Polyaniline Based Field Effect Transistor for Humidity Sensor

In this present research work, we presented a new Bottom Gate P-Type Organic Field Effect Transistor (OFET) humidity sensor and its applicability towards humidity has been experimentally demonstrated. P-type organic semiconductor polyaniline (PANI) has been used in a variety of applications, including logic circuit components, electromagnetic shielding, chemical sensing, and anticorrosion. Humidity sensor can be used to monitor relative humidity (RH) in various environments. We only focus on the fabrication of conducting polymer OFETs with top contact to measure humidity and verify I-V properties. The current saturation (ISat) of p-type OFETs was 0.8 μA, while the threshold voltage VTh was 2.2 V. The results of FESEM have been perform to confirm that deposited thin film grown on the substrate is purely uniform. The Proposed sensor shows that organic gate dielectrics are a low-cost alternative to inorganic gate dielectrics with good electrical performance. The proposed OFET-based sensors have a number of benefits, such as high sensitivity, low cost, quick response, and physical flexibility.


Introduction
Sensor technology has been use in advanced application, resulting in low power, miniaturized, high-speed, and costeffective system. Organic electronics is becoming a prominent topic in the fields of displays, sensor arrays, and photovoltaics, among other things. OFETs provided high-quality components for active matrix displays, radio frequency identification tags, and a variety of other small-scale integrated systems. Conducting polymers (CPs) are organic compounds with π -orbital system that allows electrons to travel from one end of the polymer to the other. Because of its mechanical flexibility, ease of synthesis and high electrical conductivity, conducting polymer (CP) is used. At room temperature, CPs can help sensors to improve their speed, sensitivity, and sort response time. Polyaniline (PANI), Polypyrrole (PPy), Poly (phenyl vinlene) (PPV), Poly (3, 4-ethylenedioxythiophene), and PEDOT are only a few of the organic conducting polymers available. PANI, a p-type organic semiconductor material with good electrical conductivity and stability, is one of these organic compounds. Because of its ease of synthesis, room temperature operation, and relative stability, polyaniline (PANI) has been use for a variety of applications including logic circuit components, electromagnetic shielding, chemical sensing, and anticorrosion. This device can be used in low-power applications [1][2][3][4]. Humidity sensors can be used to measure relative humidity (RH) in a variety of weather. These sensors will have relative merits for the low-cost and low-temperature fabrication of mechanically flexible electronics on flexible substrates. PANI has been used in humidity sensor devices based on the electrical conductivity with water vapor. Polymers are the most common type of material used to fabricate the humidity sensors. Humidity is an important environmental characteristic that can be used for a variety of applications including instrumentation, automated systems, agriculture, and climatology. Plantation protection and soil moisture monitoring are two applications of humidity sensors in agriculture. These sensors will also include some relative demerits in terms of conductive polymers have high resistance and therefore are not good conductors of electricity, poor electronic behavior (lower mobility), which have much smaller bandwidths. To get high performance of humidity, sensing materials must have high surface to volume ratio to interact with the water molecules repeatedly for longer life cycle and faster response [5,6]. In this paper, we propose a bottom gate OFET device composed of PANI as a conducting channel and SiO 2 act as an insulator on n-type silicon substrate, which will be used as a humidity sensor based on organic field-effect transistor (OFET). This device is compatible with low power applications. In this work, we only concentrate on fabrication of conducting polymer OFET with top contact to evaluate the humidity and verify I-V characteristics. The current saturation (I Sat ) of p-type OFETs was 0.8 μA, while the threshold voltage V Th was 2.2 V. The FESEM (Field Emission Scanning Electron Microscopy) have been presented. The Proposed sensor shows that organic gate dielectrics are a low-cost alternative to inorganic gate dielectrics with good electrical performance.

