Anatomy and Physiology in Health and Illness

Continuous, real-time monitoring of the level of pH and free chlorine in drinking water is of great importance to public health. However, it is challenging when conventional analytical instruments, such as bulky pH electrodes and expensive free chlorine meters, are used. These instruments have slow response, are difficult to use, prone to interference from operators, and require frequent maintenance. In contrast, microfabricated electrochemical sensors are cheaper, smaller in size, and highly sensitive. Therefore, these sensors are desirable for online monitoring of pH and free chlorine in water. In this review, we discuss different physical configurations of microfabricated sensors. These configurations include potentiometric electrodes, ion-sensitive field-effect transistors, and chemo-resistors/transistors for electrochemical pH sensing. Also, we identified that micro-amperometric sensors are the dominant ones used for free chlorine sensing. We summarized and compared the structure, operation/sensing mechanism, applicable materials, and performance parameters in terms of sensitivity, sensing range, response time and stability of each type of sensor. We observed that novel sensor structures fabricated by solution processing and operated by smart sensing methodologies may be used for developing pH and free chlorine sensors with high performance and low cost. Finally, we highlighted the importance of the concurrent design of materials, fabrication processes, and electronics for future sensors.

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Microfabricated electrochemical pH and free chlorine sensors for water quality monitoring: recent advances and research challenges

Ultra thin NiO nanosheets for high performance hydrogen gas sensor device

This paper presents an overview of semiconductor materials used in gas sensors, their technology, design, and application. Semiconductor materials include metal oxides, conducting polymers, carbon nanotubes, and 2D materials. Metal oxides are most often the first choice due to their ease of fabrication, low cost, high sensitivity, and stability. Some of their disadvantages are low selectivity and high operating temperature. Conducting polymers have the advantage of a low operating temperature and can detect many organic vapors. They are flexible but affected by humidity. Carbon nanotubes are chemically and mechanically stable and are sensitive towards NO and NH3, but need dopants or modifications to sense other gases. Graphene, transition metal chalcogenides, boron nitride, transition metal carbides/nitrides, metal organic frameworks, and metal oxide nanosheets as 2D materials represent gas-sensing materials of the future, especially in medical devices, such as breath sensing. This overview covers the most used semiconducting materials in gas sensing, their synthesis methods and morphology, especially oxide nanostructures, heterostructures, and 2D materials, as well as sensor technology and design, application in advance electronic circuits and systems, and research challenges from the perspective of emerging technologies.

https://www.mdpi.com/1424-8220/20/22/6694/pdf

Semiconductor Gas Sensors: Materials, Technology, Design, and Application

Improved selectivity and low concentration hydrogen gas sensor application of Pd sensitized heterojunction n-ZnO/p-NiO nanostructures

A Pd/AlGaN/GaN heterostructure field-effect transistor (HFET)-type hydrogen gas sensor, based on the sensitization, activation, and electroless plating (EP) deposition processes, is fabricated and studied. Due to the used sensitization and activation approaches, a dense and uniform EP seed layer could be achieved. Good dc and microwave characteristics, including the higher turn-on voltage, lower reverse leakage current, improved thermal stability of drain current, enhanced unity current gain cutoff frequency, and maximum oscillation frequency, are obtained for a 1-m-gate-length device. Moreover, the significant hydrogen gas sensing performance, such as larger drain current variation and higher hydrogen detection sensitivity, are found under 1% and 5 ppm H2/air ambiences, respectively. Consequently, the studied EP-based Pd/AlGaN/GaN HFET gives the promise for high-performance electronic device and hydrogen gas sensor applications.

On an Electroless Plating (EP)-Based Pd/AlGaN/GaN Heterostructure Field-Effect Transistor (HFET)-Type Hydrogen Gas Sensor

Hydrogen sensing characteristics of Pd/SiO2-nanoparticles (NPs)/AlGaN metal-oxide-semiconductor (MOS) diodes

A GaN-based light-emitting diode (LED) with a stair-like ITO layer is studied. The stair-like ITO structure is achieved by using a simple wet etching process. The current distribution could be adjusted by the different thickness of each step in this stair-like ITO layer. The current injected from p-pad is forced to be spread outside by the thicker ITO layer below the p-pad, which exhibits larger parasitic series resistance, instead of flowing downward directly. The current spreading effect could be enhanced. As compared with a conventional LED, at 20 mA, the studied device with a stair-like ITO layer shows 13.8% improvement in light output power. A lower turn-on voltage is also achieved.

Improved current spreading performance of a GaN-based light-emitting diode with a stair-like ITO layer

A GaN-based light-emitting diode (LED) grown on a nanocomb-shaped patterned sapphire substrate (PSS) is fabricated and studied. Nanocomb-shaped patterns are transferred on a sapphire substrate using a well-ordered anodized aluminum oxide (AAO) thin film as a mask for the inductively coupled plasma etching process. This well-ordered AAO thin film with a high aspect ratio is grown on a sapphire substrate by an oxalic acid-based electrochemical system and a three-step anodization. The strain state generated during epitaxial growth could be effectively alleviated by the use of nanocomb-shaped PSS. The treading dislocation density could be reduced. Thus, the enhanced crystalline quality is obtained. In addition, due to the presence of photonic crystal-like air buffer layer, part of reflected photons upward the top side could be scattered by this layer. Therefore, more photons could be extracted outside. Experimentally, at 20 mA, as compared with a conventional LED grown on a planar sapphire substrate, the studied LED grown on a nanocomb-shaped PSS shows 53.8% and 43.7% enhancements in light output power and external quantum efficiency as well as a reduced leakage current.

Implementation of a High-Performance GaN-Based Light-Emitting Diode Grown on a Nanocomb-Shaped Patterned Sapphire Substrate

The temperature-dependent characteristics of meta morphic high electron mobility transistors (MHEMTs) with double and single δ-doped structure are studied and demonstrated. Due to the use of double δ-doped sheets, the current density in the channel layer and two-dimensional electron gas could effectively be increased. The excellent turn-on voltage of 1.18 (0.80) V, max imum drain saturation current of 544 (524) mA/mm, maximum extrinsic transconductance of 361 (312) mS/mm, unity current gain cutoff frequency of 55.06 GHz, and maximum oscillation frequency of 129.17 GHz are obtained at 300 (510) K for a 0.6 × 100 μm2 gate dimension double δ-doped MHEMT. In addition, using wide-bandgap InAlAs Schottky, spacer, and buffer layers, the carrier confinement could significantly be improved at high temperature. Therefore, excellent thermal stability is achieved for double δ-doped MHEMT. The device with a double δ-doped structure exhibits a considerably low temperature coefficient on threshold voltage (∂Vth/∂T) of 0.06 mV/K when the temperature is increased from 330 to 510 K, which is superior to previous reports of related high electron mobility transistors.

Chun-Chia Chen

Papers

On an Electroless Plating (EP)-Based Pd/AlGaN/GaN Heterostructure Field-Effect Transistor (HFET)-Type Hydrogen Gas Sensor

Hydrogen sensing characteristics of Pd/SiO2-nanoparticles (NPs)/AlGaN metal-oxide-semiconductor (MOS) diodes

Improved current spreading performance of a GaN-based light-emitting diode with a stair-like ITO layer

Implementation of a High-Performance GaN-Based Light-Emitting Diode Grown on a Nanocomb-Shaped Patterned Sapphire Substrate

Comprehensive Temperature-Dependent Studies of Metamorphic High Electron Mobility Transistors With Double and Single $\delta$ -Doped Structures