Search

We report a latent fluorescence probe based on 2-hydroxycinnamate for the selective and senstive detection of cyanide. The probe underwent a rapid cyanide-catalyzed cinamate-to-coumarin transformation reaction and displayed strong fluorescence selectively to the cyanide anion over other various anions with a micromolar limit of detection (LOD = 1.3 μM) in 5% water-acetonitrile solvent, which was easily detectable under a portable UV spectroscope. [ABSTRACT FROM AUTHOR]

Poly-silicon has a wide range of applications because of its interesting properties in the thermoelectric effect. We propose an optimal field-effect transistor-type (FET-type) gas sensor platform with an embedded poly-silicon micro-heater designed considering both power consumption and reliability. Since the poly-silicon micro-heater has a thermoelectric effect, the reliability of the micro-heater can be improved without a significant increase in power consumption by adopting asymmetric etch-holes (finally asymmetric air-gap) along the heater electrode. The sensor with the shorter etch-hole (10 µm) around one electrode of the heater electrode increases power consumption by only 1.22 times but can increase lifetime by 14.2 times. The heat distribution and contact temperature of symmetric and asymmetric air-gap heaters are compared using COMSOL Multiphysics. Sensors with different lengths of etch-holes are fabricated, and the thermal response and reliability of the fabricated sensors are measured and analyzed. In addition, in a sensor in which a thin film of indium oxide is deposited as a sensing material on the gas sensor platform, the effect of air-gap design change on the gas sensing characteristics is investigated. Depending on the polarity of the voltage applied between the two terminals of the heater, the difference in void formation and damage at the heater contacts is confirmed by TEM and EDS analysis. • FET-type gas sensors with different lengths of etch-holes are fabricated. • The electrical and thermal characteristics of fabricated sensors are investigated. • The lifetime of the micro-heater depending on the direction of current flow is investigated. • The gas sensor with an asymmetric air-gap heater shows improved reliability. [ABSTRACT FROM AUTHOR]

Semiconductor-based gas sensors have been applied to a variety of applications, including environmental, safety, and health monitoring. Extensive efforts have been made to improve sensing performance outcomes, with the majority of these efforts focusing on the sensor response, sensitivity, and selectivity issues, mainly by optimizing the sensing materials and sensor structures. However, low-frequency noise (LFN), which has a considerable impact on the stability and reliability of sensors, has received far less attention in gas sensor research. In gas sensing applications, the noise in the sensing signal is determined by the LFN due to the slow reaction process. Thus, it is necessary to characterize the LFN in semiconductor-based gas sensors. This review article presents an overview of the LFN in semiconductor-based gas sensors. First, the history of LFN in gas sensor studies is explored briefly. Then, we discuss noise generation mechanisms in resistor-type, thin-film transistor-type, and horizontal floating-gate field-effect-transistor-type gas sensors. On the basis of this information, the signal-to-noise ratio, which determines the limit of detection, is examined, and the method to optimize the SNR in each sensor platform is discussed. Finally, LFN spectroscopy for selective gas detection is introduced, and its working principle is analyzed. This review article provides a foundation for understanding the LFN in semiconductor-based gas sensors and methods to control it based on application requirements. [ABSTRACT FROM AUTHOR]

The design of the FET-type gas sensor affects various sensor performance factors such as sensitivity, noise, and power consumption. However, few studies comprehensively consider the impact of sensor design on the factors. Here, we show how the design of FET-type gas sensors influences a variety of performance factors and provide design guidelines for FET-type gas sensors. Sensors with several sensing material areas and FET channel areas are fabricated, and fabricated sensors are modeled using Technology Computer-Aided Design (TCAD) simulation to obtain the coupling ratio between the sensing material and FET channel and to investigate the gas response mechanism of the FET-type sensor. As the area of the sensing material increases, the sensitivity of the sensor increases due to the increase in the coupling ratio. Meanwhile, the area to be heated by the micro-heater also increases, thereby increasing power consumption. As the FET channel area decreases, the coupling ratio increases, resulting in better sensitivity, but the noise of the sensor increases. Therefore, it is desirable to design the FET-type gas sensor using sensitivity/power and signal-to-noise ratio (SNR) as indicators in consideration of sensitivity, power consumption, and noise. A FET-type gas sensor can be optimized by considering the performance factors of trade-offs, setting an optimization indicator suitable for the application, and then selecting design parameters that maximize the indicator. As a proof of concept, we show processes to optimize the horizontal floating-gate FET (HFGFET)-type gas sensor using Δ I D /power, SNR, SNR/power, and (SNR/power)/size as optimization indicators. • The effect of the design of FET-type gas sensors on the sensitivity is examined. • Device and gas sensing characteristics are analyzed using TCAD device simulation. • The response can be expressed using design factors of FET-type gas sensor. • Device characteristics, sensitivity, power, noise, and SNR are compared. • Design guidelines for FET-type gas sensors are presented. [ABSTRACT FROM AUTHOR]

