High Electron Mobility Transistor Sensor Research Articles

Beyond the Limit of Ideal Nernst Sensitivity: Ultra-High Sensitivity of Heavy Metal Ion Detection with Ion-Selective High Electron Mobility Transistors

Exposure to heavy metal ions poses grave danger to public health and reliable and affordable water quality monitoring system that can rapidly screen for heavy metal ion contamination is necessary. In this research, we have developed a unique sensing methodology to detect heavy metal ions such as Pb2+ and Hg2+ in water sources, using ion-selective high electron mobility transistor sensor (ISHEMT). A detailed investigation of the sensing and selectivity characteristics of ISHEMT is carried out and a theoretical model is proposed for the illustration of the enhanced sensitivity and selectivity. The high field modulated ISHEMT sensor displays very high sensitivity, much beyond the ideal Nernstian slope, offering very low detection limit (10−10 M for Pb2+ and 10−11 M for Hg2+). The sensing characteristics are not affected by the presence of interfering ions and the selective sensor response has been validated using fixed interference and separate solution methods. These sensor characteristics are superior than the traditional ISE or ISFET sensors, and at par with laboratory standard technologies like ICP-MS. The miniaturized, inexpensive and user-friendly sensor technology can provide consumers with an affordable and convenient means of securing safe and contamination free water and food consumption.

Enumeration of circulating tumor cells and investigation of cellular responses using aptamer-immobilized AlGaN/GaN high electron mobility transistor sensor array

This study reports the fabrication and characterization of electrical double layer gated AlGaN/GaN High electron mobility (HEMT) biosensor array to capture, detect and count circulating tumor cells (CTCs) of colorectal cancer (CRC). GaN HEMT chips were assembled into a sensor array on a thermo-curable polymer substrate in a simple and robust packaging process. The array had multiple aptamer immobilized sensing sites and was highly sensitive and selective with up to single cell resolution. The present method can detect cells in their native electrolyte composition, in a very short time with extremely low cost compared to the conventional circulating tumor cell assays. The effect of cellular binding in different test environments was further studied and reported. The electrical response of the sensor was discovered to be relevant to the transmembrane potential of the cell. This whole cell sensor array has the potential to be used as a point-of-care diagnostic tool in varied applications such as detection of rare cells, pathogens and studying the dynamics of cellular interactions.

Investigation of DNA Detection Mechanism with AlGaN/GaN High Electron Mobility Transistor (HEMT) Biosensor in High Ionic Strength Solution

In this research, the electrical double layer HEMT device is used to serve as the cardiovascular disease (CVD) RNA biomarkers biosensor. In this experiment, we try to use the DNA equivalent to miRNA-126 to test the ability of the novel DNA sensors. Short Debye length has been a haunted problem among all the field effect transistor (FET) biosensor for years. As the Debye length is extremely short in high ionic strength solutions, the traditional FET may not be directly detecting the biomolecule. The novel HEMT sensors using AlGaN/GaN can perform high sensitivity and great specificity in DNA sequences testing. The detection limit can be down to the 1fM and the specificity measurement can identify the signals between two DNA sequences with six-base mismatch. Furthermore, the mechanism of the sensor structure has also been studied. Since the structure of the device has the separation between source drain channel and gate electrode, the comparison with different electrode area and gap distance has been done as well. With repeating electrical measurement confirming, the reusability can also be seen after the 95℃ dehybridization process. With the Gibbs free energy ∆G of the specific sequences, we can also predict the equilibrium reaction constant and the binding ratio of probe DNA with the target DNA. The comparison between the thermal dynamics with the experiment consequence has been demonstrated too.

Investigation of DNA Detection Mechanism with AlGaN/GaN High Electron Mobility Transistor (HEMT) Biosensor in High Ionic Strength Solution

In this research, the electrical double layer HEMT device is used to serve as the DNA biosensor. In this experiment, we try to use the DNA sequence of miRNA-126, which serve as a novel biomarkers of cardiovascular disease (CVD), to test the ability of the novel DNA sensors. Debye length has been a haunted problem among all the field effect transistor (FET) biosensor for years. As the Debye length is extremely low in high ionic strength solutions, the traditional FET may not be direct detect the biomolecular. The novel HEMT sensors using AlGaN/GaN can performs high sensitivity and great specificity in DNA sequences testing. The sensitivity can be down to the 1fM and the specificity measurement can separate the signals between two DNA sequences with two mismatch differences. Furthermore, the mechanism of the sensor structure has also been studied. Since the structure of the device has the separation between source drain channel and gate electrode, the comparison with different electrode area and gap distance has been done as well. With repeating electrical measurement confirming, the reusability can also be seen after the 95℃ dehybridization process. With the Gibbs free energy of the specific sequences, we can also predict the equilibrium reaction constant and the binding ratio of probe DNA with the target DNA. The comparison between the thermal dynamics with the experiment consequence has been demonstrated too. Figure 1