Device Structure and Operation
The cross section of our proposed device, Bottom Gate OFET (BGOFET) is illustrated in Fig. 1 in which organic polymer PANI act as an active conducting layer. In the bottom gate structure, the electrodes can be deposited after the PANI coating. Here organic semiconductor is p-type PANI conducting polymer. Metal (Al) is deposited on source, drain and gate region as electrode [7][8][9][10]. N-type heavily doped silicon is placed between gate electrode and oxide layer, thickness of this silicon layer 0.5 mm and oxide thickness is 2 nm. The insulator can be made from a variety of dielectric materials, even though SiO 2 is a common choice. A voltage is applied to the gate metal to control the amount of current flow from drain to source region. In our proposed OFET, the relatively high hole mobility p-type channel is used. A much higher negative voltage causes a p-type channel to form at the semiconductor-insulator interface. A drain to source negative voltage (V DS ) causing holes to flow from the source to the drain and hence a negative current started from the drain to the source (I DS ). As the magnitude of the drain-source voltage (V DS ) is increased, I DS is enhanced until "pinch-off," at which point the p-channel pinches closed on one side and the drain current saturates at its maximum value [11,12].
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Materials and Instruments
Polyaniline (PANI) was purchased from Sigma-Aldrich which was in liquid form (product number 650013). Figure 2 depicts the chemical structure of PANI and Table 1 shows its specifications in liquid form. The substrates used are commercially available. The thickness of silicon layer is 0.5 mm with resistivity 10-1Ωcm; the thickness of the oxide layer on the Si is 0.5 μm. It is highly Phosphorus-doped Si in (100) orientation. KEITHLEY 2635A system source was used to measure current-voltage characteristics of OFET. Scanning Electron Microscope (FESEM) was used to confirm that deposited thin film grown on the substrate is purely uniform and does not have any other scattered crystals on the thin layer of PANI. The properties of polyaniline in liquid form are enlisted below:

Synthesis of Polyaniline
Polyaniline (PANI) solution was prepared using Sol-gel method. Polyaniline was dissolved in10ml N,N-Dimethylformamide (DMF). Then, the mixture is stirred properly up to 1 h.The colour of the solution is green.  for the oxidation step. This insulator was grown on silicon wafer using wet oxidation and it took 30 min at 1000 °C in a horizontal furnace. In nitrogen, the furnace is set for a 10 min temperature ramp-up. In the case of the wet oxidation after temperature ramp-up the water valve of the furnace should be open and it took 1 h and then again dry oxidation for 30mins. These steps which follow for cleaning, are standard recommended process. After cleaning process, Si wafer should be hydrophobic (water is not staying at surface) in nature. The surface contaminants that may be present on silicon wafer. Before fabrication process, the wafers have to be clean to remove any adhering particles and organic/inorganic impurities. Improper cleaning causes to bio contaminants on the wafer surface, causing unpredictable electrical properties that result in defective or low-quality semiconductor output.

Polyaniline Deposition Using Synthesis Process
Before depositing the semiconductor layer, sample are cleaned with 2-propanol and then the sample were taken out of the solution and dry in a vacuum oven for 2 min. PANI solutions were deposited on dielectric layer i.e. SiO 2 by using spin coating at 4000 rpm for 40 s, as shown in   Fig. 3(c). The channel length is 100 μm and channel width is 2 mm.

Electrode Formation
After the PANI deposition, source and drain electrodes were deposited by Al with thickness of 150 nm over the PANI layer using the thermal evaporation method under a pressure of 4 × 10 −6 mbar, as shown in Fig. 3(d). In the same method, the gate electrode (Al) was thermally evaporated on Si wafer, as shown in Fig. 3(e). Then, inside the Ag paste, three copper wires were placed one by one and heated at 150 °C for 15 min. The whole sensors PANI resistance measured by using digital multimeter. The sensor is a resistive type, and it produces a change in resistance when the sensitive film absorbs or desorbs water vapor. The copper leads are taken for connection to perform the characteristic measurements of OTFT through the multimeter, as depicted in Fig. 4. I-V characteristic of the OFET was measured at different gate voltages (V g ) applied with DC power supply between source and gate in different humidity levels range of 65% RH.

