High Electron Mobility Transistor Sensor Research Articles

Application of a Discrete AlGaN/GaN HEMT Sensor for in‐situ Regenerative Soot Particulate Detection

AbstractAn in‐situ regenerative soot particulate matter sensor for automotive exhaust systems based on a discrete AlGaN/GaN high electron mobility transistor (HEMT) was developed and tested, where charge‐sensitive AlGaN/GaN HEMT was utilized to detect the signal variations caused by the presence of soot particulate matters, and the micro‐hotplate serves as soot deposition, while the self‐heating unit was used for sensing and in‐situ regeneration. The two parts were connected for electrical signal transmission. Soot particulate were generated by a laminar diesel flame with an adsorption rate of 23.29 μg cm−2 min−1 on the sensor surface, resulting in 1.32% sensing response after 20 s deposition and reached saturation after a 12 minute deposition of soot. After soot deposition, the sensor was successfully regenerated when a voltage of 2 V was applied to the micro‐hot plate, causing in‐situ heat‐up to reach 600°C for 30 seconds. Moreover, benefiting from the advantage of discrete structure, the sensor exhibited consistent and stable response characteristics even after undergoing more than 10 consecutive regeneration cycles, and the replaceable micro‐hotplate could further extend the service life of the sensor. These results show that the discrete AlGaN/GaN HEMT sensor can be a promising soot particulate sensing platform for automotive exhaust systems.

Advanced Potentiometric Configuration to Enhance Stability and Reliability of AlGaN/GaN HEMT-Based Water-Gated pH Sensor

High-electron mobility transistor (HEMT) sensors show great promise for achieving direct, real-time, and label-free detection. Leveraging the distinctive characteristics of the two-dimensional electron gas (2DEG), HEMT sensors can amplify current fluctuations related to potential changes when molecules are introduced, making them extremely sensitive to surface charge variations. The introduction of a water-gated HEMT (WGHEMT) sensor represents a significant advancement, enabling precise detection of pH levels across a broad spectrum, from acidic pH 2 to basic pH 12. Comprising an inner and outer pool with an ion-selective membrane situated in between, the WGHEMT pH sensor exhibits excellent voltage and current sensitivity of 43.71 mV pH−1 and 144.42 μA pH−1, respectively. It features a rapid response time of 12 s and demonstrates remarkable stability and reliability, with coefficients of variation (C.V.) of 0.29%, 0.12%, 0.26%, 0.24%, 0.28%, and 0.28% for pH 2, pH 4, pH 6, pH 8, pH 10, and pH 12, respectively. The sensor maintains its performance with only 10% degradation after 5 months of repeated measurements and exposure to various pH levels. The WGHEMT pH sensor’s excellent sensitivity, linearity, hysteresis, rapid response time, stability, and reliability highlight its potential for practical deployment in real-world applications.

A salivary urea sensor based on a microsieve disposable gate AlGaN/GaN high electron mobility transistor.

The abundant bio-markers in saliva provide a new option for non-invasive testing. However, due to the presence of impurities in the saliva background, most of the existing saliva testing methods rely on pre-processing, which limits the application of saliva testing as a convenient means of testing in daily life. Herein, a disposable-gate AlGaN/GaN high electron mobility transistor (HEMT) biosensor integrated with a micro-sieve was introduced to solve the problem of signal interference caused by charged impurities in saliva for HEMT based biosensors, where the micro-sieve was utilized as a pre-treatment unit to remove large particles of impurities from saliva through the size effect and thus greatly improving the accuracy of detection. The experimental results showed that the HEMT based biosensor has excellent linearity (R2 = 0.9977) and a high sensitivity of 6.552 μA dec-1 for urea sensing from 1 fM to 100 mM in 0.1× PBS solution. When it comes to artificial saliva detection, compared to the HEMT sensor without the micro-sieve (sensitivity = 3.07432 μA dec-1), the sensitivity of the HEMT sensor integrated with the micro-sieve showed almost no change. Moreover, to verify that urea can be detected in actual saliva, urea is sensed directly in human saliva. The addition of the microsieve module provides a new way for biosensors to detect specific markers in saliva in real time, and the designed HEMT biosensor with the microsieve function has a wide range of application potential in rapid saliva detection.