Aptamer Functionalized AlGaN/GaN HEMT Biosensor Array for Electrical Enumeration of Circulating Tumor Cells

This paper demonstrates a novel high sensitive FET biosensor array that can capture as well as count the number of captured Circulating Tumor Cells (CTCs) by the aptamer immobilized on the sensor. The FET used for this purpose is AlGaN/GaN HEMT (High Electron Mobility Transistor) because of its high sensitivity. Using a new and robust sensing technique the cell capture was able to be measured electrically and analyzed. The array enhances the capturing and detection capability of the sensors. The advantage of this sensor array is that the detection can be extended to any type of cells and cell lines to differentiate the normal and abnormal cells among them, and the behavior of cells can be studied in different kinds of mediums in which the cells are suspended. Biosensors can be used as point of care testing devices. In order to do so, they must be compact, portable, highly sensitive and user friendly. Along with being highly sensitive, GaN HEMT also are biocompatible, high temperature tolerant, chemically resistive and have excellent electrical properties. These properties of HEMT makes it a candidate for rapid biosensing. CTC enumeration has been used as a prognostic marker for metastatic colorectal cancer (m-CRC). CTCs are migrating cells that are shed into the blood stream from primary and metastatic tumors. But cancer patients have only 1-10 CTCs per cc of blood which makes them hard to be detected among plethora of other blood cells. The HEMT biosensor array developed by our group overcomes this drawback by capturing CTCs and counting them electrically, with an accuracy of single cell binding. The fabrication of AlGaN/GaN HEMT is done by forming mesa islands by ICP etching of epi-wafer which is followed by ohmic metal deposition, annealing and probing pad deposition. Finally individual chips of size 1mm X 1mm are embedded onto an epoxy substrate and interconnects are deposited making a planar sensing device. This fabricated chip is then passivated and only the gate electrode and channel regions are opened with an area of 10X60µm2 using photolithography. Aptamer is immobilized on the gate electrode and CTCs are allowed to be captured for 5~10 minutes. The cells bound on the aptamer are observed and the capture can be confirmed electrically by measuring the change in current gain compared to the aptamer current level and the change in current gain of HEMT sensor corresponds to the number of CTCs captured by the aptamer. In this way, CTC enumeration can be carried out using electrical measurement. This sensing technique is highly sensitive and can provide up to single cell resolution. Apart from rare migrating cells, abnormal cells and other cell based disease biomarkers can be detected and enumerated using our GaN HEMT based biosensor array. Figure 1

Direct Detection of NT- Pro BNP As a Cardiac Biomarker Using High Electron Mobility Transistors in Physiological Salt Environment

Cardiovascular disease (CVD) is one of the major causes to claim millions of lives all across the globe every year. While the chances of CVD increase with age, it can also occur in any age group of people, both male and female. Despite all efforts and tremendous accomplishments in the precaution and treatment of CVD, its detection at an early stage is still a big challenge for scientists in this present modern world. Here, NT-pro BNP plays a crucial role in acute congestive heart failure since it is biologically inactive, binds better with its receptors and has a longer half-life than any other, making it an ideal biomarker. The detection method is based upon the ligand-receptor interaction, also called as immunoassay, which can be successfully detected with the use of High Electron Mobility Transistors (HEMTs) with short duration pulse applied at the gate electrode. With its unique characteristic features like good electrical property, chemical stability, biocompatibility, thermal stability and low power consumption HEMT sensors also have been used in several other applications like ion-sensing, pH sensing, gas sensing, etc. Therefore, the detection method becomes simple, fast, convenient and cost effective.

