Oxide semiconductor gas sensors are widely used for detecting toxic, explosive, and flammable gases due to their simple structure, cost-effectiveness, and potential integration into compact devices. However, their reliable gas detection is hindered by a longstanding issue known as humidity dependence, wherein the sensor resistance and gas response change significantly in the presence of moisture. This problem has persisted since the inception of oxide semiconductor gas sensors in the 1960s. This paper explores the root causes of humidity dependence in oxide semiconductor gas sensors and presents strategies to address this challenge. Mitigation strategies include functionalizing the gas-sensing material with noble metal/transition metal oxides and rare-earth/rare-earth oxides, as well as implementing a moisture barrier layer to prevent moisture diffusion into the gas-sensing film. Developing oxide semiconductor gas sensors immune to humidity dependence is expected to yield substantial socioeconomic benefits by enabling medical diagnosis, food quality assessment, environmental monitoring, and sensor network establishment.

Direct exposure to toxic and hazardous gases has always been considered as the most pervasive problem worldwide, leading to a gradual increase in the number of asthma patients due to NO_x/SO_x gases inhaling and exposure to 50 ppm formaldehyde gases. Therefore, the development of accurate gas sensors is a key issue for resolving these problems. To address such issues, the development of membranes for selective filtering of target molecules as well as nanocatalyst for enhancing the sensing selectivity is highly crucial. In this review, the research progress for porous membrane materials (e.g. MOFs, and graphene) and nanocatalyst technology for the development of selective and accurate gas sensors will be discussed.

Atmospheric environmental problems have a major impact on human health and lifestyle. In humans, inhalation of nitrogen oxides causes respiratory diseases, such as bronchitis. In this paper, thermal analysis of a gas sensor was carried out to design and fabricate a wearable nylon-yarn gas sensor for the detection of NO_x gas. In the thermal analysis method, the thermal diffusion process was analyzed while operating the sensors at 40 and 60℃ to secure a temperature range that does not cause thermal runaway due to temperature in the operating environment. Thermal diffusion analysis was performed using the COMSOL software. The thermal analysis results could be useful for analyzing gas adsorption and desorption, as well as the design of gas sensors. The thermal energy diffusion rate increased slightly from 10.05 to 10.1 K/mm as the sensor temperature increased from 40 to 60℃. It was concluded that the sensor could be operated in this temperature range without thermal breakdown.

In this study, we produced a light, flexible, wearable gas sensor by depositing MWCNTs (Multi-walled Carbon Nanotubes) into nylon. MWCNTs are widely used as a gas sensor material due to their excellent mechanical, electrical and physical characteristics. We produced a gas sensor to detect NOx gases by depositing nylon yarn in a MWCNT solution. The MWCNT solution was made by mixing 3 mg MWCNT in 5 ml of ethanol. Nylon yarn was placed in the manufactured solution and ultrasonic waves were applied using an ultrasonicator for 3 h, resulting in MCWNT deposition. The MWCNT-deposited nylon yarn was dried at room temperature for 24 h. The MWCNT-thin-film-coated nylon yarn was masked 1 mm apart, and gold was then deposited on the masked nylon yarn to create the gas sensor. The sensor then was installed in a chamber with a controlled atmospheric environment and exposed to NOx gas. The changing signal from the sensor was amplified to analyze its gas detection characteristics.

In particular, gas sensors require characteristics such as high speed, sensitivity, and selectivity. In this study, we fabricated a NO_X gas sensor by using a multi-walled carbon nanotube (MWCNT)/zinc oxide (ZnO) composite film. The fabricated MWCNT/ZnO gas sensor was then treated by a 450℃ temperature process to increase its detection sensitivity for NOx gas. We compared the detection characteristics of a ZnO film gas sensor, MWCNT film gas sensor, and the MWCNT/ZnO composited film gas sensor with and without the heat-treatment process. The fabricated gas sensors were used to detect NO_X gas at different concentrations. The gas sensor absorbed NO_X gas molecules, exhibiting increased sensitivity. The sensitivity of the gas sensor was increased by increasing the gas concentration. Additionally, while changing the temperature inside the chamber for the MWCNT/ZnO composite film gas sensor, we obtained its sensitivity for detecting NO_X gas. Compared with ZnO, the MWCNT film gas sensor is excellent for detecting NO_X gas. From the experimental results, we confirmed the enhanced gas sensor sensing mechanism. The increased effect by electronic interaction between the MWCNT and ZnO films contributes to the improved sensor performance.

