J. C. Roberts

12 W/mm AlGaN-GaN HFETs on silicon substrates

Al/sub 0.26/Ga/sub 0.74/N-GaN heterojunction field-effect transistors were grown by metal-organic chemical vapor deposition on high-resistivity 100-mm Si (111) substrates. Van der Pauw sheet resistance of the two-dimensional electron gas was 300 /spl Omega//square with a standard deviation of 10 /spl Omega//square. Maximum drain current density of /spl sim/1 A/mm was achieved with a three-terminal breakdown voltage of /spl sim/200 V. The cutoff frequency and maximum frequency of oscillation were 18 and 31 GHz, respectively, for 0.7-/spl mu/m gate-length devices. When biased at 50 V, a 2.14-GHz continuous wave power density of 12 W/mm was achieved with associated large-signal gain of 15.3 dB and a power-added efficiency of 52.7%. This is the highest power density ever reported from a GaN-based device grown on a silicon substrate, and is competitive with the best results obtained from conventional device designs on any substrate.

Enzymatic glucose detection using ZnO nanorods on the gate region of AlGaN∕GaN high electron mobility transistors

ZnO nanorod-gated AlGaN∕GaN high electron mobility transistors (HEMTs) are demonstrated for the detection of glucose. A ZnO nanorod array was selectively grown on the gate area using low temperature hydrothermal decomposition to immobilize glucose oxidase (GOx). The one-dimensional ZnO nanorods provide a large effective surface area with high surface-to-volume ratio and provide a favorable environment for the immobilization of GOx. The AlGaN∕GaN HEMT drain-source current showed a rapid response of less than 5s when target glucose in a buffer with a pH value of 7.4 was added to the GOx immobilized on the ZnO nanorod surface. We could detect a wide range of concentrations from 0.5nMto125μM. The sensor exhibited a linear range from 0.5nMto14.5μM and an experiment limit of detection of 0.5nM. This demonstrates the possibility of using AlGaN∕GaN HEMTs for noninvasive exhaled breath condensate based glucose detection of diabetic application.

Pressure-induced changes in the conductivity of AlGaN∕GaN high-electron mobility-transistor membranes

AlGaN∕GaN high-electron-mobility transistors (HEMTs) show a strong dependence of source∕drain current on the piezoelectric-polarization-induced two-dimensional electron gas. The spontaneous and piezoelectric-polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The changes in the conductance of the channel of a AlGaN∕GaN high electron mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The conductivity of the channel shows a linear change of −(+)6.4×10−2mS∕bar for application of compressive (tensile) strain. The AlGaN∕GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications.

Effect of Gate Leakage in the Subthreshold Characteristics of AlGaN/GaN HEMTs

This letter studies the effect of gate leakage on the subthreshold slope and ON/OFF current ratio of AlGaN/GaN high-electron mobility transistors (HEMTs). We found a strong correlation between the gate leakage current and the transistor subthreshold characteristics: the lower the gate leakage, the higher the ON/OFF ratio and the steeper the subthreshold slope. To improve the subthreshold characteristics in GaN HEMTs, the gate leakage current was reduced with an O2 plasma treatment prior to the gate metallization. The O2 plasma treatment effectively reduces the gate leakage current by more than four orders of magnitude, it increases the ON/OFF ratio to more than seven orders of magnitude and the improved AlGaN/GaN HEMT shows a nearly ideal subthreshold slope of 64 mV/dec.

Electrical detection of deoxyribonucleic acid hybridization with AlGaN∕GaN high electron mobility transistors

Au-gated AlGaN∕GaN high electron mobility transistor (HEMT) structures were functionalized in the gate region with label-free 3′-thiol-modified oligonucleotides. This serves as a binding layer to the AlGaN surface for hybridization of matched target deoxyribonucleic acid (DNA). X-ray photoelectron spectroscopy shows the immobilization of thiol-modified DNA covalently bonded with gold on the gated region. Hybridization between probe DNA and matched or mismatched target DNA on the Au-gated HEMT was detected by electrical measurements. The HEMT drain-source current showed a clear decrease of 115μA as this matched target DNA was introduced to the probe DNA on the surface, showing the promise of the DNA sequence detection approach for biological sensing.

