Abstract: 3,4-polyethylenedioxythiophene (PEDT) was prepared by chemical oxidation polymerization and its film was characterized by SEM and infrared absorption spectroscopy. At the same time, it was deposited on an N-channel depletion-type metal-free gate field-effect transistor to form a chemical field-effect transistor (ChemFET) gas sensor with PEDT film replacing the gate metal of MOSFET. Its sensitivity to ammonia and temperature was studied. The results showed that when the ammonia concentration was below 54 ppm, the leakage current of ChemFET decreased with the increase of ammonia concentration, and its change (ΔIDS) was linear with the change of ammonia concentration. At the same time, the leakage current decreased with the increase of negative substrate voltage and decreased with the increase of temperature. Keywords: PEDT; chemical field-effect transistor; ammonia sensor Introduction Field-effect transistor (FET) is an active device that uses electric field to control the conductivity of solid materials. It has the advantages of small size, light weight, energy saving and good thermal stability and is widely used in circuits [1]. However, the application of field-effect transistors in gas sensors has only attracted the interest of many researchers since Lundstrom first reported it [2]. Field-effect transistor gas sensors have emerged in recent years with the development of microelectronic IC technology and MEMS. It replaces the gate metal of the field-effect transistor with a sensitive film. When gas molecules exchange electrons with the thin film, the surface of the thin film is polarized, which changes the surface potential of the semiconductor under the gate, causing surface inversion and conduction of the source and drain current. The sensitive signal can be obtained by detecting the change of the source and drain current [3]. At present, most of the research on the sensitive materials of ChemFET gas sensors focuses on conductive polymers. They have the advantages of easy deposition and various polymer structures, easy modification, and molecular design and synthesis according to functional requirements, and have attracted much attention. 3,4-polyethylenedioxythiophene (PEDT) is a promising conductive polymer that has been widely used in the electronics and other fields, but its application in chemical sensors is rare. In this paper, PEDT is synthesized by chemical oxidation polymerization and used as a sensitive material to prepare an N-channel depletion type ChemFET gas sensor. The ammonia sensitivity and temperature characteristics of the sensor are tested. II. Experiments 1. PEDT Thin Film Preparation and Device Structure Design (1) PEDT Thin Film Preparation PEDT is derived from EDT monomer polymerization, and its polymerization reaction is shown in Figure 1. The PEDT film is prepared by chemical oxidative polymerization. First, acetone and isopropanol are mixed in proportion as the reaction solvent, and then EDT monomer and oxidant are added in proportion. The reaction is completed at 5°C for several minutes. The prepared ChemFET is immersed in the reaction solution, left to stand for 15 minutes, and then taken out and formed at room temperature. (2) Device Structure Design An N-channel depletion-type metal-gate-free field-effect transistor was fabricated on a (100) crystal p-type silicon substrate with a resistivity of 14 W × cm using semiconductor planar process technology. The channel length is 10 μm and the width is 100 μm. The basic structure of the device is shown in Figure 2. According to the literature [8], the quality and thickness of the gate insulating layer directly affect the current input impedance of the device. The thinner the gate insulating layer, the larger the response current. Therefore, combined with the actual process conditions, the thickness of the insulating layer (the total thickness of the SiO2 layer plus the Si3N4 layer) of the chemical field effect transistor is about 50nm. 2. Gas testing device The sensitive mechanism of the chemical field effect transistor gas sensor is to use the penetration or adsorption of gas molecules on the conductive polymer sensitive film to change the work function of the gate sensitive film, thereby affecting the modulation effect of the gate voltage on the channel current, and using the current change to detect the gas concentration [4-7]. The testing device used in the experiment is shown in Figure 3, including gas cylinder, test chamber and Keithley 4200 semiconductor tester, etc. 3 Results and discussion 1. Characterization of PEDT (1) Scanning electron