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Chemical sensors can be used to analyze a wide variety of environmental and biological gases and liquids and may need to be able to selectively detect a target analyte. Different methods, including gas chromatography, chemiluminescence, selected ion flow tube, and mass spectroscopy, have been used to measure biomarkers. These methods show variable results in terms of sensitivity for some applications and may not meet the requirements for a handheld biosensor. A promising sensing technology utilizes AlGaN/GaN high electron mobility transistors (HEMTs). HEMT structures have been developed for use in microwave power amplifiers due to their high two dimensional electron gas (2DEG) mobility and saturation velocity. The conducting 2DEG channel of AlGaN/GaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. HEMT sensors can be used for detecting gases, ions, pH values, proteins, and DNA. In this paper we review recent progress on functionalizing the surface of HEMTs for specific detection of glucose, kidney marker injury molecules, prostate cancer, and other common substances of interest in the biomedical field. [ABSTRACT FROM AUTHOR]

The performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with either uncapped surfaces or with low-temperature (LT) GaN or SiO2 or SiNx as gate insulators is reported. The sheet carrier concentrations of AlGaN/GaN HFETs with any of these surface insulating layers are similar to each other and in each case about 50% higher than that in an AlGaN/GaN HFET with a free surface. This result is consistent with the insulator layers providing passivation of surface states that cause the depletion of the channel layer. Due to the closer lattice match with the AlGaN surface layer, the HFET with a LT-GaN layer as the gate insulator shows the best dc and rf device performance, demonstrating that this material is an effective insulator for nitride electronic devices. [ABSTRACT FROM AUTHOR]

Presents a study which compared Hall-effect measurements combined with annealing and photoluminescence as characterization techniques for the optimization of the growth of pseudomorphic InGaAS channel modulation-doped field-effect-transistor structures. Growth of the epitaxial layers; Results and discussion; Conclusion.

Describes a single-energy, implant isolation scheme for thick semiconductor device structures such as heterojunction bipolar transistors. Advantages of III-V semiconductors over silicon-based technologies; Details on the experiment; Discussion on the results of the study.

Deep traps were studied in multiple-finger AlGaN/GaN transistors with broad periphery by means of current-voltage and capacitance-voltage characteristics, reverse deep level transient spectroscopy, deep level transient spectroscopy with electrical (DLTS) and optical DLTS injection, and current DLTS (CDLTS) with gate voltage and drain voltage pulsing. Deep electron traps with activation energies of 0.25, 0.36, 0.56, and 0.8-0.9 eV are found to be responsible for variations in threshold current with applied gate bias. These traps also give rise to current transients observed at different temperatures in CDLTS. The 0.25 and 0.56 eV centers are most likely located at the AlGaN/GaN interface. The 0.25 eV state is believed to be nitrogen vacancy-related, the 0.36 eV level is of unknown origin, the 0.56 eV level is likely due to point defects enhanced in the presence of dislocations, and responsible for low luminescence efficiency of nonpolar n-GaN and for degradation of GaN transistors and light-emitting diodes and light-emitting diodes, while the 0.8 eV-level has been attributed to either interstitial Ga or N. Hole-trap-like relaxations in CDLTS were observed at temperatures higher than ~300K and attributed to hole traps near Evþ0.9 eV located in the buffer layer. These have previously been assigned to either gallium vacancy-oxygen acceptor complexes (VGa-O)2- or carbon on nitrogen site acceptor (CN) complexes. These results show that the stability of GaN-based high electron mobility transistors on Si is still dominated by the high concentration of point defects present in the material. [ABSTRACT FROM AUTHOR]-) complexes. These results show that the stability of GaN-based high electron mobility transistors on Si is still dominated by the high concentration of point defects present in the material. [ABSTRACT FROM AUTHOR]

A review of electron, proton and neutron damage in GaN and AlGaN materials and devices such as high electron mobility transistors and lightemitting diodes is presented. A comparison of theoretical and experimental threshold displacement energies is given, along with a summary of energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects. Many studies have shown that GaN is several orders of magnitude more resistant to radiation damage than GaAs, i.e., it can withstand radiation doses at least two orders of magnitude higher than those degrading GaAs of similar doping level. In terms of heterostructures, the initial data suggests that the radiation hardness decreases in the order AlN/GaN >AlGaN/GaN > InAlN/GaN, consistent with the average bond strengths in the Al-based materials. Many issues still have to be addressed. Among them are the strong asymmetry in carrier removal rates in n-and p-type GaN and interaction of radiation defects with Mg acceptors, and the poor understanding of the interaction of radiation defects in doped nitrides with the dislocations always present. [ABSTRACT FROM AUTHOR]

