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Sputtered p-NiO films were used to suppress gate leakage and produce a positive shift in gate voltage of AlGaN/GaN high electron mobility transistors for e-mode operation. A direct comparison with Schottky gate devices fabricated on the same wafer shows the utility of the NiO in increasing the on-off ratio and shifting the threshold voltage from -0.95 V (Schottky gated) to +0.9 V (NiO gated). The breakdown voltage was 780 V for a 40 µm drain-source separation. The sub-threshold swing decreased from 181 mV/dec for Schottky gate HEMTs to 128 mV/dec on NiO-gated devices. The simple fabrication process without any annealing or passivation steps shows the promise of NiO-gates for e-mode AlGaN/GaN HEMT operation.

The characteristics of NiO/ β-(Al0.21Ga0.79)2O3/Ga2O3 heterojunction lateral geometry rectifiers with the epitaxial layers grown by Metal Organic Chemical Vapor Deposition were measured over the temperature range from 25-225°C. The forward current increased with temperature, while the on-state resistance decreased from 360 Ω•cm2at 25°C to 30 Ω.cm2at 225°C. The forward turn-on voltage was reduced from 4 V at 25°C to 1.9 V at 225°C. The reverse breakdown voltage at room temperature was ~4.2 kV, with a temperature coefficient of -16.5 V/K. This negative temperature coefficient precludes avalanche being the breakdown mechanism and indicates that defects still dominate the reverse conduction characteristics. The corresponding power figures-of-merit were 0.27-0.49 MW.cm-2. The maximum on/off ratios improved with temperature from 2105at 25°C to 3107at 225°C when switching from 5 V forward to 0 V. The high temperature performance of the NiO/ β-(Al0.21Ga0.79)2O3/Ga2O3lateral rectifiers is promising if the current rate of optimization continues.

Ga2O3 is a promising wide bandgap semiconductor for applications including power electronics and photodetectors and is available in large diameter, high quality bulk crystalline form. The high-power/high-voltage market is currently primarily served by Si and SiC devices. Ga2O3 is the leading candidate to address the ultra-high power market (>1kW). We will discuss recent progress in developing Ohmic contacts using interlayers of ITO or AZO, high density plasma etching using ICP discharges and the resultant etch rates, morphologies and near-surface damage, behavior of hydrogen incorporated during ion implantation or plasma exposure, measurement of band offsets with dielectrics and application of these processes to vertical and lateral transistor structures.

The fabrication and switching characteristics of large diameter (1mm) Ga2O3heterojunction rectifiers with bilayers of NiO as the p-type layer are reported. The band alignment shows this is a staggered-gap system. Selective wet and dry etching was developed to allow fabrication of vertical geometry heterojunction rectifiers. A forward current of 2.5 Amperes was achieved, with a reverse breakdown voltage of 2.7 kilovolts. The on/off ratio was above 108over a broad range of switching voltages, with a measurement-limited reverse recovery time of 100 nanoseconds.

NiO/ β-(AlxGa1-x)2O3/Ga2O3heterojunction lateral geometry rectifiers with diameter 50-100 µm exhibited maximum reverse breakdown voltages >7kV, showing the advantage of increasing the bandgap using the β-(AlxGa1-x)2O3alloy. This Si-doped alloy layer was grown by Metal Organic Chemical Vapor Deposition with an Al composition of ~21 %. On state resistances were in the range 50-2180 Ω.cm2, leading to power figures-of-merit up to 0.72 MW.cm-2. The forward turn-on voltage was in the range 2.3-2.5 V, with maximum on/off ratios >700 when switching from 5V forward to reverse biases up to -100V. Transmission line measurements showed the specific contact resistance was 0.12 Ω.cm2. The breakdown voltage is among the highest reported for any lateral geometry Ga2O3-based rectifier.