Result and Discussions
The Fig. 5(a) illustrates the drain current I D vs drain voltage V D curve of a p-channel OFET at different gate voltage in various atmospheric environment. The active channel's I D increases with proportion to the negative gate voltage (Vg). The saturation current (I Sat) is determined to be 0.8 μA, while the threshold voltage V Th is 2.2 V at V gs = -1 V. When higher gate voltage (-V G ) is applied, then current flows from source to drain electrodes via holes. The I DS are linear with -V DS under this circumstance, corresponding to the linear region. The transfer characteristic curves of the OFET device generated by dual changing the gate voltage for V DS = -2 V and -5 V are shown in Fig. 5(b). The space charge region under the channel and near the drain region becomes widens as the -V DS increases, finally reaching a pinch-off point condition where the channel width is zero. The Ion/Ioff ratio, transconductance (g m ), and conductance (g d ) are calculated from these curves to be 10 4 , 13.7 X 10 −4 ℧ and 10 −6 ℧ respectively. Figure 6 is an FESEM image displaying the cross-section image of PANI/ SiO 2 thin film on silicon substrate at room temperature. The surface of the thin-film of PANI has grain sizes of few nanometers with an average scale size of 1 μm. FESEM image confirms that deposited thin film grown on the substrate is purely uniform and does not have any other scattered crystals on the thin layer of PANI. Figure 7. shows that resistance of PANI increases in the region A-B and then decreases with the increase of the  where R a represents the resistance in air and R v represents the resistance in vapor in different temperature [ Ref 11]. Figure 8 shows that the PANI based OFET gives better performance at higher RH% in terms of good humidity dependence over wide humidity ranges (45%-75% RH) with good RH (%) = R a − R v ∕R a x 100   long term stability. It is seen that as relative response (relative humidity) is increased from 45% to 75%, response is improved significantly. But beyond that limit relative response is degraded continuously. Temperature decreased when relative humidity is high. However, relative humidity is inversely proportional to temperature as air warms it can hold more moisture so that the percentage of potential humidity decreases. Figure 9(a) shows the drain current shows increasing tendency with the increase of relative humidity up to 75% (at room temperatures 25 °C and V gs = -1 V). However, beyond that limit drain current drastically reduces with increase of relative humidity as evident from Fig. 9(a). Figure 9(b) shows the variation of threshold with the change of the values of 45-75 RH%. This figure also supports the results of Fig. 9(a) reflecting that their fabricated OFET is best usable in the range of RH%. Further, it has been observed that the fabricated device shows good reversibility with increase and decrease in humidity. If RH is beyond that limit 75% relative response is degraded continuously. At higher values of relative humidity temperature decreased. However, relative humidity is inversely proportional to temperature as air warms it can hold more moisture so that the percentage of potential humidity decreases. Moreover, a comparative overview of a few recently reported humidity sensor can be seen in Table 2. We observe that the stability of fabricated OFET acting as humidity sensor is superior compared to other humidity sensors.

Conclusion
In conclusion, it can be mentioned that we have fabricated a PANI/SiO 2 based OFET for using it as a humidity sensor. We have investigated the I D -Vgs, I D -V DS characteristics of the fabricated OFET. In state of varying humidity, we have varied temperature of the system (as relative humidity is inversely proportional to temperature because it has air warmers it can hold moisture). We have also measured relative response also called relative humidity with change of temperature. Hence we have studied the performance of the fabricated OFET as a humidity sensor with I d vs relative humidity in Fig. 9(a) and V th vs. relative humidity in Fig. 9(b). It has been observed that the fabricated OFET shows better response in the RH% range of 45 to 75. Further, the stability of current OFET act as humidity sensor has also investigated and found superior compared to other humidity sensor. The major application areas of this RH sensor is crucial ambient parameter, which can be measured for a wide range of applications such Domestic electric Appliance: Air conditioning system; Industry: Web printing, clean room, web printing, automated systems and Agriculture. The fabricated sensors available in commercially competitive price. However, purchased costly materials and complicated fabrication process to make the sensor competitively high in cost for lab purpose. Otherwise, it can be available in commercially competitive price.
Authors' Contributions Professor Subir kumar Sarkar analyzed data after taking humidity sensor data and guided us to complete the whole work. Anup Das helps in oxidation process. Mandira Biswas has a major contribution in writing the manuscript and after getting the data to plot the figure. All authors read and approved the final manuscript.

Declarations
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