A finite element simulation was used to estimate the temperature rise of laser lift-off AlGaN/GaN high electron mobility transistors (HEMTs) mounted on AlN substrates via silicone elastomer under various conditions of power density on the transistor and for a range of elastomer thicknesses. The thermal response was compared to HEMTs fabricated on SiC and sapphire substrates. The maximum temperature of the transistor occurred in the gate area at a power level of 15 W, and was only 25 K higher than HEMTs fabricated on SiC. The transit time required for steady-state temperature of the flip-chip bonding device is in the range of a millisecond, which is faster than that of most power switch applications. [ABSTRACT FROM AUTHOR]

AlGaN/GaN structures grown on Si substrates by metal organic chemical vapor deposition have been processed into high power transistors with maximum input power density of 12 W/mm. The transistor showed low concentration of bulk electron traps in the GaN buffer and of interfacial traps at the AlGaN/GaN interface. A new version of contactless thermography method based on microcathodoluminescence spectra for transistors under operation was shown to produce similar results to more established techniques based on measurements of the E2(high) phonon frequency shift in the GaN buffer and of the TO phonons in the Si substrate. The results of temperature measurements are in good agreement with modeling results. [ABSTRACT FROM AUTHOR]

We have demonstrated significant improvements of AlGaN/GaN high electron mobility transistors (HEMTs) dc performance by employing Pt/Ti/Au instead of the conventional Ni/Au gate metallization. During off-state bias stressing, the typical critical voltage for HEMTs with Ni/Au gate metallization was ∼-45 to -65 V. By sharp contrast, no critical voltage was observed for HEMTs with Pt/Ti/Au gate metallization, even up to -100 V, which was the instrumental limitation in this experiment. After the off-state stressing, the drain current of Ni/Au gated-HEMTs decreased by ∼15%. For the Pt-gate HEMTs, no degradation of the drain current occurred and there were minimal changes in the Schottky gate characteristics for both forward and reverse bias conditions. The HEMTs with Pt/Ti/Au metallization showed an excellent drain on/off current ratio of 1.56×108. The on/off drain current ratio of Ni-gated HEMTs was dependent on the drain bias voltage and ranged from 1.16×107 at VDS=5 V and 6.29×105 VDS=40 V due to the larger gate leakage current at higher drain bias voltage. [ABSTRACT FROM AUTHOR]

Reliability studies of InAlAs/InGaAs metamorphic high electron mobility transistors (MHEMTs) grown on GaAs substrates for high frequency/power applications are reported. The MHEMTs were stressed at a drain voltage of 3 V for 36 h, as well as undergoing a thermal storage test at 250 °C for 48 h. The drain current density of the MHEMTs at zero gate bias dropped about 12.5% after either the thermal storage experiment or dc stress. The gate current of the MHEMT devices with thermal storage was much higher than that of devices after dc stress. In the latter case, significant gate sinking was observed by transmission electron microscopy. The main degradation mechanism during thermal storage was the reaction of the Ohmic contact with the underlying semiconductor. [ABSTRACT FROM AUTHOR]

Transparent dual-gate (DG) InGaZnO4 thin film transistors for OR logic operation were fabricated on a glass substrate. A 100-nm-thick SiO2 layer used as both top and bottom gate dielectrics was deposited by plasma enhance chemical vapor deposition at 200 °C. Compared to bottom gate, top gate thin film transistors (TFTs) exhibited better device performance with higher saturation mobility, drain current on-to-off ratio, lower threshold voltage, and subthreshold gate-voltage swing. This improved performance was mainly attributed to low process-induced damage or low parasitic capacitance between gate and source/drain and low parasitic resistance between channel and source/drain in top-contact TFT configuration (coplanar type). DG-mode TFTs showed saturation mobility of ∼16.9 cm2 V-1 s-1, drain current on-to-off ratio of ∼1×106, subthreshold gate-voltage swing of ∼0.33 V decade-1, and threshold voltage of ∼1.25 V. The results demonstrate that DG InGaZnO4 TFTs are effective in improving the device performance because the channel layer is modulated independently by a top or, bottom gate signal and are well suited for OR gate operation. [ABSTRACT FROM AUTHOR]

