A novel metallization scheme for Ohmic contact (Ti/Al/ W 2 B /Ti/Au) to n-GaN using high temperature boride was studied using contact resistance, scanning electron microscopy and Auger Electron Spectroscopy measurements. A minimum contact resistance of 7×10 −6 Ω.cm 2 was achieved for W 2 B based scheme at an annealing temperature of 800 °C. Contact resistances were found to be essentially independent of measurement temperature, indicating that tunneling plays a dominant role in the current transport. The outdiffusion of Ti to the surface at temperatures of ~500°C, and at 800°C the onset of intermixing of Al within the contact was found to occur. By 1000°C, the contact showed a reacted appearance and AES showed almost complete intermixing of the metallization. The reliability measurements for the contact resistance of W 2 B based contact showed excellent stability for extended periods at 200°C, which simulates the type of device operating temperature that might be expected for operation of GaN-based power electronic devices.

In this work we focus on the fabrication of ohmic contacts and of Schottky barrier devices (SBD) on non-polar cubic GaN epilayers grown by molecular beam epitaxy (MBE). A Ti/Al/Ni/Au metallization was used for ohmic contacts and the contact resistance was measured by transmission line measurements (TLM). Ni, Pd, Ag and NiSi Schottky barrier devices 300 urn in diameter were fabricated by thermal evaporation using contact lithography on cubic GaN epilayers. The current-voltage (I-V) and the capacity-voltage (C-V) characteristics were studied at room temperature in detail. A clear rectifying behavior was measured in all SBDs. In the Ni and Ag SBDs an abnormal large leakage current under reverse bias was observed. Isochronal thermal annealing of these Ni and Ag based SBDs at 200°C in air improved the reverse characteristics by up to three orders of magnitude. This is in contrast to the Pd contacts, where the as grown contact showed already good performance and thermal annealing had nearly no influence on the I-V characteristics. For all SBDs the magnitude of the reverse current is generally larger than that expected due to thermionic emission and an exponential increase of the reverse current is observed with increasing reverse voltage. In-depth analysis of the I-V characteristic showed that a thin surface barrier is formed at the metal semiconductor interface and that crystal defects like dislocations may be the reasons for the discrepancy between experimental data and thermionic emission theory.

The carbon monoxide (CO) detection sensitivities of ZnO nanorod-gated AlGaN/GaN high electron mobility transistors were measured over a range of temperatures from 25–150 °C. Once the sensor was exposed to the CO-containing ambient, the drain current, I, of the high electron mobility transistors increased due to chemisorbed oxygen on the ZnO surface reacting with CO, forming CO2 and releasing electrons to the oxide surface. Although the sensor could detect CO as low as 100 ppm at room temperature, the detection sensitivity, ΔI/I, was only around 0.23%. By increasing the sensor temperature to 150 °C, the detection sensitivity was improved by a factor of over 30% to 7.5%.

A promising sensing technology utilizing AlGaN/GaN high electron mobility transistors (HEMTs) has been developed to analyze a wide variety of environmental and biological gases and liquids. The conducting 2DEG channel of GaN/AlGaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. Examples of detecting mercury ions, perkinsus, lactic acid, carbon dioxide, and vitellogenin are discussed in this paper.

The long-term stability of antibody-functionalized, Au-gated AlGaN/GaN high electron mobility transistors for detecting botulinum toxin is reported in this study. The botulinum toxin sensor, which initially showed good data reproducibility and recyclability, was repeatedly tested over a 9-month period. The botulinum sensor was packaged and stored in phosphate buffered saline (PBS) at 4°C in a refrigerator for long-term storage. The sensor was tested over time at room temperature and we found sensitivity losses of 2%, 12% and 28% after 3, 6 and 9 months, respectively. These results clearly demonstrate a significant step towards the realization of electronic detection of biomolecules by field-deployed sensor chips based on AlGaN/GaN HEMTs.

The effects of relative humidity on sensing characteristics of Pt-gated AlGaN/GaN high electron mobility transistor diode based hydrogen sensors were investigated. The absorbed water and oxygen molecules blocked available Pt surface adsorption sites for H2 absorption and reduced the hydrogen sensing sensitivity compared to low humidity conditions. The hydrogen sensing sensitivity decreased proportional to the relative humidity. However, the presence of humidity improved the sensor recovery characteristics after exposure to the hydrogen ambient.

