The drain current fluctuation of ungated AlGaN/GaN high electron mobility transistors (HEMTs) measured in different fluids at a drain-source voltage of 0.5 V was investigated. The HEMTs with metal on the gate region showed good current stability in deionized water, while a large fluctuation in drain current was observed for HEMTs without gate metal. The fluctuation in drain current for the HEMTs without gate metal was observed and calculated as standard deviation from a real-time measurement in air, deionized water, ethanol, dimethyl sulfoxide, ethylene glycol, 1,2-butanediol, and glycerol. At room temperature, the fluctuation in drain current for the HEMTs without gate metal was found to be relevant to the dipole moment and the viscosity of the liquids. A liquid with a larger viscosity showed a smaller fluctuation in drain current. The viscosity-dependent fluctuation of the drain current was ascribed to the Brownian motions of the liquid molecules, which induced a variation in the surface dipole of the gate region. This study uncovers the causes of the fluctuation in drain current of HEMTs in fluids. The results show that the AlGaN/GaN HEMTs may be used as sensors to measure the viscosity of liquids within a certain range of viscosity.

Hydrogen peroxide is a metabolic by-product and a kind of stable reactive oxygen species (ROS). When the ROS levels increase, it may causes several harmful effects of ROS on the cell structure, such as damage of DNA and protein oxidation, and this is known as oxidative stress. The oxidative stress is also the important clinical indicators of cause of aging, Alzheimer disease, and kidney diseases. Now the common test method of oxidative stress is Enzyme-linked immunosorbent assay (ELISA). ELISA needs color giving dyes, complicated preparation of sample, and optical system, so it is very expensive and inconvenient. In our study, we present a high sensitivity hydrogen peroxide sensor with ultra-low detection limit. The sensor can directly test the sample and only need very small amount of sample. And because of the simple structural design and fabrication, the sensor can be used as a cheap, efficient, and portable sensor system. In the future, the novel hydrogen peroxide sensor has various applications in studying oxidative stress and detecting reactive oxygen species for cells. Furthermore, we can combine the PANI sensor with different enzymes to fabricate other highly sensitive sensors to detect diverse materials, such as glucose, lactic acid and cholesterin. In our study, the PANI layer and gold electrodes were deposited on silicon nitride substrate, and the PANI was applied to fabricate a thin film between two electrodes. Then the PANI layer was sultonated by propane sultone and modified with HRP. During the measurement, the sensor was operated at 100mV and different concentrations of hydrogen peroxide citrate buffer solutions (pH=5.4) were dropped on it. The current change was measured when the hydrogen peroxide reacted with the HRP Immobilized PANI thin film. We tested the hydrogen peroxide solution from 0.1 nm to 1mM. The HRP-modified resistive sensors based on n-alkylated polyaniline(PANI) detect hydrogen peroxide in solution with very high sensitivity, ultra-low limit, and short response time. The sensitivity is higher than that of other sensing methods, such as electrochemical sensors or transistor sensors. The detection limit of PANI sensor is 0.7 nM (Figure 1. a, b). It is three orders smaller than that of other common methods with detection limit around 1 μM. To the best of our knowledge, it is the lowest detection limit that has ever been reported. And We combine the hydrogen peroxide sensor with glucose oxidase to build up the glucose sensor. In summary, the hydrogen peroxide sensor can provide a more exact hydrogen peroxide concentration and quick detection. The simple process for the sensor fabrication also allows the sensor to be cheap, disposable and combinable with other sensors to build up sensing system. This work was partially supported by National Science Council grant (No.99B20495A & 101-2221-E-007-102-MY3) and by the research grant (100N2049E1) at National Tsing Hua University.

Free radicals have been demonstrated in related to various human health problems. Among these free radicals, hydroxyl radical is the highest oxidized species. We developed a novel, simple and cheap polyaniline microchip sensor that can detect hydroxyl radical generated from Fenton reaction in real time. The notable current (or conductance) change of sensor was induced when ferrous ion was added, indicating that hydroxyl radical react with polyaniline to cause the current decreased. There are many advantages of polyaniline microchip sensor, including simple fabrication of sensor, easy measurement and portable design. Furthermore, the polyaniline microchip sensor is a potential tool for cancer biomarkers, cholesterol and glucose detection.

In this study, an ultra-sensitive hydrogen peroxide sensor was fabricated by using horseradish peroxidase (HRP)-immobilized conducting polymer, polyaniline (PANI). With the proposed detection mechanism, hydrogen peroxide first oxidizes HRP, which then oxidizes polyaniline, thus resulting in decreased conductivity of the polyaniline thin film. The reduced HRP can be further oxidized by hydrogen peroxide and the cycle of the oxidation/reduction interaction would proceed until all hydrogen peroxide are reacted. It leads to the high sensitivity of the sensor due to the signal contributed from all reacted hydrogen peroxide. The detection limit of the hydrogen peroxide sensor is 0.7 nM. The detecting range of concentration of H 2 O 2 is from 0.7 nM to 1 µM. The simple fabrication for the sensor allows the sensor to be costeffective and disposable. This ultra-sensitive hydrogen peroxide sensor is promising in applications for low concentration hydrogen peroxide detections, such as the reactive oxygen species (ROS) in oxidative stress studies.

Variation of temperature, pressure, and pH values has been demonstrated for affecting the stability of AlGaN/GaN HEMTs sensors. In addition to the factors above mentioned, we found out other factors which are related to the stability of AlGaN/GaN HEMTs sensors. In this study, we found out the current variation of AlGaN/GaN HEMTs without gate metal sensors is inversely proportional to the viscosity of liquids. We also found out that dipole moment of liquids is related to the current variation of AlGaN/GaN HEMTs without gate metal sensors. IntroductionThe III-nitride based devices, such as GaN, AlN and InN, were commonly used for sensor applications at harsh environment. Among various III-nitride based sensors, AlGaN/GaN HEMT based sensors have been demonstrated for gas, chemical, and biosensing applications 1-5 . Furthermore, AlGaN/GaN HEMT sensors have been also demonstrated for temperature and pressure sensing application. The current-voltage response to the temperature and pressure is linear 6, 7 . In addition to the application above mentioned, AlGaN/GaN HEMTs sensors have been demonstrated for measuring pH value and showed linear response to pH value between pH=2 and pH=12 8 . Therefore, the temperature, pressure and pH value are the significant factors in regard to the stability of sensors.