The performance of amorphous InGaZnO4 thin film transistors with three different gate dielectrics (SiO2, SiON, and SiNx) is reported. The gate dielectric films were prepared by plasma enhanced chemical vapor deposition with SiH4, N2O, and NH3 gases at 250 °C. Current-voltage (C-V) and refractive index characterization showed that different film compositions were achieved when changing the flow rate of N2O in the precursor gas mixture. The use of SiNx films reduced the interfacial roughness of the channel/gate dielectric by a factor of 2.3 compared to SiO2. This results in an improvement of saturation mobility of the thin film transistors (TFTs) by a similar factor. An enhancement of subthreshold gate-voltage swing and drain current on-to-off ratio for TFTs with SiNx was also attributed to the reduction of the trap density at the channel/gate-dielectric interface. [ABSTRACT FROM AUTHOR]

A process for achieving high yield of SiC through wafer via holes without trenching or micromasking and with excellent electrical connection after subsequent metal plating across full wafers was developed for use in high electron mobility transistors (HEMTs) and microwave monolithic integrated circuits (MMICs) using an inductively coupled plasma etch. Consideration was given to the choice of wafer platen, hard mask, gas chemistry, surface treatments, and plasma parameters in order to achieve an acceptable etch rate while at the same time minimizing trenching and micromasking that can harm via yield. In addition, the issue of wafer thickness variation and etch nonuniformity leading to punch through of Au pads at the bottom of the vias was addressed by the addition of a metal layer to the front side of the wafer. The etch rate achieved for 25% of a 2 in. diameter wafer is approximately 3800 Å/min while demonstrating acceptable levels of trenching and micromasking with little or no Au punch through. The final process has been demonstrated to achieve >95% yield across a full 2 in. diameter, 100 μm thick wafer with a high density of vias. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors were grown by molecular beam epitaxy on Si on poly-SiC substrates formed by the Smart Cut™ process. The Smart Cut™ approach is an alternative solution to provide both a high resistivity and an excellent thermal conductivity template needed for power applications. Although the structure has not been optimized, devices with 0.7 μm gate length show breakdown voltage of >250 V, fT of 18 GHz, and fmax of 65 GHz. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors were irradiated with 60Co gamma-rays to doses up to 1000 Gy, in order to analyze the effects of irradiation on the devices' transport properties. Temperature-dependent electron beam-induced current measurements, conducted on the devices before and after exposure to gamma-irradiation, allowed for the obtaining of activation energies related to radiation-induced defects due to nitrogen vacancies. DC current-voltage measurements were also conducted on the transistors to assess the impact of gamma-irradiation on transfer, gate, and drain characteristics. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with zinc oxide (ZnO) nanowires modified gate exhibit significant changes in channel conductance upon expose to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition (CVD) with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change in surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced 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 in the open cavity with continuous gas flow using a 50×50 μm2 gate sensing area. [ABSTRACT FROM AUTHOR]

Nitrogen ion-implanted AlGaN/GaN high electron mobility transistor structures showed an isolation blocking voltage of 900 V with a leakage current at 1 μA/mm across an implanted isolation-gap of 10 μm between two Ohmic pads. The effect of implanted gap distance (1.7, 5, or 10 μm) between two Ohmic contact pads was evaluated. The isolation current density was determined to be solely dependent on the applied field between the contact pads. A model using a combination of resistive current and Poole-Frenkel current is consistent with the experimental data. The resistance of the isolation implantation region significantly decreased after the sample was annealed at temperatures above 600 °C. [ABSTRACT FROM AUTHOR]

We investigated the transfer characteristics and the gate-bias stability of amorphous indium-gallium-zinc oxide thin-film transistors when the channel layer was exposed to hydrogen, oxygen, air, or vacuum at room temperature during measurements. The threshold voltage and the drain current were changed by the ambient atmospheres. The threshold voltage shift (ΔVth) under gate-bias stress was faster in hydrogen than in oxygen and vacuum. It is suggested that hydrogen exposure degrades the gate-bias stress stability due to surface accumulation layer creation. The characteristic trapping times, τ, in H2, O2, air, and vacuum were 5×103, 1.5×104, 2×104, and 6.3×104 s, respectively. [ABSTRACT FROM AUTHOR]