Direct Detection of NT- Pro BNP As a Cardiac Biomarker Using High Electron Mobility Transistors in Physiological Salt Environment

Cardiovascular disease (CVD) is one of the major causes to claim millions of lives all across the globe every year. While the chances of CVD increase with age, it can also occur in any age group of people, both male and female. Despite all efforts and tremendous accomplishments in the precaution and treatment of CVD, its detection at an early stage is still a big challenge for scientists in this present modern world. Here, NT-pro BNP plays a crucial role in acute congestive heart failure since it is biologically inactive, binds better with its receptors and has a longer half-life than any other, making it an ideal biomarker. The detection method is based upon the ligand-receptor interaction, also called as immunoassay, which can be successfully detected with the use of High Electron Mobility Transistors (HEMTs). With its unique characteristic features like good electrical property, chemical stability, biocompatibility, thermal stability and low power consumption HEMT sensors also have been used in several other applications like ion-sensing, pH sensing, gas sensing, etc. Therefore, the detection method becomes simple, fast, convenient and cost effective. AlGaN/ GaN are the most promising candidates for HEMT sensor materials. A thin film of undoped Al0.25Ga0.75N layer (150 Å) is deposited onto the surface of 3 μm-thick undoped GaN buffer layer, which in turn is deposited on the top of Silicon substrate by metal-organic chemical vapor deposition (MOCVD) process. Using mesa ICP etching, active device area is formed. Then four different metals of various thicknesses, i.e. Ti, Al, Ni and Au, are deposited to make an ohmic contact and is annealed at an elevated temperature of 850ºC. Later, the third mask helps in making the final metal layer and the gate electrode used for immobilizing the receptor. Finally, the fourth mask lets us open some portion of the gate and the transistor region to allow liquid to cross through. The next step to sensor fabrication and device gate opening is making the surface ready for immobilization of receptor, or so called surface functionalization. The antibody disulfide bridges between the heavy chains are cleaved in the presence of mercaptoethylamine (MEA) and EDTA whereas the disulfide bridges between the heavy chain and the light chain remains unaffected. This leaves behind a number of disulfide bonds which can be bound onto gold surface easily because of the strong interactions between the native thiol group and the gold surface. Finally, the sensor measurement is performed with Agilent B1530 parameter analyzer. The figure 1 (c) represents the detection of NT- pro BNP at various concentrations. The graph shows the total charge accumulated against various concentrations of target proteins prepared in physiological environment (1X PBS). From the experiment performed, it can be observed that there is an increase in the change of charge with increase in the concentration of target proteins, which confirms the immobilization of antibody and ultimately the detection of target proteins. Moreover, the calibration curve for the sensor can be drawn by plotting change in the total charge against the concentrations with repeated testing of the device. Therefore, this method of detection is cheap, robust and time efficient making it also ideal for multi-sensor array and point-of-care diagnostics. Figure 1

Investigation of the Performance of HEMT-Based NO, NO₂ and NH₃ Exhaust Gas Sensors for Automotive Antipollution Systems.

We report improved sensitivity to NO, NO and NH gas with specially-designed AlGaN/GaN high electron mobility transistors (HEMT) that are suitable for operation in the harsh environment of diesel exhaust systems. The gate of the HEMT device is functionalized using a Pt catalyst for gas detection. We found that the performance of the sensors is enhanced at a temperature of 600 °C, and the measured sensitivity to 900 ppm-NO, 900 ppm-NO and 15 ppm-NH is 24%, 38.5% and 33%, respectively, at 600 °C. We also report dynamic response times as fast as 1 s for these three gases. Together, these results indicate that HEMT sensors could be used in a harsh environment with the ability to control an anti-pollution system in real time.

Enzymatic lactic acid sensing by In-doped ZnO nanowires functionalized AlGaAs/GaAs high electron mobility transistor

Indium (In) doped zinc oxide (ZnO) nanowires-gated AlGaAs/GaAs high electron mobility transistor (HEMT) was demonstrated for the detection of lactic acid. The In-doped ZnO nanowires were synthesized via chemical vapor deposition (CVD) method. Such In-doped ZnO nanowires offered an effective surface area with high surface area-to-volume ratio as well as a favorable environment for the immobilization of lactate oxidase (LOX) on the HEMT gate area. The In-doped ZnO nanowires have better conductivity than pure ZnO nanowires and retained the electroactivity of enzymes. Due to the novel structure of the Si-doped GaAs cap layer, the drain–source current of the AlGaAs/GaAs HEMT sensor showed a rapid response when lactic acid solutions at various concentrations were introduced to the gate area of the HEMT. The fabricated sensor exhibited a wide detection range from 3pM to 3mM and a low detection limit of 3pM. The result indicated that a portable, fast response and high sensitivity lactic acid detector can be realized by AlGaAs/GaAs HEMT sensor.