In this study, we fabricated an NOX gas sensor using a composite film of multi-walled carbon nanotubes (MWCNT)/zinc oxide (ZnO). Carbon nanotubes (CNTs) show good electronic conductivity and chemical-stability, and zinc oxide (ZnO) is a wide band gap semiconductor with a large exciton binding energy. Gas sensors require characteristics such as high speed, sensitivity, and selectivity. The fabricated gas sensor was used to detect NOX gas at different NOX concentrations. The sensitivity of the gas sensor increased with increasing gas concentrations. Additionally, while changing the temperature inside the chamber containing the MWCNT/ZnO gas sensor, we obtained the sensitivity and normalized responses for detecting NOX gas in comparison to ZnO and MWCNT film gas sensors. From the experimental results, we confirmed that the gas sensor sensing mechanism was enhanced in the composite-film gas-sensor and that the electronic interaction between MWCNT and ZnO contributed to the improved sensor performance.

The power law is very important in gas sensing for the determination of gas concentration. In this study, the resistance of a gas sensor based on poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate＋graphene oxide composite was found to exhibit a power law dependence on hydrogen concentration at 150℃. Experiments were carried out in the gas concentration range of 30~180 ppm at which the sensor showed a sensitivity of 6~9% with a response and recovery time of 30s.

Using a vanadium dioxide (VO₂) source, highly pure and amorphous vanadium oxide (VO) thin films were deposited using an e-beam evaporator at room temperature and high vacuum (<10^-7 Torr). Then, by controlling the post-annealing conditions such as N₂：O₂ pressure ratio and annealing time, we could easily synthesize a homogeneous VO₂ thin film and also mixed-phase VO thin films, including VO₂, V₂O₅, V₃O₇, V₅O₉, and V₆O₁₃. The crystallinity and phase of these were characterized by X-ray diffraction, and the surface morphology by FE-SEM. Moreover, the electrical properties and ethanol sensing measurements of the VO thin films were analyzed as a function of temperature. In general, mixed-phases as a self-doping effect have enhanced electrical properties, with a high carrier density and an enhanced response to ethanol. In summary, we developed an easy, scalable, and reproducible fabrication process for VO thin films that is a promising candidate for many potential electrical and optical applications.

MOS-FET structured gas sensors were manufactured using MWCNTs for application as NOx gas sensors. As the gas sensors need to be heated to facilitate desorption of the gas molecules, heat dispersion plays a key role in boosting the degree of uniformity of molecular desorption. We report the desorption of gas molecules from the sensor at 150℃ for different sensor electrode gaps (30, 60, and 90 μm). The COMSOL analysis program was used to verify the process of heat dispersion. For heat analysis, structure of FET gas sensor modeling was proceeded. In addition, a property value of the material was used for two-dimensional modeling. To ascertain the degree of heat dispersion by FEM, the governing equations were presented as partial differential equations. The heat analysis revealed that although a large electrode gap is advantageous for effective gas adsorption, consideration of the heat dispersion gradient indicated that the optimal electrode gap for the sensor is 60 μm.

Fe₂O₃ is one of the most important metal oxides for gas sensing applications because of its low cost and high stability. It is well-known that the shape, size, and phase of Fe₂O₃ have a significant influence on its sensing properties. Many reports are available in the literature on the use of Fe₂O₃-based sensors for detecting gases, such as NO₂, NH₃, H₂S, H₂, and CO. In this paper, we investigated the gas-sensing performance of a Pt-doped ε-phase Fe₂O₃ gas sensor. Pt-doped Fe₂O₃ nanoparticles were synthesized by a Sol-Gel method. Platinum, known as a catalytic material, was used for improving gas-sensing performance in this research. The gas-response measurement at 300℃ showed that Fe₂O₃ gas sensors doped with 3%Pt are selective for NO₂ gas and exhibita maximum response of 21.23%. The gas-sensing properties proved that Fe₂O₃ could be used as a gas sensor for nitrogen dioxide.

In this study, we fabricated a TFT gas sensor with ZnO nanorods grown by hydrothermal synthesis. The suggested devices were compared with the conventional ZnO film-type TFTs in terms of the gas-response properties and the electrical transfer characteristics. The ZnO seed layer is formed by atomic-layer deposition (ALD), and the precursors for the nanorods are zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine ((CH₂)6N₄). When 15 ppm of NO gas was supplied in a gas chamber at 150°C to analyze the sensing capability of the suggested devices, the sensitivity (S) was 4.5, showing that the nanorod-type devices respond sensitively to the external environment. These results can be explained by X-ray photoelectron spectroscopy (XPS) analysis, which showed that the oxygen deficiency of ZnO nanorods is higher than that of ZnO film, and confirms that the ZnO nanorod-type TFTs are advantageous for the fabrication of high-performance gas sensors.