The focus of gas sensor research has been on improving the response of individual sensors. However, in order to integrate sensors with CMOS circuits, a sensing system composed of an amplifier circuit with a voltage output is required. As such, in this work, we propose a novel gas sensing amplifier circuit composed of resistor- and FET-type gas sensors. Indium-gallium-zinc oxide (IGZO) is used as a sensing material for the detection of hydrogen sulfide (H 2 S) gas. The H 2 S gas sensing mechanism of the amplifier circuit is determined by the interplay between the resistor- and FET-type gas sensors. Also, the low-frequency noise characteristics of the resistor- and FET-type gas sensors are analyzed using carrier number fluctuation and Hooge's mobility fluctuation models, respectively. The signal-to-noise ratio of the amplifier circuit is accurately characterized as a function of the input voltage. The optimal operating bias condition is proposed, and the limit of detection of the amplifier circuit is obtained under this condition. The methodology demonstrated in this work can be applied to other types of amplifier circuits, contributing to the advancement of knowledge about integrated gas sensing systems. • A novel gas sensing amplifier circuit composed of resistor- and FET-type gas sensors is proposed. • The signal-to-noise ratio of the amplifier circuit is accurately characterized as a function of the input voltage. • The SNR can be modulated by tuning the V in , and is maximized at the V in 0.53 V where the Δ V out is also the largest. • The sensor can detect 29 ppb of H 2 S with optimized SNR performance. [ABSTRACT FROM AUTHOR]

Gas sensors with micro-heaters are being widely studied in various fields. However, there are few systematic studies examining the effect of embedded heaters on their reliability and gas detection features. In this work, we investigate the change in electrical characteristics of the heater that occurs when thermal stress and current stress are applied to the embedded micro-heater for the FET-type gas sensor. Four different patterns of micro-heaters exhibited the same resistance change. Also, we investigate the effect of the increase in contact resistance between the poly-silicon heater and the metal (Cr/Au) electrodes on the gas sensing characteristics. As the heater resistance increases by 12%, the power consumption for optimal gas response increases by 58%, and the gas response is reduced by 46% without compensation of the heater voltage. The result indicates that the heating efficiency of the poly-silicon heater also changes during the degradation of the heater. The reason for the increase in the heater resistance and the change in the heating efficiency is explained by the inter-diffusion of Cr atoms to the surface through the Au layer. The diffusion of Cr atoms is confirmed by TEM and EDS analysis in the contacts at both ends of the heater. The results suggest that the temperature at the same voltage applied across the heater should also be considered in the reliability of the sensor platform. • The contact resistance of the heater increases when thermal stress or current stress is applied to the heater. • The increase in heater resistance comes from an increase in contact resistance. • The heating efficiency of the gas sensor platform is degraded as current stress is applied to the heater. • The chromium oxide layer is found on the top of the contact area. [ABSTRACT FROM AUTHOR]

Sensing performances of the Si-based field-effect transistor (FET)-type gas sensor are not only affected by sensing material characteristics but also by electrical properties of a transducer. Therefore, the optimization of transducer properties, including subthreshold swing, transconductance, and low-frequency noise (LFN) characteristics, is necessary for improving the sensing performances. In this paper, NO 2 gas sensing properties, LFN characteristics, and signal-to-noise ratio (SNR) of the FET-type gas sensor having different channel structures (surface and buried channel FETs) are investigated. An n -type indium-gallium-zinc oxide (IGZO) thin-film is used as a sensing layer. The LFN characteristics of both sensors are explained using a carrier number fluctuation model with correlated mobility fluctuation. The flat-band voltage fluctuation (S Vfb) value of the buried channel (4.54 × 10−10 V2/Hz) is smaller than that of the surface channel (2.73 × 10−9 V2/Hz). Thus, the SNR of the sensor with a buried channel shows ~10 times larger SNR than that with a surface channel. Also, the optimal bias conditions for both sensors are suggested. The sensor with buried channel FET has a limit of detection (LOD) of 44.2 ppt to NO 2 gas. • The NO 2 gas sensing performances are investigated in the FET-type gas sensors with surface and buried channel structures. • The LFN characteristics of the sensors are analyzed using CNF model with CMF. • The sensor with buried channel FET has a limit of detection (LOD) of 44.2 ppt to NO 2 gas. [ABSTRACT FROM AUTHOR]