microscopy (SEM) analysis In order to observe the surface morphology of PEDT, we deposited it on a treated quartz substrate and analyzed it with the S-450 scanning electron microscope of Hitachi, Japan. During the film formation process, we studied the influence of the preparation process conditions on the surface morphology of the film. Figure 4 shows the surface morphology of the PEDT film formed under different conditions observed under a scanning electron microscope. It is easy to see from the comparison of A, B, C and D that as the amount of oxidant increases, the surface morphology of the film tends to be smooth and uniform. The number of mesh pores in the film increases rapidly, but the size of the pores decreases continuously. This change in the mesh structure of the film improves the conductivity of the film and increases the conductivity of the PEDT film. (2) Infrared absorption spectrum analysis of PEDT film The infrared absorption spectrum of the conductive polymer PEDT film was determined by FTIR and its main characteristic peaks were assigned, as shown in Figure 5: 685 cm-1 is the absorption peak of the deformation vibration of the thiophene ring, 1509 cm-1 and 1425 cm-1 are the absorption peaks of the stretching vibration of the ring, 1090 cm-1 is the absorption peak of the puzzle bond of the ethylenedioxy group on the thiophene ring, and 1184 cm-1 is the absorption peak of some dopants. Ferric chloride, the oxidant used in the preparation of the PEDT film, also serves as a dopant. It is an electron-accepting p-dopant, thereby increasing the film conductivity. 2. The ammonia sensitivity test was conducted in an environment with 62% RH and a temperature of 15°C. The drain-source voltage (VDS) was fixed at 3V, and the substrate was grounded. The study found that the leakage current of the field-effect transistor (FET) gradually decreased as the ammonia concentration increased from 0 to 54 ppm. The relationship between the change in leakage current of the PEDT film ChemFET and the ammonia concentration is shown in Figure 6. It can be seen from the figure that the change in leakage current is almost linear with the increase of ammonia concentration. To study the effect of substrate voltage on leakage current, different voltages were applied to the substrate of the FET at an ammonia concentration of 36 ppm. The output characteristic curves of the ChemFET are shown in Figure 7. Figure 7 shows that the sensitivity of the PEDT film FET to ammonia is very similar to that of a MOSFET to gate voltage. It can also be seen that the leakage current of the FET decreases with the increase of a negative substrate voltage. 3. Temperature Characteristics To test the effect of temperature on the sensor's sensitivity, the drain-source voltage VDS was fixed at 3V, with the source and substrate grounded. The leakage current of the ChemFET was tested at different temperatures, as shown in Figure 8. The results show that the leakage current gradually decreases as the temperature increases from -20°C to 60°C. Therefore, to reduce temperature interference, a temperature compensation circuit needs to be designed, and this work is currently underway. IV. Conclusion PEDT was prepared using chemical oxidation polymerization, and its thin film was analyzed and characterized. The results show that the film has a dense and uniform network structure, indicating good film formation. A chemical field-effect transistor ammonia sensor was prepared using PEDT as the sensing material, and its gas-sensing characteristics were tested. It was found that the leakage current of the ChemFET decreases with increasing ammonia concentration, and its change (ΔIDS) is linearly related to the change in ammonia concentration. Simultaneously, the leakage current decreases with increasing negative substrate voltage, and temperature has a certain influence on sensor performance. These research results have certain reference value for the research of chemical field-effect transistor sensors. References [1] Zhou Guijiang, Ye Cheng. Organic/polymer field effect transistors [J], Chemical Bulletin: 2002, (4): 227-233 [2] JACovington, JWGardner, D.Briard, NFde Rooij, A polymer gate FET sensor array for detecting organic vapors [J], Sensors and actuators B, 2001 (77): 155-162 [3] Hodge-Miller A, Perkins FK, Peckerar M., et al. Gateless Depletion Mode Field Effect Transistor For Macromolecule Sensing [A]. Proceedings of the 2003 International Symposium- Circuits and Systems, 2003, ISCAS '03 [C], 2003. 918-921 [4] Burgmair M., Eisels I. 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