There is continued interest in developing more stable contacts to a variety of GaN-based devices. In this paper we give two examples of devices that show improved thermal stability when boride, nitride or Ir diffusion barriers are employed in Ohmic contact stacks. AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/X /Ti/Au source/drain Ohmic (where X is TiB2, ZrN, TiN, TaN or Ir) contacts and subjected to long-term annealing at 350°C. For GaN layers with an electron concentration of ∼3×1017 cm-3, the minimum specific contact resistance achieved is 6×10-5 Ω cm2 for Ti/Al/TiN/Ti/Au after annealing at 800°C. The specific contact resistance was found to strongly depend on the doping level, suggesting that tunneling is the dominant mechanism of current flow. By comparison with companion devices with conventional Ti/Al/Ni/Au Ohmic contacts, the HEMTs with boride-based Ohmic metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C. The gate current for standard HEMTs increases during aging and the standard Ohmic contacts eventually fail by shorting to the gate contact. Similarly, InGaN/GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni/Au/TiB2/Ti/Au or Ni/Au/Ir/Au p-Ohmic contacts. Both of these contacts showed superior long-term thermal stability compared to LEDs with conventional Ni/Au contacts.

We report on the demonstration of light emission from hybrid CdZnO quantum-well light emitting diodes. A one-dimensional drift-diffusion method was used to model the expected band structure and carrier injection in the device, demonstrating the potential for 90% internal quantum efficiency when a CdZnO quantum well is used. Fabricated devices produced visible electroluminescence that was found to redshift from 3.32 to 3.15 eV as the forward current was increased from 20 to 40 mA. A further increase in the forward current to 50 mA resulted in a saturation of the redshift.

AlGaN/ GaN High Electron Mobility Transistors (HEMTS) were used to detect pH value and glucose concentration in the exhaled breath condensate (EBC). A Peltier element mounted on the backside of the AlGaN/GaN HEMT was used to lower the temperature of the sensor in order to condense the exhaled breath. The measured current change of the HEMT shows that the pH of the condensate from the exhaled breath is within the range of 7- 8, corresponding to the range of interest for human blood. The sensor exhibited a repeatable change of 1.24 mA/mm of the drain current at a drain bias of 0.25V, when the surface was exposed to repeated exhalations. ZnO nanorod-gated AlGaN/GaN HEMTs were used to detect glucose in EBC. A ZnO nanorod array was selectively grown on the gate area using low temperature hydrothermal decomposition to immobilize glucose oxidase (GOx).

We have demonstrated a wireless hydrogen sensing system using commercially available wireless components and AlGaN/GaN high electron mobility transistor (HEMTs) differential sensing diodes as the sensing devices. The active device in the differential pair is coated with 10nm of Pt to enhance catalytic dissociation of molecular hydrogen, while the reference diode is coated with Ti/Au. Our sensors have a wide range of detection from ppm levels to ∼30%, with the added advantages of a very rapid response time within a couple of seconds, and rapid recovery. The sensors have shown good stability for more than 18 months in an outdoor field test. Currently, the wireless sensing system consists of six wireless sensor nodes and a base station. The wireless sensor node consists of a sensor, a power management system with back-up batteries in case of power outages and a wireless transceiver. The base station consists of a high sensitivity receiver and an in-house developed intelligent monitoring software that does basic data logging and tracking of each individual sensor. The software defines and implements the monitoring states, transitions, and actions of the hydrogen sensor network. Also, the software is able to warn the user of potential sensor failure, power outages and network failures through cell phone network and Internet. Real-time responses of the sensors are displayed through a web site on the Internet. (http://ren.che.ufl.edu/app/default.aspx).

Thioglycolic acid functionalized Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect mercury(II) ions. The drain current of the HEMT sensors monotonically increased with the mercury(II) ion concentration from 1.5×10−8 to 4×10−8M. The drain current reached equilibrium around 15–20s after the concentrated Hg ion solution added to the gate area of the HEMT sensors. The effectiveness of the thioglycolic acid functionalization was evaluated with a surface contact angle study. The results suggest that portable, fast response, and wireless-based heavy metal ion detectors can be realized with AlGaN/GaN HEMT-based sensors.