• Effects of IGZO film thickness on the H 2 S gas sensing performance in resistor- and FET-type gas sensors are investigated. • A transition of the dominant sensing area from the surface to the bulk is observed with increasing film thickness. • Excessive recovery is observed and its detailed mechanism is analyzed for the first time. • The SNR of the FET-type gas sensor is the largest when the IGZO film thickness is 60 nm. In this work, we investigate the effects of IGZO film thickness on the H 2 S gas sensing performance, such as response, excess recovery, low-frequency noise (LFN), and signal-to-noise ratio (SNR) in the resistor- and FET-type gas sensors. A transition of the dominant sensing area from the surface to the bulk of the IGZO film is observed with increasing film thickness. Also, excessive recovery is observed and its detailed mechanism is analyzed for the first time. The resistor-type gas sensor with thicker IGZO film has a smaller 1/ f noise due to the decreased trap density and impurity scattering, which guarantees the largest SNR and the lowest limit of detection (LOD). In the case of the FET-type gas sensor, the thicker film with high porosity allows H 2 S gas to diffuse more easily into the IGZO-O/N/O interface and increases the response. Also, the LFN characteristics of the FET-type gas sensor have no dependence on the IGZO film thickness. Thus, the SNR of the FET-type gas sensor is the largest when the IGZO film thickness is 60 nm. [ABSTRACT FROM AUTHOR]

• Humidity sensing characteristics of the FET-type humidity sensor with GQDs sensing layer are analyzed. • A GQDs sensing layer is deposited by ink-jet printing method. • The response and recovery characteristics of the FET-type sensor could be maximized by adopting the pre-bias scheme. • The humidity sensor has highly stable and reproducible characteristics in long-term measurements for 5 months. This paper investigates humidity sensing characteristics of a silicon metal oxide semiconductor field effect transistor (Si MOSFET)-based humidity sensor having horizontal floating-gate (FG) interdigitated with control-gate (CG). The sensing material of the humidity sensor is graphene quantum dots (GQDs), deposited locally on the interdigitated CG-FG area by an ink-jet printing method using a small amount of the GQDs solution. The humidity sensing characteristics of the sensor are measured as a parameter of relative humidity (RH). The response of the humidity sensor is 78 % to the humid air of 81.3 % RH. We also adopt a pulsed pre-bias method to improve the response and recovery characteristics of the humidity sensor. The response and recovery characteristics of the sensor can be improved 30 % and 40 % respectively by applying a pre-bias of 2 V and -1 V to the CG. In all relative humidity ranges, the FET-type humidity sensor has highly stable and reproducible characteristics in long-term measurements for 5 months. [ABSTRACT FROM AUTHOR]

• The LFN characteristics, response, SNR, LOD of gas sensors using three different transducers are compared. • For a fair comparison, the MOSFET, resistor, and TFT-type gas sensors are fabricated on the same substrate. • The response is the smallest in the MOSFET-type gas sensor and the largest in the TFT-type gas sensor. • The LODs for NO 2 gas of TFT-, MOSFET- and resistor-type gas sensors are 0.017, 0.300, and 26.3 ppb respectively. The semiconductor-type gas sensor consists of a transducer and a sensing material. Most of the research on the sensor has focused on sensing materials. In this study, the characteristics of gas sensors using three different transducers (MOSFET, resistor, and TFT) are compared. For a fair comparison, the three types of gas sensors are fabricated through the same process on one chip and have the same sensing material. Indium oxide (In 2 O 3) and NO 2 gas are used as the sensing material and the target gas, respectively. The low-frequency noise (LFN) characteristics, response, signal to noise ratio (SNR), and limit of detection (LOD) of the three types of gas sensors are investigated and compared. Since the MOSFET-type gas sensor uses crystalline Si as the current path, the current normalized noise of the sensor is ∼105 smaller than that of the resistor- and TFT-type gas sensors using polycrystalline In 2 O 3 as the current path. However, when reacting to 500 ppb NO 2 gas, the response is the smallest in the MOSFET-type gas sensor and the largest in the TFT-type gas sensor (MOSFET: ∼2.3 × 102, resistor: 3.3 × 103 and TFT: ∼3.2 × 107). The SNR is large in order of TFT-, MOSFET-, resistor-type gas sensor over the entire range of measured gas concentrations (20 ∼ 500 ppb). The LODs of TFT-, MOSFET- and resistor-type gas sensors are 0.017, 0.300, and 26.3 ppb respectively. Considering the response, SNR and LOD, the TFT-type gas sensor is the best among the three-types of sensors. [ABSTRACT FROM AUTHOR]

