-controlled
data-acquisition program.
3. Results and Discussion
To investigate the gas-sensing properties, ChemFET sensor was exposed to NO2 continuously at
room temperature. Fig. 2 shows the relationship between the drain-source current (IDS) and drain
voltage (VDS) of the ChemFET gas sensor with 0.3m-thick Sm[Pc*]2 LB film on exposure to
different NO2 concentrations. It is found that drain current increases with the increase of drain
voltage. Under the same drain voltage (VDS=3V), the drain-source current increases with the increase
of NO2 concentrations, which indicates that gas concentration has great effect on the field-effect of
ChemFET device. The function of NO2 gas is equivalent to the gate voltage of MOSFET. Here, gas
makes the conductive state of drain-source channel turn on or change, hence, resulting in the variation
of IDS.
1.6
1.4
CNO =40ppm
2
IDS(A)
1.2
1.0
CNO =30ppm
0.8
CNO =20ppm
2
2
0.6
CNO =10ppm
0.4
2
CNO =5ppm
0.2
0.0
2
0
2
4
6
8
VDS(volts)
Fig. 2: Output characteristics of ChemFET gas sensor with 0.3m-thick Sm[Pc*] 2 LB film
To the ChemFET with 0.3m-thick Sm[Pc*]2 film, when the concentration of NO2 gas was less than
1.0 ppm, IDS didn’t change evidently; but when the NO2 concentration reached 2.5ppm, IDS changed
obviously to 1.610-7A. It is demonstrated that Sm[Pc*]2 LB film/SiO2/Si structural ChemFET shows
the preferable sensitivity that it can detect NO2 gas down to 2.5ppm. The detection sensitivity is
higher than that of the sensor with microelectrodes.
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Fig. 3: Transfer characteristics of Sm[Pc*] 2 LB film with various thickness/SiO2/Si structural
ChemFET gas sensor
From the curves of Fig. 2, it is found that the gas-sensing properties of the ChemFET sensor to NO2
gas is similar to the output characteristic of MOSFET to different gate voltages. Keeping the drain
voltage unchanged, the relation of IDS and NO2 concentrations of ChemFET device can be obtained,
the curves of which are similar to the transfer characteristic curve of MOSFET. Therefore, they also
can be named as transfer characteristic curves, as showed in Fig. 3. These measurements were
performed with a drain voltage of 3 volts. In order to compare the relative characteristics with the
standard MOSFET, the output characteristic and transfer characteristic of MOSFET were also
measured (Fig. 4 (a) and (b)). It can be estimated that the responsivity of the ChemFET device with
0.4m-thick Sm[Pc*]2 LB film to 10ppm NO2 gas is equivalent to the effect of 0.2V gate voltage on
MOSFET. It is found from Fig. 3 that IDS increases with the increase of thickness of LB films. The
thinner the films, the fewer the number of charges in the conductive channel becomes, therefore, the
weaker the IDS gets. The obvious field effect can be observed only when the thickness of LB film is
above 0.3m at least, especially when NO2 gas concentration down to 5ppm. It is suggested that both
the thickness of LB film and the concentrations of NO2 have effect on the drain current.
1.4
IDS(A)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.8-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
VGS(volts)
Fig. 4: Output characteristic (a) and transfer characteristic (b) of MOSFET
Even though the NO2 concentration makes IDS of such ChemFET change, in fact, ChemFET device is
still an electric field turn-on FET. Compared with the usual MOSFET, LB film ChemFET has the
basic structure of the generic FET, only with LB film replacing the metal gate electrode.
Because phthalocyanine is a kind of p-type organic semiconductor, it is prone to react with certain
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oxidizing gases. When the ChemFET gas sensor with Sm[Pc*]2 LB film sensitive gate is exposed to
NO2 gas, electrons and holes within LB film are produced through the charge transfer interaction with
NO2 molecules. It will cause the increase of the LB film conductivity [11,12]. This results in many
electric dipoles in the LB film, which change the electrical potential of the semiconductor surface in
the gate area, leading to the change of IDS. When NO2 gas concentration or the thickness of LB film
varies, the number of electrons and holes of LB film in the gate area will change too, resulting in the
variation of IDS. It is more convenient to measure the change of current than that of resistance for
higher resistive materials. Therefore, it can be expected that such ChemFET gas sensor have better
gas-sensing properties to NO2 gas compared with microelectrodes gas sensor.
The response-recovery properties of Sm[Pc*]2 LB film based ChemFET with various thickness to
NO2 gas were monitored. The results show that the signal magnitude is related to both film thickness
and NO2 concentration. Thinner films provide weaker but faster response. The higher the
concentration of NO2, the faster the response becomes. Fig.5 shows the response properties of
Sm[Pc*]2 LB film based ChemFET sensor to 20ppm and 40ppm NO2 gas. The response time of
ChemFET sensor with 0.3m-thick Sm[Pc*]2 LB film to 40ppm and 20ppm NO2 is about 15 s and 50
s, while the FET with 0.2m-thick Sm[Pc*]2 LB film is close to10 s and 30 s, respectively.
Fig. 3: The plot of response time of ChemFET gas sensor to 20ppm and 40ppm NO2 vs. the
thickness of Sm[Pc*] 2 LB films
Further experiments show that the gas-sensing characteristics of the device in air are reversible, even
though the complete recovery time is relatively longer. The recovery time of ChemFET sensor with
0.3m-thick Sm[Pc*]2 LB film to 40ppm NO2 is about 3 min. This may be due to the rapid desorption
of the NO2 molecules coated on the LB film surface at initial recovery stage. During the latter longer
recovery stage, desorption of NO2 molecules from LB film surface and diffusion into the film is a
complex process. Of course, the whole interaction process between LB film and the adsorption gas is
a more complicated dynamical process: when being exposed to NO2, the adsorption and desorption
processes simultaneously occur [13,14].
5. Conclusions
In summary, a new microsensor for detecting NO2 gas has been fabricated by incorporating the
multilayer Sm[Pc*]2 LB film onto the gate area of a MOSFET, forming a Sm[Pc*]2 LB film/SiO2/Si
structural ChemFET device with a sensitive gate area of 50m×50m. It is found that the ChemFET
gas sensor with 0.3m-thick Sm[Pc*]2 LB film can detect NO2 gas down to 2.5ppm. The thinner the
LB film or the higher the concentration of NO2 gas, the faster the response becomes. The response
and recovery time of the sensor to 40 ppm NO2 is about 15 s and 3 min, respectively. Detection to
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different gases with lower concentration can be realized by using such ChemFET device, which can
overcome the problem associated with the use of interdigitated electrode device. It is feasible to
achieve the miniaturization and integration of all kinds of sensors integrating with microelectronic
fabrication process. Therefore, ChemFET sensor with gas-sensing organic film deposited on the gate
area as a sensitive gate is a promising device structure to develop micro-sensors, which show more
improved properties.
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