Ultrasensitive Real-Time Detection of Pb2+ Ions Using g-C3N4 Nanosheets

The quick and easy monitoring of heavy metals in drinking water is utmost important due to their harmful effects on human health. In this work, GaN based high electron mobility transistor (HEMT) sensor has been fabricated using optical lithography and explored as a prospective sensor for the determination of trace Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions present in the water. Graphitic carbon nitride was synthesized by single step combustion method. The g-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanosheets were used to functionalise the gate area of the fabricated GaN HEMT sensor to investigate the presence of Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> metal ions in water. The g-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> functionalized sensor exhibited sensitivity of around 0.46 μA/ppb with 0.32 ppb as Limit of detection (LoD) much below the international set standards. Moreover, the real time measurements on lake water were performed using the developed GaN HEMT sensor to detect the presence of Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions in real samples in a fast and ultrasensitive manner. We anticipate that the reported work will certainly serve as proof-of-concept to develop heavy metal ion sensors.

Lead ion detection using glutathione-functionalized aluminum gallium nitride/gallium nitride high-electron-mobility transistors

This work presents a new approach for lead ion detection (Pb2+) using an aluminum gallium nitride/gallium nitride (AlGaN/GaN) high-electron-mobility transistor (HEMT) sensor. The AlGaN/GaN HEMT structure of the sensor was realized by functionalizing the gate area with glutathione (GSH). The crystalline and surface qualities of the AlGaN film were measured through X-ray diffraction and atomic force microscopy. The response of the sensor was measured in terms of the source–drain current with varying concentrations of Pb2+ ions at a fixed drain-to-source voltage. The sensitivity of the sensor was 29.3 μA/(mg/L), and it exhibited high selectivity toward Pb2+. The results show that using the GSH-functionalized AlGaN/GaN HEMT sensor is a promising strategy for Pb2+ ion detection.

Investigation of Thin-Barrier AlGaN/GaN HEMT Heterostructures for Enhanced Gas-Sensing Performance

In this study, a theoretical relationship between two-dimensional electron gas (2DEG) properties of AlGaN/GaN high-electron-mobility transistor (HEMT) hetero- structure and various gas-sensing characteristics using this structure was established. Based on the analytical study, it is proposed that a thinner barrier layer in AlGaN/GaN HEMT heterostructure should result in lower detection limits, higher sensing response, and a faster response time. To prove the analytical study, a thin-barrier (~8 nm) AlGaN/GaN HEMT structure was designed and grown to study its sensing characteristics. The results are compared with the thick-barrier (~18 nm) gas sensor structure. For the thin-barrier NO2 sensor, the sensing response increased by almost ten times when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sf \Delta {I}$ </tex-math></inline-formula> was changed by more than 25%. The sensor also showed a faster response time of 10 s when compared to the thick barrier sensor. Similarly, the thin-barrier NH3 sensor showed a remarkable improvement in sensing response by 17 times and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sf \Delta {I}$ </tex-math></inline-formula> by two times. Based on the response time, the thin-barrier NH3 sensor was also found to be 40% more faster compared to the thick-barrier AlGaN/GaN HEMT sensor.

The Sensing Mechanism of InAlN/GaN HEMT

The sensing mechanism of InAlN/GaN high electron mobility transistors (HEMTs) is investigated systematically by numerical simulation and theoretical analysis. In detail, the influence of additional surface charge on device performance and the dependence of surface sensing properties on InAlN barrier thickness are studied. The results indicate that the saturation output drain current Idsat and two-dimensional electron gas (2DEG) concentration increase with the increase of positive surface charge density, which decrease with the increase of negative surface charge. The influence of negative surface charge on device performance is more remarkable than that of positive surface charge. Additionally, the modulation ability of surface charge on device performance increases with the decrease ofInAlN barrier thickness. The modulation of surface charge on device performance and the influence of barrier thickness on surface sensing sensitivity are mainly attributed to the variation of the energy band structure, surface potential and 2DEG concentration in the HEMT heterostructure. This work provides important support for structural optimization design of GaN-based HEMT sensors.

Carbon monoxide detection down to ppb-level realized by O2 plasma treated TiO2-gated AlGaN/GaN HEMT sensor

The AlGaN/GaN high electron mobility transistor (HEMT) based sensors with O2 plasma treated TiO2 film as the gate electrode were fabricated. The ultrafast ppb-level carbon monoxide detection was achieved under dry air atmosphere. It is revealed that the O2 plasma treatment on TiO2 film significantly increases the effective work function of TiO2 film due to the strong dipole layer formation which is induced by the adsorption of O2 bonding states. As a result, the HEMT drain current observably decreases. Thus, when CO gas is introduced, the reduction reaction between O2 bonding states and CO molecules weakens the strength of the formed dipole layer, resulting in the decrease of the effective work function of TiO2 electrode and the significant increase of HEMT conduction current. It is demonstrated that with O2 plasma treatment the gas sensor based on TiO2 gated AlGaN/GaN HEMT can detect CO concentration down to 10 ppb with a response time of ~100 s and 0.15 mA conduction current variation. Furthermore, the ultrafast response time of ~4 s for ≥ 500 ppb CO, high resolution of CO concentration, and long-term stability are also demonstrated.