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 5s when the target toxin in a buffer was added to the antibody-immobilized surface. We could detect a range of concentrations from 1to10ng∕ml. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN∕GaN HEMTs for botulinum toxin detection.

Abstract not Available.

Bare Au-gated and thioglycolic acid functionalized Au-gated AlGaN/GaN high electron mobility transistors were used to detect mercury (II) and copper (II) ions. Fast detection of < 5 s was achieved for thioglycolic acid functionalized sensors. The thioglycolic acid functionalization increased the sensitivity for detection of mercury by 2.5 times over the bare Au-gated surface. Both surfaces had a selectivity of approximately 100-fold over other contaminating ions of sodium, magnesium, and lead and can be easily recycled. Our results show that portable, selective, and fast Cu2+ and Hg2+ sensors can be realized by combining bare Au-gated and thioglycolic acid-functionalized surface in one sensor. (c) 2007 The Electrochemical Society. All rights reserved.

Electrical properties, admittance, and microcathodoluminescence spectra are compared for p-GaN samples grown by hydride vapor phase epitaxy (HVPE) and by molecular beam epitaxy (MBE). The former are characterized by a high 300K hole concentration and a weak temperature dependence of conductivity. The latter samples show strongly temperature-activated conductivity due to ionization of Mg acceptors. The main effects of neutron irradiation were similar for the p-HVPE and the p-MBE materials: a compensation of p-type conductivity starting with neutron fluences exceeding 2×1016cm−2 and conversion to high resistivity n type with the Fermi level pinned near Ec-(0.8–0.9)eV after irradiation with high doses of 1018cm−2. For the heavily neutron irradiated p-HVPE samples, a strong increase was observed in the c-lattice parameter which indicates an important role for interstitial-type defects.

We report on p-type AlGaN/GaN and GaN/InGaN superlattice (SL) designs with significantly improved vertical and lateral electrical conductivity (σv and σL). Composition-graded p–AlGaN layers were used to produce eight fold reduction in barrier height and a ~40% increase in the sheet hole density in the p-GaN wells compared to typical SL structures. Thirteen orders of magnitude and 35 times improvement is obtained for σv and σL compared to typical SL and bulk p-GaN, respectively. A similar approach for p-GaN/InGaN SL resulted in seven fold reduction in barrier height and a ~30% increase in the sheet hole density in the p-InGaN wells compared to a typical SL structures. σL is strongly dependent on hole mobility in the well and about 19 times improvement is obtained for the optimized design, SL-B, with µp=30 cm2 V-1s−1 compared to bulk-InGaN. More than 10 orders of magnitude improvement in σV is obtained for SL-B compared to modulation doped SL.

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 4μA as this protein was introduced to the surface, showing the promise of this all-electronic detection approach for biological sensing.

Al Ga N ∕ Ga N high electron mobility transistors (HEMTs) both with and without a Au gate are found to exhibit significant changes in channel conductance upon exposing the gate region to various halide ions. The polar nature of the halide ions leads to a change of surface charge in the gate region on the HEMT, producing a change in the surface potential at the semiconductor∕liquid interface. HEMTs with a Au-gate electrode not only doubled the sensitivity of changing the channel conductance as compared to gateless HEMT, but also showed the opposite conductance behavior. When anions adsorbed on the Au, they produced a counter charge for electrovalence. These anions drag some counter ions from the bulk solution or create an image positive charge on the metal for the required neutrality. The gateless HEMTs can be used as sensors for a range of chemicals through appropriate modification with covalently bonded halide functional groups on the Au surface. This creates many possibilities to functionalize the surface for a wide range of integrated biological, chemical, and fluid monitoring sensors.

Iron- and carbon-doped GaN and iron-doped Al0.2Ga0.8N irradiated by low energy electron beam of scanning electron microscope were studied by cathodoluminescence and electron beam-induced current techniques. Irradiation is shown to induce a systematic decay of the cathodoluminescence intensity, which is accompanied by increased electronic carrier diffusion length, indicating the increase of carrier lifetime. Temperature-dependent cathodoluminescence measurements yielded activation energies for irradiation-induced effects of 210, 230, and 360meV for GaN:C, GaN:Fe, and Al0.2Ga0.8N:Fe, respectively. These observations are consistent with trapping of non-equilibrium electrons on deep, non-ionized acceptor levels.