An AuNPs-functionalized AlGaN/GaN high electron mobility transistor sensor for ultrasensitive detection of TNT

In this work, we developed a gold nanoparticle functionalized AlGaN/GaN HEMT sensor for fast and ultrasensitive detection of TNT. This HEMT sensor showed attractive properties for TNT detection in terms of speed, sensitivity and miniaturization.

Label free electrical detection of prostate specific antigen with millimeter grade biomolecule-gated AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistor (HEMT) was successfully fabricated by complementary metal-oxide semiconductor field effect transistor-compatible fabrication method, and the label-free, electrical detection of prostate specific antigen in real time using the biomolecule-gate AlGaN/GaN HEMT sensor was presented. It shows a rapid response when target prostate biomarker in buffer solution was added to the antibody-immobilized sensing area. The linear range for target prostate specific antigen detection has been demonstrated from 0.1 pg/ml to 10.269 ng/ml and a low detection below 0.1 pg/ml level is estimated, which is the best result of AlGaN/GaN HEMT biosensor for prostate specific antigen (PSA) detection till now. The sensitivity of 0.027 % is determined for 0.1 pg/ml prostate specific antigen solution. The electrical result of the biomolecule-gated AlGaN/GaN HEMT biosensor suggested that this biosensor might be a useful tool for the prostate cancer screening.

Detection of prostate-specific antigen with biomolecule-gated AlGaN/GaN high electron mobility transistors

In order to improve the sensitivity of AlGaN/GaN high electron mobility transistor (HEMT) biosensors, a simple biomolecule-gated AlGaN/GaN HEMT structure was designed and successfully fabricated for prostate specific antigen (PSA) detection. UV/ozone was used to oxidize the GaN surface and then a 3-aminopropyl trimethoxysilane (APTES) self-assembled monolayer was bound to the sensing region. This monolayer serves as a binding layer for attachment of the prostate specific antibody (anti-PSA). The biomolecule-gated AlGaN/GaN HEMT sensor shows a rapid and sensitive response when the target prostate-specific antigen in buffer solution was added to the antibody-immobilized sensing area. The current change showed a logarithm relationship against the PSA concentration from 0.1 pg/ml to 0.993 ng/ml. The sensitivity of 0.215% is determined for 0.1 pg/ml PSA solution. The above experimental result of the biomolecule-gated AlGaN/GaN HEMT biosensor suggested that this biosensor might be a useful tool for prostate cancer screening.

Biological detection by high electron mobility transistor (HEMT) based AlGaN/GaN

AbstractMaterials based on gallium nitride are characterized by a wide bandgap. The excellent chemical, physical and electronic properties give them a place in various fields: electronics, optoelectronics, photovoltaics, military, biological and medical. The use of biosensors based III‐N has been confirmed for the detection of biological substances in cells, blood or in solutions. This effect of detection results in various chemical and electric changes in a medium that is contacted with the devices without gate metallization of a high electron mobility transistor (HEMT). HEMT sensors can thus be used to detect gases, ions, pH values, proteins and DNA. In this work, we are interested in HEMT‐based heterostructure AlGaN/GaN used in the construction of biosensors. The principle of operation of the HEMT is based on the properties of a high‐mobility two‐dimensional gas (2DEG) and high‐speed saturation channel of a 2DEG AlGaN HEMT is very close to the surface that is very sensitive to absorption of analytes. We use this microsystem for detection of substances found in biological solutions. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Research on biomolecule-gate AlGaN/GaN high-electron-mobility transistor biosensors

In order to enhance the performance of AlGaN/GaN high electron mobility transistor (HEMT) biosensor, millimeter grade AlGaN/GaN HEMT structure have been designed and successfully fabricated. Factors influencing the capability of the AlGaN/GaN HEMT biosensor are analyzed. UV/ozone is used to oxidize GaN surface and then 3-aminopropyl trimethoxysilane (APTES) self-assembled monolayer can be bound to the sensing region. This serves as a binding layer in the attachment of prostate specific antibody (anti-PSA) for prostate specific antigen detection. The millimeter grade biomolecule-gated GaN/AlGaN HEMT sensor shows a quick response when the target prostate specific antigen in a buffer solution is added to the antibody-immobilized sensing area. The detection capability of this biomolecule-gate sensor estimated to be below 0.1 pg/ml level using a 21.5 mm2 sensing area, which is the best result of GaN/AlGaN HEMT biosensor for PSA detection till now. The electrical result of the biomolecule-gated GaN/AlGaN HEMT biosensor suggests that this biosensor might be a useful tool for the prostate cancer screening.