Detection of heavy metal ions using meander gated GaN HEMT sensor

We have developed a fast and simplistic AlGaN/GaN high electron mobility transistor (HEMT) for the detection of heavy metals such as mercury, lead, copper and zinc in water with a dynamic range of 1 nM to 1 mM. Four device designs have been fabricated where the gate structure and the distance between the source and the drain have been varied. It is observed that the device with meander gated (MG) structure and larger sensing area showed better response. A detailed analysis of the surface of the samples has been also investigated using SEM, EDS and Raman spectroscopy which confirms the re-usability of the sensor for a longer period of time. The response of the sensor stabilizes at 35 and 27 s for mercury and copper ions respectively. The response time of the sensor also shows a concentration dependent behavior. The sensor exhibits the high sensitivities of 31, 38, 35, and 24 mV/decade [M] for mercury, copper, zinc and lead ions respectively.

Model and Simulation of GaN-Based Pressure Sensors for High Temperature Applications—Part II: Sensor Design and Simulation

In this part, design and optimization guidelines are presented for an Aluminium Gallium Nitride (AlGaN)/GaN-on-Silicon (Si) High Electron Mobility Transistor (HEMT)-based pressure sensor, for high temperature applications. The work presented here is based on the compact model developed and presented in Part I. The nature of the developed model allows not only the simulation of the sensor behavior for the case of a single HEMT sensor, but also for the case of a Wheatstone bridge configuration. Both configurations are analyzed in terms of temperature compensation, pressure sensitivity, and mechanical failure limits. Based on the presented analysis, we propose an optimized design of a temperature compensated pressure sensor for a pressure operation range between −10 bar and 10 bar, and for temperatures up to 500 °C. The optimization includes the epitaxial design, HEMT placement on the sensor membrane, membrane thickness and geometry, and the optimal biasing points for a maximum sensitivity. Strong temperature compensation can be achieved with less than 0.3 % sensitivity deviation up to 500 °C. The maximum sensitivity of 2.6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mV</sup> / <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">barV</sub> can be achieved by applying a gate voltage near the pinch-off voltage of the device or as proposed here, by recessing the barrier locally under the gate. In addition, design guidelines for other pressure ranges are given. The approach taken here can be applied to a different epitaxial design with different barrier composition and substrate using the same compact model.

Boronate probe-based hydrogen peroxide detection with AlGaN/GaN HEMT sensor.

The results from this study demonstrate the potential of an AlGaN/GaN high electron mobility transistor sensor for the detection of reactive and transient biological molecules such as hydrogen peroxide. A boronate-based fluorescent probe was used with this device to detect the presence of micromolar levels of hydrogen peroxide typically associated with intracellular processes. The real-time electrical response of the high electron mobility transistor sensor showed a gradual decrease in the two-dimensional electron gas current as the reaction proceeded over time. A corresponding increase in the emission intensity was measured from the fluorescent probe with the progression of the reaction. The fluorescence from the boronate probe was used as an indicator to confirm the detection of hydrogen peroxide. These results demonstrate the dynamic measurement capability of AlGaN/GaN high electron mobility transistor sensors in monitoring real-time reactions of reactive oxygen species such as hydrogen peroxide.

Ultrasensitive Detection of Mercury Ions Under UV Illumination of MoS2 Functionalized AlGaN/GaN Transistor