In this work, one dimension In₂O₃ nanostructures as detecting materials for indoor toxic gases were synthesized by an electrospinning process. The morphology of electrospun In₂O₃ nanofibers was controlled by electrolyte composition, applied voltage and working distance between a nozzle and a substrate. The synthesized In₂O₃ nanofibers-based paste with/without carbon black additives was prepared for the integration on a sensor device. The integration of In₂O₃ sensing materials was conducted by a hand-printing of the paste into the interdigit Au electrodes patterned on Si wafer. Gas sensing properties on CO and HCHO gases were characterized at 300℃. The evaluated sensing properties such as sensitivity, response time and recovery time were improved in In₂O₃ nanofiber pastes with carbon black, compared to the paste without carbon black.

In this paper, we have studied about the optimum fabrication condition of the printed Indium Tin Oxide (ITO) layers for the electrical resistance-type sensor application. We have investigated on the substrates surface treatments, mixing ratio of organic binder/ITO powder, and viscosity of the printing paste to determine the optimum condition of the screen printed ITO layer. Also, we found that the printing condition is closely related with the sensor performance. To know the feasibility of printed ITO layer as an electrical resistance-type sensor, we have fabricated the ITO sensors with a printed and sputtered ITO layers. The printed ITO films revealed 102 times higher sensitivity than the sputtered ITO layer. Also, the sputtered ITO layer exhibited an operating temperature of 127℃ at the operating voltage of 5 V. While, in case of the printed ITO layer showed the operating temperature of 27.6℃ in high operating voltage of 30 V. We found that the printed ITO layer is suitable for the various sensor applications.

In this study, we fabricated NOX gas sensor by using multi-walled carbon nanotubes(MWCNT)/zinc oxide(ZnO) composite film. Carbon nanotubes (CNTs) have good electronic, chemical-stability, and sensitivity characteristics. And zinc oxide (ZnO) is a wide band gap and large exciton binding energy semiconductor. In particular, gas sensors require characteristics such as high speed, sensitivity, and selectivity. The fabricated gas sensor was used to detect NOX gas for different values of the NOX gas concentrations. The gas sensor that absorbed NOX gas molecules showed a increasing in resistance. The sensitivity of the gas sensor was increased by increasing the gas concentrations. Additionally, while changing the temperature inside the chamber for the MWCNT/ZnO composite film gas sensor, we obtained the sensitivity. And the comparison analysis to ZnO film gas sensor for detecting NOX gas. From the experiment result, we confirmed improvement of NOX gas detection characteristics using the MWCNT/ZnO composite film.

Generally, MWCNT, with thermal, chemical and electrical superiority, is manufactured with CVD (Chemical Vapor Deposition). Using MWCNT, it is comonly used as gas sensor of MOS-FET structure. In this study, in order to repeatedly detect gases, the author had to effectively eliminate gases absorbed in a MWCNT sensor. So as to eliminate gases absorbed in a MWCNT sesor, the sensor was applied heat of 423[K], and in order to observe how the applied heat was diffused within the sensor, the author interpreted the diffusion process of heat, using COMSOL interpretation program. In order to interpret the diffusion process of heat, the author progressed modeling with the structure of MWCNT gas sensor in 2-dimension, and defining heat transfer velocity(u=△T/△χ), accorded to governing equation within the sensor, the author proposed heat transfer mechanism.

Carbon nanotubes(CNT) has chemical stability and great sensitivity characteristics. In particular, the gassensor required characteristics such as rapid, selectivity and sensitivity sensor. Therefore, CNT are ideal materialsto gas sensor. So, we fabricated the NOx gas sensors of MOS-FET type using the MWCNT (multi-walled carbonnanotube). The fabricated sensor was used to detect the NOx gas for the variation of Vgs (gate-source voltage)and electrode changed electrode spacing=30, 60, 90[㎛]. The gas sensor absorbed with the NOx gas moleculesshowed the decrease of resistance, and the sensitivity of sensor was increased by magnification of electrodespacing. Furthermore, when the voltage(Vgs) was applied to the gas sensor, the decrease in resistance wasincreased. On the other hand, the sensor sensitivity for the injection of NOx gas was the highest value at theelectrode spacing 90[㎛]. We also obtained the adsorption energy(Vgs) using the Arrhenius plots by the reduction ofresistance due to the voltage variations. As a result, we obtained that the adsorption energy was increased withthe increment of the applied voltages.