Sc2O3/AlGaN/GaN metal-oxide-semiconductor (MOS) high electron mobility transistors were fabricated on (1120) a-plane, Si-doped epitaxial layers grown by metal organic chemical vapor deposition on (1 1_20) r-plane sapphire substrates. The gate finger direction was varied from parallel to the in-plane c-axis [0001] to the m-axis [1 01_0] direction perpendicular to the observed striations in surface morphology. The [0001] gate devices show lower drain-source current, mobility, and transconductance than their [1 01_0] counterparts, with a systematic dependence on gate finger direction. The symmetrical gate current-voltage characteristics and low gate current (∼3×10-9 A at ±5 V) confirm the effectiveness of the MOS gate, which is an attractive approach for these structures containing heavily Si-doped AlGaN. [ABSTRACT FROM AUTHOR]

Enhancement mode AlN/GaN high electron mobility transistors (HEMTs) were fabricated from originally depletion-mode structures using oxygen plasma treatment on the gate area prior to the gate metallization. Starting with a depletion mode AlN/GaN HEMT, the threshold voltage of the HEMT could be shifted from -3.2 to 1 V depending on the oxygen plasma treatment time to partially convert the AlN barrier layer into Al oxide. The gate current was reduced and the current-voltage curves show metal-oxide semiconductor diodelike characteristics after oxygen plasma treatment. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with a polarized polyvinylidene difluoride (PVDF) film coated on the gate area exhibited significant changes in channel conductance upon exposure to different ambient pressures. The PVDF thin film was deposited on the gate region with an ink-jet plotter. Next, the PDVF film was polarized with an electrode located 2 mm above the PVDF film at a bias voltage of 10 kV and 70 °C. Variations in ambient pressure induced changes in the charge in the polarized PVDF, leading to a change in surface charges on the gate region of the HEMT. Changes in the gate charge were amplified through the modulation of the drain current in the HEMT. By reversing the polarity of the polarized PVDF film, the drain current dependence on the pressure could be reversed. Our results indicate that HEMTs have potential for use as pressure sensors. [ABSTRACT FROM AUTHOR]

Antibody-functionalized, Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect botulinum toxin. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN/GaN HEMT drain-source current showed a rapid response of less than 5 s when the target toxin in a buffer was added to the antibody-immobilized surface. We could detect a range of concentrations from 1 to 10 ng/ml. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN/GaN HEMTs for botulinum toxin detection. [ABSTRACT FROM AUTHOR]

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 167 nM to 139 μM. Our results show that portable, fast response, and wireless-based lactic acid detectors can be realized with AlGaN/GaN HEMT based sensors. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with GaN cap layers were irradiated with high-energy (17 MeV) protons to doses up to 2 x 1016 cm-2. There was no significant degradation in dc electrical parameters such as drain-source current (IDS) and extrinsic transconductance (gm) of the HEMTs up to a fluency of 7.2 x 1013 protons/cm². At the highest dose of 2 x 1016 protons/cm² there was a decrease of 43% in IDS and a 29% decrease in gm. The data are consistent with the introduction of defect centers in the HEMT structure by the high-energy protons leading to a reduction in carrier concentration and mobility in the two-dimensional electron gas channel. These results show that AIGaN/GaN HEMTs are attractive for space-based applications where high-energy proton fluxes are present. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) functionalized with polyethylenimine/starch were used for detecting CO2 with a wide dynamic range of 0.9%–50% balanced with nitrogen at temperatures from 46 to 220 °C. Higher detection sensitivity to CO2 gas was achieved at higher testing temperatures. At a fixed source-drain bias voltage of 0.5 V, drain-source current of the functionalized HEMTs showed a sublinear correlation upon exposure to different CO2 concentrations at low temperature. The superlinear relationship was at high temperature. The sensor exhibited a reversible behavior and a repeatable current change of 32 and 47 μA with the introduction of 28.57% and 37.5% CO2 at 108 °C, respectively. [ABSTRACT FROM AUTHOR]

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 5 s 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 0.5 nM to 125 μM. The sensor exhibited a linear range from 0.5 nM to 14.5 μM and an experiment limit of detection of 0.5 nM. This demonstrates the possibility of using AlGaN/GaN HEMTs for noninvasive exhaled breath condensate based glucose detection of diabetic application. [ABSTRACT FROM AUTHOR]