In this work, the MoS2 functionalized AlGaN/GaN high-electron mobility transistor (HEMT) was utilized for the first time to detect toxic mercury (Hg2+) ions under ultraviolet (UV) illumination. The AlGaN/GaN HEMT was fabricated on the sapphire substrate, and corresponding structural and electrical properties were investigated. Subsequently, the optimization of MoS2 concentration for device functionalization was carried out by performing sensing analysis of Hg2+ ions on three devices, and it was observed that the 20 mg/mL concentration of MoS2 is optimum for the detection of Hg2+ ions. Furthermore, the detection of Hg2+ ions was performed under UV exposure, where the developed sensor showed much-improved sensitivity of $548.07~\mu \text{A}$ /ppb compared with normal light. The comparative analysis indicates the increase of three orders of magnitude in sensitivity under UV irradiation, and it is the highest sensitivity ever observed by the AlGaN/GaN HEMT sensor for Hg2+ ion detection. Moreover, the sensor also exhibits the limit of detection (LoD) of 6.14 parts per trillion (ppt) that is much lower than the World Health Organization (WHO) standard limit for Hg2+ ions in drinking water. Due to the photoexcitation process, the generation of electron–hole pairs provides more binding sites on the MoS2 surface, which results in the ultrasensitive detection of the Hg2+ ions at trace and ultratrace levels.

The Impact of Gate Recess on the H₂ Detection Properties of Pt-AlGaN/GaN HEMT Sensors

The present work reports on the hydrogen gas detection properties of Pt-AlGaN/GaN high electron mobility transistor (HEMT) sensors with recessed gate structure. Devices with gate recess depths from 5 to 15 nm were fabricated using a precision cyclic etching method, examined with AFM, STEM and EDS, and tested towards H2 response at high temperature. With increasing recess depth, the threshold voltage ( ${V}_{\textit {TH}}$ ) shifted from −1.57 to 1.49 V. A shallow recess (5 nm) resulted in a 1.03 mA increase in signal variation ( $\Delta {I}_{\textit {DS}}$ ), while a deep recess (15 nm) resulted in the highest sensing response ( ${S}$ ) of 145.8% towards 300 ppm H2 as compared to reference sensors without gate recess. Transient measurements demonstrated reversible H2 response for all tested devices. The response and recovery time towards 250 ppm gradually decreased from 7.3 to 2.5 min and from 29.2 to 8.85 min going from 0 nm to 15 nm recess depth. The power consumption of the sensors reduced with increasing recess depth from 146.6 to 2.95 mW.

High sensitivity detection of glucose with negatively charged gold nanoparticles functionalized the gate of AlGaN/GaN High Electron Mobility Transistor

A high sensitivity glucose sensor using AlGaN/GaN based high electron mobility transistor (HEMT) was fabricated. The 3-aminopropyltriethoxysilane (APTES) was aligned to the GaN surface by forming the covalent linkage between them. The negatively charged gold nanoparticles (AuNPs) can be effectively self-assembled on the positively charged APTES surface. The morphology of the immobilized AuNPs was observed by the field emission scanning electron microscope (FE-SEM). The chemical groups after APTES/AuNPs functionalization were confirmed by X-ray photoelectron spectroscopy (XPS). Glucose oxidase (GOx) was immobilized on the APTES/AuNPs functionalized the gate surface through electrostatic attraction. From the analysis of the current signals, our glucose sensor exhibited wide detection range from 0.001 to 9 mM with higher sensitivity of above 1 × 106 μA mM-1 cm-2. It showed a quick response time of less than 8 s and a detection limit of 1 nM. Furthermore, it was found that the performance of detection after AuNPs attachment can be increased by 15 % as compared to the HEMT sensor without AuNPs. The apparent Michaelis-Menten constant KMαpp was calculated about 0.06 mM, indicating high affinity of GOx to glucose. Consequently, proposed strategy for the AuNPs-gated AlGaN/GaN HEMT could be an efficient immobilization platform for biosensing of the different bio-analytes.

A Highly Porous and Conductive Composite Gate Electrode for NO, NO2, O2, H2 and NH3 Exhaust Gas Sensors

The real-time knowledge of the NO, NO2 and NH3 concentration at high temperature, would allow manufacturers of automobiles to meet the upcoming stringent EURO7 anti-pollution measures for diesel engines. Knowledge of the concentration of each of these species will also enable engines to run leaner (i.e. more fuel efficient) while still meeting the anti-pollution requirements. Our proposed technology is promising in the field of automotive sensors. It based on AlGaN/GaN high electron mobility transistor (HEMT) sensors incorporating highly porous platinum in the gate. Compared to platinum full gate AlGaN/GaN HEMT sensor, the Highly porous gate sensor has dramatically improved current response to O2, NO, NO2 H2 and NH3, to the NO concentration in the range of 0–2000 ppm, 0-2500 ppm NO2, 0-300 ppm NH3, (1.5-10%) H2 and O2 (1.5-100%) species at high temperature (500 °C). This improvement in sensitivity is the result of the increase in catalytic surface area interaction between gas and platinum. Keywords—HEMT, sensors, GaN, H2, O2, NO2, NO, NH3 high temperature, highly porous gate.