Carbon nanotubes (CNT) has the excellent physical characteristics in the sensor, medicine, manufacturing and energy fields, and it has been studied in those fields for the several years. We fabricated the NOx gas sensors of MOS-FET type using the MWCNT. The fabricated sensor was used to detect the NOx gas for the variation of Vgs (gate-source voltage) with the ambient temperature. The gas sensor absorbed the NOx gas molecules showed the decrease of resistance, and the sensitivity of sensor was reduced by the NOx gas molecules accumulated on the MWCNT surface. Furthermore, when the voltage (Vgs) was applied to the gas sensor, the term of the decrease in resistance was increased. On the other hand, the sensor sensitivity for the injection of NOx gas was the highest value at the ambient temperature of 40℃. We also obtained the adsorption energy (Vgs)using the Arrhenius plots by the reduction of resistance due to the Vgs voltage variations. As a result, we obtained that the adsorption energy also was increased with the increasement of the applied Vgs voltages.

Carbon nanotubes(CNT) has strength and chemical stability, greatly conductivity characteristics. In particular, MWCNT (multi-walled carbon nanotubes) show rapidly resistance sensitive for changes in the ambient gas, and therefore they are ideal materials to gas sensor. So, we fabricated NOx gas sensors structured MOS-FET using MWCNT (multi-walled carbon nanotubes) material. We investigate the change resistance of NOx gas sensors based on MOS-FET with ultra lean NOx gas concentrations absorption. And NOx gas sensors show sensitivity on the change of gate-source voltage (V98 =0[V] or V98=3.5[V]). The gas sensors show the increase of sensitivity with increasing the temperature (largest value at 40℃). On the other hand, the sensitivity of sensors decreased with increasing of NOx gas concentration. In addition, We obtained the adsorption energy(Ua), Ua = 0.06714[eV] at the NOx gas concentration of 8[ppm], Ua = 0.06769[eV] at 16[ppm], Ua = 0.06847[eV] at 24[ppm] and Ua = 0.06842[eV] at 32[ppm], of NOx gas molecules concentration on the MWCNT gas sensors surface with using the Arrhenius plots. As a result, the saturation phenomena is occurred by NOx gas injection of concentration for 32[ppm].

ZnO nanowires on the a-, c- and m-plane oriented 4H-SiC substrates were grown by using a high temperature tube furnace, Ti/Au electrodes were deposited on ZnO nanowires and a-, c- and m-plane 4H-SiC substrates, respectively. The shape and density of the ZnO nanowires were inestigated by field emission scanning electron microscope. It was found that the growth direction of nanowires depends strongly on growth parameters such as growth temperature and pressure. In this work, The sensitivity of nanowires formed a-, c- and m-plane oriented 4H-SiC gas sensor was measured at 300℃ with CO gas concentration of 80%. The nanowires grown on a-plane oriented 4H-SiC show improved performance than those on c- and m-plane oriented 4H-SiC due to the increased density of nanowire on a plane 4H-SiC.

Carbon nanotubes(CNT) have excellent electrical, chemical stability and mechanical properties. These can be used in a variety of fields. MWCNT are extremely sensitive for minute changes in the ambient gas, namely, their sensing properties varies greatly with the absorption of gas such as NOx and H2. We investigate the electrical properties of CNTs and make a NOx gas sensor based on Multi-walled carbon nanotubes (MWCNT) materials. We obtained the NOx gas sensor of MWCNT based on P-type Si wafer that has the resistivity of 1.667×10-1 [Ω·cm]. We knew that the sensitivity of sensor decreased with increasing of NOx gas concentration. And the sensitivity of sensor shows the largest value at 20℃. The sensitivity of sensor decrease with increasing the temperature. Also absorption energy of NOx gas molecule on the MWCNT surface decreases with increasing concentration of NOx gas.

SnO2 nanoparticles were synthesized by flame spray pyrolysis, which were directly deposited on Pt interdigitated substrates. Gas sensing performance was evaluated for various gases such as H2, CO, H2S, and NH3, and it was compared with that of commercial SnO2 nanopowder. The synthesis of SnO2 nanoparticles was also conducted in various solvents. As a result, the primary particle size was changed with the solvent of precursor solution, and their H2 sensing properties were significantly affected.