The origin of hysteresis in capacitance-voltage (C-V) characteristics was studied for Schottky diodes prepared on AlGaN/GaN transistor structures with GaN (Fe) buffers. The application of reverse bias leads to a shift of C-V curves toward higher positive voltages. The magnitude of the effect is shown to increase for lower temperatures. The phenomenon is attributed to tunneling of electrons from the Schottky gate to localized states in the structure. A technique labeled “reverse” deep level transient spectroscopy was used to show that the deep traps responsible for the hysteresis have activation energies of 0.25, 0.6, and 0.9 eV. Comparison with deep trap spectra of GaN buffers and Si doped n-GaN films prepared on GaN buffers suggests that the traps in question are located in the buffer layer. [ABSTRACT FROM AUTHOR]

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 5 nm 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 1 ng/ml using a 20×50 μm2 gate sensing area. This approach shows potential for both preclinical and clinical kidney injury diagnosis with accurate, rapid, noninvasive, and high throughput capabilities. [ABSTRACT FROM AUTHOR]

An enhancement of ∼0.18 eV (an 18% increase) in Schottky barrier height was obtained for Au deposited at cryogenic temperatures on n-type GaN relative to conventional deposition at 300 K (barrier height of 1.0 eV). Enhancements of 0.04–0.11 eV were achieved for Au deposition on p-GaN under the same conditions. The increase in barrier height on n-GaN persists for annealing temperatures up to ∼200 °C. At higher annealing temperatures, both types of diodes show a deterioration in rectifying behavior. The reverse current of low temperature deposited diodes was approximately two orders of magnitude lower than conventional Au/n-GaN diodes. The ideality factor of the cryogenically processed n-type devices (∼1.06) was similar to that for room temperature diodes (1.13). This simple process method has potential for improving output resistance and power gain and lowering gate leakage current and noise in GaN-based transistors. [ABSTRACT FROM AUTHOR]

Pt-gated AlGaN/GaN high electron mobility transistors can be used as room-temperature hydrogen gas sensors at hydrogen concentrations as low as 100 ppm. A comparison of the changes in drain and gate current-voltage (I-V) characteristics with the introduction of 500 ppm H2 into the measurement ambient shows that monitoring the change in drain-source current provides a wider gate voltage operation range for maximum detection sensitivity and higher total current change than measuring the change in gate current. However, over a narrow gate voltage range, the relative sensitivity of detection by monitoring the gate current changes is up to an order of magnitude larger than that of drain-source current changes. In both cases, the changes are fully reversible in <2–3 min at 25 °C upon removal of the hydrogen from the ambient. [ABSTRACT FROM AUTHOR]

MgO has proven effective in the past as a surface passivation layer to minimize current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). However, MgO is not environmentally stable and more stable oxides need to be developed. MgCaO can be produced that is lattice matched to the GaN. Three samples were grown with 0%, 50% and 75% of Ca, which had respective lattice mismatches of -6.5% for MgO, -1% for Mg0.50Ca0.50O and +4% for Mg0.25Ca0.75O. Drain saturation current in HENTs had increases of 4.5% and 1%, respectively, for Mg0.5Ca0.5O and Mg0.25Ca0.75O passivated devices. However, there was a 10% decrease for the device passivated with pure MgO. This was due to strain applied on the nitride HEMT by the oxide, which is consistent with the piezoelectric effect in the nitride HEMT by the oxide, which is consistent with the piezoelectric effect in the nitride form the lattice mismatch between AlGaN and GaN. From pulsed measurements, HEMTs passivated with Mg0.5Ca0.5O and Mg0.25Ca0.75O showed higher passivation effectiveness (90% of dc current) then the MgO passivated HEMTs (83% dc current). This is due to the closer lattice matching of these calcium containing oxides and the reduction in interface traps associated with lattice mismatch. [ABSTRACT FROM AUTHOR]

Ungated AlGaN/GaN high-electron-mobility transistor (HEMT) structures were functionalized in the gate region with aminopropyl silane. This serves as a binding layer to the AlGaN surface for attachment of fluorescent biological probes. Fluorescence microscopy shows that the chemical treatment creates sites for specific absorption of probes. Biotin was then added to the functionalized surface to bind with high affinity to streptavidin proteins. The HEMT drain-source current showed a clear decrease of 4 μA as this protein was introduced to the surface, showing the promise of this all-electronic detection approach for biological sensing. [ABSTRACT FROM AUTHOR]