Characteristics of carbon monoxide sensors made by polar and nonpolar zinc oxide nanowires gated AlGaN/GaN high electron mobility transistor

AlGaN/GaN high electron mobility transistors (HEMTs) with polar and nonpolar ZnO nanowires modified gate exhibit significant changes in channel conductance upon exposure to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition, with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change of surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezo-induced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm and 3200 ppm in the open cavity with continuous gas flow using a 50 × 50 μm2 gate sensing area for polar and nonpolar ZnO nanowire gated HEMTs sensor, respectively.

AlGaN/GaN HEMT based hydrogen sensor with platinum nanonetwork gate electrode

AlGaN/GaN high electron mobility transistor (HEMT) based hydrogen sensors incorporating platinum nanonetworks in the gate region were demonstrated. Pt nanonetworks with 2–3 nm diameter were synthesized by a simple and low-cost solution phase method, and applied to the gate electrode of transistor sensor. The HEMT with physically and electrically connected Pt nanonetwork gate showed good pinch-off and modulation of drain current characteristics. Compared to conventional Pt thin film AlGaN/GaN HEMT sensor, the Pt nanonetwork sensor has dramatically improved current response to hydrogen. Relative current change of Pt nanonetwork gated sensor in 500 ppm H2 balanced with Air ambient was 3.3 × 106% at VGS of −3.3 V, while 2.5 × 102% at VGS of −2.9 V for Pt film. This results from large increase in channel conductance induced by huge catalytic surface area of nanostructured Pt networks.

SnO2-gated AlGaN/GaN high electron mobility transistors based oxygen sensors

Hydrothermally grown SnO2 was integrated with AlGaN/GaN high electron mobility transistor (HEMT) sensor as the gate electrode for oxygen detection. The crystalline of the SnO2 was improved after annealing at 400 °C. The grain growth kinetics of the SnO2 nanomaterials, together with the O2 gas sensing properties and sensing mechanism of the SnO2 gated HEMT sensors were investigated. Detection of 1% oxygen in nitrogen at 100 °C was possible. A low operation temperature and low power consumption oxygen sensor can be achieved by combining the SnO2 films with the AlGaN/GaN HEMT structure.

SnO2 functionalized AlGaN/GaN high electron mobility transistor for hydrogen sensing applications

In this study, we report on a demonstration of hydrogen sensing at low temperature using SnO2 functionalized AlGaN/GaN high electron mobility transistors (HEMT). The SnO2 dispersion was synthesized via a hydrothermal method and selectively deposited on the gate region of a HEMT device through a photolithography process. The high electron sheet carrier concentration of nitride HEMTs provides an increased sensitivity relative to simple Schottky diodes fabricated on GaN layers. The morphology and crystalline properties of the SnO2-gate, together with the texture of the multilayer films on the device were investigated by SEM, HRTEM, EDS and XRD. The effects of annealing treatment on the crystalline properties of the SnO2-gate, and gas sensing properties of SnO2-gated HEMT sensors were studied. The SnO2-gated HEMT sensor showed fast and reversible hydrogen gas sensing response at low temperature.

RF GaN HEMT Sensors for Detection of Caustic Chemicals

For future wireless sensor network applications, high-speed RF GaN HEMTs (high electron mobility transistor) are investigated for toxic industrial chemical detection, for the first time. RF GaN HEMTs with a Pt-based gate metal show reliable, repeatable, and distinctive responses toward Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas and HCl vapor at room temperature.

Oxygen gas sensing at low temperature using indium zinc oxide-gated AlGaN/GaN high electron mobility transistors

Indium zinc oxide (IZO)-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect oxygen gas. Amorphous IZO films with high carrier concentration of 1021 cm−3 were deposited on the gate region of the HEMTs by cosputtering from ZnO and In2O3 targets. The changes in IZO gated-AlGaN/GaN HEMT drain current were used to monitor the presence of oxygen. The IZO gated AlGaN/GaN HEMT sensors were tested with O2 at room temperature, 50 °C, and 120 °C. There was no response to O2 at room temperature. At 50 °C, the sensors could sense O2 but gradually saturated. The sensor showed a strong response to the oxygen gas at 120 °C, which is a much lower temperature than with conventional oxide-based oxygen sensors that typically operate in the range of 400–700 °C. This enhanced oxygen sensing sensitivity was due to the amplification effect of the AlGaN/GaN HEMT. A preannealing step at 350 °C was also found to improve the sensitivity and response time of O2 sensing at 120 °C.