Prostate specific antigen detection using AlGaN∕GaN high electron mobility transistors

Antibody-functionalized Au-gated AlGaN∕GaN high electron mobility transistors (HEMTs) were used to detect prostate specific antigen (PSA). The PSA antibody was anchored to the gate area through the formation of carboxylate succinimdyl ester bonds with immobilized thioglycolic acid. The AlGaN∕GaN HEMT drain-source current showed a rapid response of less than 5s when target PSA in a buffer at clinical concentrations was added to the antibody-immobilized surface. The authors could detect a wide range of concentrations from 10pg∕mlto1μg∕ml. The lowest detectable concentration was two orders of magnitude lower than the cutoff value of PSA measurements for clinical detection of prostate cancer. These results clearly demonstrate the promise of portable electronic biological sensors based on AlGaN∕GaN HEMTs for PSA screening.

Fast electrical detection of Hg(II) ions with AlGaN∕GaN high electron mobility transistors

Bare Au gated and thioglycolic acid functionalized Au-gated AlGaN∕GaN high electron mobility transistors (HEMTs) were used to detect mercury (II) ions. Fast detection of less than 5s was achieved for thioglycolic acid functionalized sensors. This is the shortest response time ever reported for mercury detection. Thioglycolic acid functionalized Au-gated AlGaN∕GaN HEMT based sensors showed 2.5 times larger response than bare Au-gated based sensors. The sensors were able to detect mercury (II) ion concentration as low as 10−7M. The sensors showed an excellent sensing selectivity of more than 100 for detecting mercury ions over sodium or magnesium ions. The dimensions of the active area of the sensor and the entire sensor chip are 50×50μm2 and 1×5mm2, respectively. Therefore, portable, fast response, and wireless based heavy metal ion detectors can be realized with AlGaN∕GaN HEMT based sensors.

Enzyme-based lactic acid detection using AlGaN∕GaN high electron mobility transistors with ZnO nanorods grown on the gate region

The detection of lactic acid with ZnO nanorod-gated AlGaN∕GaN high electron mobility transistors (HEMTs) was demonstrated. The array of ZnO nanorods provided a large effective surface area with a high surface-to-volume ratio and a favorable environment for the immobilization of lactate oxidase. The HEMT drain-source current showed a rapid response when various concentrations of lactic acid solutions were introduced to the gate area of the HEMT sensor. The HEMT could detect lactic acid concentrations from 167nM to 139μM. Our results show that portable, fast response, and wireless-based lactic acid detectors can be realized with AlGaN∕GaN HEMT based sensors.

Electrical detection of kidney injury molecule-1 with AlGaN∕GaN high electron mobility transistors

AlGaN∕GaN high electron mobility transistors (HEMTs) were used to detect kidney injury molecule-1 (KIM-1), an important biomarker for early kidney injury detection. The gate region consisted of 5nm gold deposited onto the AlGaN surface. The gold was conjugated to highly specific KIM-1 antibodies through a self-assembled monolayer of thioglycolic acid. The HEMT source-drain current showed a clear dependence on the KIM-1 concentration in phosphate-buffered saline solution. The limit of detection was 1ng∕ml using a 20×50μm2 gate sensing area. This approach shows potential for both preclinical and clinical kidney injury diagnosis with accurate, rapid, noninvasive, and high throughput capabilities.

pH sensor using AlGaN∕GaN high electron mobility transistors with Sc2O3 in the gate region

Ungated AlGaN∕GaN high electron mobility transistors (HEMTs) exhibit large changes in current upon exposing the gate region to polar liquids. The polar nature of the electrolyte introduced leds to a change of surface charges, producing a change in surface potential at the semiconductor/liquid interface. The use of Sc2O3 gate dielectric produced superior results to either a native oxide or UV ozone-induced oxide in the gate region. The ungated HEMTs with Sc2O3 in the gate region exhibited a linear change in current between pH 3 and 10 of 37μA∕pH. The HEMT pH sensors show stable operation with a resolution of <0.1pH over the entire pH range. The results indicate that the HEMTs may have application in monitoring pH solution changes between 7 and 8, the range of interest for testing human blood.Ungated AlGaN∕GaN high electron mobility transistors (HEMTs) exhibit large changes in current upon exposing the gate region to polar liquids. The polar nature of the electrolyte introduced leds to a change of surface charges, producing a change in surface potential at the semiconductor/liquid interface. The use of Sc2O3 gate dielectric produced superior results to either a native oxide or UV ozone-induced oxide in the gate region. The ungated HEMTs with Sc2O3 in the gate region exhibited a linear change in current between pH 3 and 10 of 37μA∕pH. The HEMT pH sensors show stable operation with a resolution of <0.1pH over the entire pH range. The results indicate that the HEMTs may have application in monitoring pH solution changes between 7 and 8, the range of interest for testing human blood.