Embossed TiO2 thin films with high surface areas are achieved using soft-templates composed of monolayer polystyrene beads. The form of links between the beads in the templates is controlled by varying the O2 plasma etching time on the templates, resulting in various templates with close-linked, nano-linked, and isolated beads. Room-temperature deposition of TiO2 on the plasma-treated templates and calcination at 550℃ result in embossed films with tailored links between anatase TiO2 hollow hemispheres. Although all the embossed films have similar surface areas, the sensitivity of films with nano-linked TiO2 hollow hemispheres to 500 ppm CO and ethanol gases are much higher than that of films with close-linked and isolated hollow hemispheres, and the detection limits of them are as low as 0.6 ppm for CO and 0.1 ppm for ethanol. The strong correlation of sensitivity with the form of links between hollow hemispheres reveals the critical role of potential barriers formed at the links. The facile, large-scale, and on-chip fabrication of embossed TiO2 films with nano-linked hollow hemispheres on Si substrate and the high sensitivity without the aid of additives give us a sustainable competitive advantage over various methods for the fabrication of highly sensitive TiO2-based sensors.

Multifunctional structures with two kinds of materials have been intensively investigated in order to improve their electrical characteristic with two functions simultaneously. However, the research regarding of multifunctional ceramic sensor is still in a preliminary stage and how to integrate them with low-cost and high-yield mass production process remains a challenge issue. In this study, we fabricated the multifunctional ceramic sensor composed of temperature and gas sensors. Moreover, we investigated the CO sensing properties of three dimensional nanostuctured Nb2O5 thin film gas sensors fabricated with silica (SiO2) nanosphere (Ø= 750 nm). Compared to plain films, the nanostructured films show enhanced gas sensing of greater sensitivity and a faster response. This result reveals that significantly increased sensitivity is an increase in the effective surface area for the adsorption of gas molecules.

ZnO thin films were deposited on p-type 4H-SiC substrate by pulsed laser deposition. ZnO nanowires were formed on p-type 4H-SiC substrate by furnace. Ti/Au electrodes were deposited on ZnO thin film/SiC and ZnO nanowire/SiC structures, respectively. Structural and crystallographical properties of the fabricated ZnO thin film/SiC and ZnO nanowire/SiC structures were investigated by field emission scanning electron microscope and X-ray diffraction. In this work, resistance and sensitivity of ZnO thin film/SiC gas sensor and ZnO nanowire/SiC gas sensor were measured at 300℃ with various CO gas concentrations (0%, 90%, 70%, and 50%). Resistance of gas sensor decreases at CO gas atmosphere. Sensitivity of ZnO nanowire/SiC gas sensor is twice as big as sensitivity of ZnO thin film/SiC gas sensor.

We have investigated the sensing properties of ethanol gas sensor with pure ZnO and Ga-doped ZnO nanowires on Au coated (0001) sapphire substrates grown by hot walled pulsed laser deposition. Randomly aligned ZnO nanowires arrays were grown on a Au-electrode patterned under ambient conditions. ZnO nanowires have various sizes and shapes with a different substrate position inside a furnace. The average of length and diameter of the ZnO nanowires were 8 ㎛ and 100 ㎚ respectively, and confirmed by field emission scanning electron microscopy. Sensitivity chanege characterization of the gas sensor was found that measured sensitivities of the ethanol gas sensors were 83.3% and 68.3% at 300℃ respectively.

Carbon nanotubes (CNTs) have excellent electrical, chemical stability, mechanical and thermal properties. In this paper, networks of Multi-walled carbon nanotube (MWCNT) materials were investigated as a resistive gas sensors for the H2 gas detection. Sensor films were fabricated by the air spray method using the multi-walled CNTs dispersion solution on the glass substrates cured with plasma and nitrocellulose. Sensors were characterized by the resistance measurements in the self-fabricated oven in order to find the optimum detection properties for the hydrogen gas molecular. The sensitivity and the linearity of the MWVNT sensors using the glass substrate cured with plasma for the H2 gas concentration of 0.06∼0.6 ppm are 0.013∼0.097%/sec and 0.131∼0.959%FS, respectively. The MWCNT film was excellent in the response for the hydrogen gas moleculars and its reaction speed was very fast, which could be using as hydrogen gas sensor. The resistance of the fabricated sensors decreases when the sensors are exposed to H2 gas.

A novel design of gas sensor using Ga-doped ZnO (GZO) thin films which are deposited on low temperature co-fired ceramic (LTCC) substrates is presented. The LTCC substrates with thickness of 400 μm are fabricated by laminating 12 green tapes which consist of alumina and glass particle in an organic binder. The GZO thin films with different thickness are deposited on LTCC substrates, by RF magnetron sputtering method. The microstructure and sensing properties of GZO gas sensing films are analyzed as a function of the film thickness. The films are well crystallized in the hexagonal (wurzite) structure with increasing thickness. The maximum sensitivity of 3.49 is obtained at 100 nm film thickness and the fastest 90% response time of 27.2 sec is obtained at 50 nm film thickness for the operating temperature of 400oC to the NO2 gas.