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 -(+)6.4×10-2 mS/bar for application of compressive (tensile) strain. The AlGaN/GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications. [ABSTRACT FROM AUTHOR]

Pt contacted AlGaN/GaN high electron mobility transistors with Sc2O3 gate dielectrics show reversible changes in drain–source current upon exposure to H2-containing ambients, even at room temperature. The changes in current (as high as 3 mA for relatively low gate voltage and drain–source voltage) are approximately an order of magnitude larger than for Pt/GaN Schottky diodes and a factor of 5 larger than Sc2O3/AlGaN/GaN metal–oxide–semiconductor (MOS) diodes exposed under the same conditions. This shows the advantage of using a transistor structure in which the gain produces larger current changes upon exposure to hydrogen-containing ambients. The increase in current is the result of a decrease in effective barrier height of the MOS gate of 30–50 mV at 25 °C for 10% H2/90% N2 ambients relative to pure N2 and is due to catalytic dissociation of the H2 on the Pt contact, followed by diffusion to the Sc2O3/AlGaN interface. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electrical and optical properties of semi-insulating GaN films with the lower part of the film doped with Fe were studied. The room-temperature sheet resistivity of the films was found to be 2×10[sup 10] Ω/square. The activation energy of the dark conductivity was ∼0.5 eV which corresponds to the depth of the dominant electron traps pinning the Fermi level. The concentration of these traps was highest in the Fe-doped portion of the films and was on the order of 3×10[sup 16] cm[sup -3]. Photoinduced current transient spectroscopy also showed the presence of a high concentration of deeper electron traps with level at E[sub C]-0.9 eV and hole traps at E[sub V]+0.9 eV. These layers look very promising as insulating buffers for AlGaN/GaN transistor structures. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We demonstrated that Sc[sub 2]O[sub 3] thin films deposited by plasma-assisted molecular-beam epitaxy can be used simultaneously as a gate oxide and as a surface passivation layer on AlGaN/GaN high electron mobility transistors (HEMTs). The maximum drain source current, I[sub DS], reaches a value of over 0.8 A/mm and is ∼40% higher on Sc[sub 2]O[sub 3]/AlGaN/GaN transistors relative to conventional HEMTs fabricated on the same wafer. The metal-oxide-semiconductor HEMTs (MOS-HEMTs) threshold voltage is in good agreement with the theoretical value, indicating that Sc[sub 2]O[sub 3] retains a low surface state density on the A1GaN/GaN structures and effectively eliminates the collapse in drain current seen in unpassivated devices. The MOS-HEMTs can be modulated to +6 V of gate voltage. In particular, Sc[sub 2]O[sub 3] is a very promising candidate as a gate dielectric and surface passivant because it is more stable on GaN than is MgO. [ABSTRACT FROM AUTHOR]

Sc[sub 2]O[sub 3]-passivated AlGaN/GaN high electron mobility transistors (HEMTs) were irradiated with 40 MeV protons to a fluence corresponding to approximately 10 years in low-earth orbit (5 × 10[sup 9] cm[sup -2]). Devices with an AlGaN cap layer showed less degradation in dc characteristics than comparable GaN-cap devices, consistent with differences in average band energy. The changes in device performance could be attributed completely to bulk trapping effects, demonstrating that the effectiveness of the Sc[sub 2]O[sub 3] layers in passivating surface states in the drain-source region was undiminished by the proton irradiation. Sc[sub 2]O[sub 3]-passivated AlGaN/HEMTs appear to be attractive candidates for space and terrestrial applications where resistance to high fluxes of ionizing radiation is a criteria. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with different gate length and widths were irradiated with [sup 60]Co γ-rays to doses up to 600 Mrad. Little measurable change in dc performance of the devices was observed for doses lower than 300 Mrad. At the maximum dose employed, the forward gate current was significantly decreased, with an accompanying increase in reverse breakdown voltage. This is consistent with a decrease in effective carrier density in the channel as a result of the introduction of deep electron trapping states. The threshold voltage shifted to more negative voltages as a result of the irradiation, while the magnitude of the drain–source current was relatively unaffected. This is consistent with a strong increase of trap density in the material. The magnitude of the decrease in transconductance of the AlGaN/GaN HEMTs is roughly comparable to the decrease in dc current gain observed in InGaP/GaAs heterojunction bipolar transistors irradiated under similar conditions. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]