Enhancement of cortisol measurement sensitivity by laser illumination for AlGaN/GaN transistor biosensor

In this study, high electron mobility transistor (HEMT) device was used as an immuno biosensor to measure concentration of a stress hormone, cortisol, by using selective binding on cortisol monoclonal antibody (c-Mab). Also, the HEMT sensor was enhanced in its sensitivity through light illumination to generate photocurrent. The optical pumping could assist the biosensor to discriminate more detailed change, which could result in an increment of limit of detection (LOD) to 1.0 pM cortisol level. It was the lowest level of detection with semiconductor device-based cortisol biosensors and the enhancement of surface potential sensitivity was induced by laser light (532 nm). Output current amplificated by photocurrent was higher than dark original current at about 3.39% when gate voltage is applied with −3 V. Since the device could be applied to not only standard cortisol solution but also real human salivary sample, it is expected to apply for in vitro direct diagnosis of point-of-care test (POCT).

Sensitive and Selective Detection of Pb2+ Ions Using 2,5-Dimercapto-1,3,4-Thiadiazole Functionalized AlGaN/GaN High Electron Mobility Transistor

We report sensitive and selective AlGaN/GaN High Electron Mobility Transistor (HEMT)-based sensor for Lead ion (Pb2+) detection. The gate region of the HEMT was functionalized by 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The response of the sensor is observed by monitoring drain to source current ( $\text{I}_{\textsf {DS}}$ ) for different concentrations of Pb2+ ions at a fixed drain to source voltage ( $\text{V}_{\textsf {DS}}$ ). Our sensor reaches the lower detection limit of 0.018 ppb, which is much lower than the standard detection limit recommended by the World Health Organization (WHO) for drinking water. Furthermore, the sensor exhibited a rapid response time of ~4 seconds and high sensitivity of $0.607~\mu \text{A}$ /ppb. Moreover, the selectivity analysis was performed and found that the sensor was highly selective towards Pb2+ ions. The change in electron concentration at 2-dimensional electron gas (2DEG) upon the capture of Pb2+ ions at gate region by DMTD, causes a change in the $\text{I}_{\textsf {DS}}$ , which showed excellent sensing response towards Pb2+ ions. The highly sensitive, selective, and rapid detection of Pb2+ ions paves the way for stable sensing performance based on DMTD functionalized AlGaN/GaN HEMT sensor.

MPA-GSH Functionalized AlGaN/GaN High-Electron Mobility Transistor-Based Sensor for Cadmium Ion Detection

This paper demonstrates a novel AlGaN/GaN high-electron mobility transistor (HEMT)-based cadmium ion (Cd2+) sensor with mercaptopropionic acid (MPA) and glutathione (GSH) functionalization. The sensing response of the sensor was analyzed by detecting Cd2+ ions at different concentrations. The AlGaN/GaN HEMT sensor exhibits excellent response with the sensitivity of $0.241~\mu \text{A}$ /ppb, a fast response time of ~ 3 s, and a lower detection limit of 0.255 ppb. The observed lower detection limit is significantly lower than the World Health Organization (WHO) standard recommended limit for Cd2+ ions in drinking water. Furthermore, the sensor showed good selectivity of Cd2+ ions toward other heavy metal ions. The results indicate that the binding properties of GSH to Cd and the sensitivity of 2-D electron gas toward the variation of charges at the gate region make the device highly sensitive with rapid detection of Cd2+ ions.

A differential extended gate-AlGaN/GaN HEMT sensor for real-time detection of ionic pollutants

In this study, we propose a differential extended gate (DEG)-AlGaN/GaN high electron mobility transistor (HEMT) sensor to detect ionic pollutants in solution.

Impact of high temperature H2 pre-treatment on Pt-AlGaN/GaN HEMT sensor for H2S detection

In this paper, a method to extend the detection range of hydrogen sulfide (H2S) gas sensor is demonstrated. The sensor is based on AlGaN/GaN high electron mobility transistors (HEMTs) with Pt gate. It is observed that the as-fabricated devices exhibited sensing signal saturation at 30 ppm H2S exposure in dry air. A pre-treatment using H2 pulses in dry air ambient at 250 °C was applied to extend the detection range of the sensor. The H2 treated H2S gas sensor was able to detect a higher H2S concentration up to 90 ppm at 250 °C without complete saturation.