• A FET-type gas sensor platform with low power consumption is proposed. • The platform offers the best power efficiency (e.g. 300 ℃ at 1.63 mW) among the reported gas sensors with external heaters. • Electrode metals with low thermal conductivity and an efficient heat insulating air gap design improve power efficiency. • The platform is sensitive and advantageous for low-concentration gas detection. Gas sensors are commercialized and widely applied in various fields. Existing commercial sensors, however, have limitations in terms of power consumption. In this work, we propose a new design guideline of the FET-type gas sensor platform to significantly improve the power efficiency of the embedded poly-Si micro-heater. The fabricated sensors are capable of low power operation while maintaining high response and stability. To our knowledge, the embedded micro-heater of the proposed platform provides the best power efficiency (e.g. 300 ℃ at 1.63 mW) among reported gas sensors with external heaters. By adopting electrode metals with low thermal conductivity (Ti/TiN instead of Cr) in contact with the heater and an efficient heat insulating air gap design (increasing air gap length under the electrode from 0 μm to 28 μm), we can increase the heater temperature by 63 % at the same power consumption. Also, the proposed platform is more sensitive and advantageous for low-concentration gas detection compared to the reported FET-type platform using hot chuck that increases the temperature of the entire sensor. [ABSTRACT FROM AUTHOR]

Gas sensing technology is becoming increasingly important in environmental monitoring, medical diagnostics, and industrial safety. Among various types of gas sensors, FET-type gas sensors are increasingly attracting attention due to their miniaturized size, low power consumption, high reliability, and good compatibility with CMOS technology. Therefore, it would be helpful to review how FET-type gas sensors have evolved and how recent trends have been. In this paper, we review the technologies of FET-type gas sensors, focusing on TFT, catalytic metal-gate FET, suspended-gate FET, capacitively-coupled FET, and horizontal floating-gate FET gas sensors. The device structure and operating principle of each type of FET-type gas sensor are explained and then the advantages and disadvantages are discussed. We also review their sensing properties such as sensitivity, selectivity and response/recovery speeds, and the methods to enhance their sensing performances. Finally, we introduce a pre-bias scheme that effectively improves the sensing characteristics of the FET-type gas sensor and its effects, and discuss the key characteristics and development direction of the FET-type gas sensor embedded with a practical low-power micro-heater. [ABSTRACT FROM AUTHOR]

• Effects of body-to-source junction bias on SNR in the FET-type gas sensor with a localized micro-heater are investigated. • Sensing material, a 12 nm-thick n -type semiconducting In 2 O 3 , is prepared through radio frequency magnetron sputtering. • When a forward body-to-source junction bias of 0.5 V is applied to the sensor, the 1/ f noise decreases by the factor of ∼10. • Limit of detection of the gas sensor for NO 2 gas can be improved from 0.55 ppb to 0.27 ppb by using a forward body-to-source junction bias of 0.5 V. This paper investigates the effects of body-to-source junction bias (V BS) on signal-to-noise-ratio (SNR) of Si metal-oxide-silicon field effect transistor (FET)-type gas sensor with a localized embedded micro-heater. Sensing material, 12 nm-thick n -type semiconducting In 2 O 3 , is prepared through radio frequency magnetron sputtering. NO 2 is used as a test gas to investigate the gas sensing characteristics of the sensor. By using the embedded micro-heater, the temperature of the sensing area can be raised to an extent required for the gas reaction (150 °C) while maintaining the temperature of the FET channel at 27 °C. The 1/ f noise of the FET-type gas sensor is dominated by the carrier number fluctuation (CNF) in all operation regions. The effects of V BS on low-frequency noise (LFN) characteristics are demonstrated by the CNF model with correlated carrier mobility fluctuation. When a forward V BS of 0.5 V is applied to the sensor, the 1/ f noise decreases by the factor of ∼10 and the SNR of the sensor increases by ∼2 times compared to when a V BS of 0 V is applied. It is shown that the limit of detection (LOD) of the gas sensor for NO 2 gas can be improved from 0.55 ppb to 0.27 ppb by applying a forward V BS of 0.5 V. [ABSTRACT FROM AUTHOR]