There is increasing interest in the alpha polytype of Ga2O3because of its even larger bandgap than the more studied beta polytype, but in common with the latter, there is no viable p-type doping technology. One option is to use p-type oxides to realize heterojunctions and NiO is one of the candidate oxides. The band alignment of sputtered NiO on α-Ga2O3 remains type II, staggered gap for annealing temperatures up to 600°C, showing that this a viable approach for hole injection in power electronic devices based on the alpha polytype of Ga2O3.The magnitude of both the conduction and valence band offsets increase with temperature up to 500°C, but then are stable to 600°C. For the as-deposited NiO/ α-Ga2O3heterojunction, ΔEV=-2.8 eV and ΔEC= 1.6 eV, while after 600°C annealing the corresponding values are ΔEV=-4.4 eV and ΔEC= 3.02 eV. These values are 1-2 eV larger than for the NiO/β-Ga2O3 heterojunction.

NiO/Ga2O3heterojunction rectifiers were exposed to 1Mrad fluences of Co-60 g-rays either with or without reverse biases. While there is small component of Compton electrons (600 keV), generated via the interaction of 1.17 and 1.33 MeV gamma-photons with the semiconductor, which in turn can lead to displacement damage, most of the energy is lost to ionization. The effect of the exposure to radiation is a 1000x reduction in forward current and 100x increase in reverse current in the rectifiers, which is independent of whether the devices were biased during this step. The on-off ratio is also reduced by almost 5 orders of magnitude. There is a slight reduction in carrier concentration in the Ga2O3 drift region, with an effective carrier removal rate of < 4 cm-1. The changes in electrical characteristics are reversible by application of short forward current pulses during repeated measurement of the current-voltage characteristics at room temperature. There are no permanent total ionizing dose effects present in the rectifiers to 1 Mad fluences, which along with their resistance to displacement damage effects indicate these devices may be well-suited to harsh terrestrial and space radiation applications if appropriate bias sequences are implemented to reverse the radiation-induced changes.

Recent developments in identification of as-grown defects and impurities in the GaN buffer region of AlGaN/GaN High Electron Mobility Transistors (HEMTs) provide an opportunity to identify their role in degradation of dynamic switching behavior. Using TCAD simulation, effect of capture-emission by electron and hole traps on drain current transients during drain lag switching experiments is presented which is otherwise difficult to predict analytically.

The effects of proton irradiation energy on Al2O3/AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOSHEMT) were studied. MOSHEMTs were irradiated at different irradiation energies of 5 MeV, 10 MeV, or 15 MeV with a fixed proton dose of 5 x 1015 cm-2. After the 5 MeV, 10 MeV and 15 MeV proton irradiation, MOSHEMTs' saturation current at gate voltage of 1V were reduced by 95.3, 68.3 and 59.8% and maximum transconductance were reduced by 88, 54.4, and 40.7% respectively. The carrier removal rate of the irradiation energy employed in this study was in the range of 127-289 cm-1, having lower energy with the highest removal rate.

The influence of different buffer layer quality on AlGaN/GaN high electron mobility transistors (HEMTs) dc characteristics was investigated. Dc measurement by parameter analyzer and gate pulse measurement were performed to compare two different buffer layer quality samples. The same Al concentrations of AlGaN with 2um and 5um GaN buffer layers on sapphire substrates from two different vendors were used. The defect densities of 2 um and 5 um GaN buffer layer's HEMTs structures measured by transmission electron microscopy (TEM) were 7 x 109 cm-2 and 5 x 108 cm-2, respectively. There was small difference in drain saturation current and transfer characteristics in HMETs for these two types of buffer. Non-passivated HEMT with 5 um GaN buffer layer showed no dispersion in gate-lag pulsed measurement at 100 kHz but HEMT with 2 um GaN buffer layer showed 71% drain current reduction at 100 kHz gate-lag pulsed measurement.

The effects of buffer oxide etchant on Ti/Al/Ni/Au Ohmic contacts for AlGaN/GaN high electron mobility transistor were studied. Scanning electron microscopy (SEM), energy dispersive spectrum (EDX), and Auger electron spectroscopy (AES) were performed to investigate the degradation after treatment. Contact resistance increase from 1x10-4 to 2.5x10-4 ohm/cm2 after treatment. Ohmic metals were found out to be divided into three regions including islands, rings surround the islands, and the remaining field area. The islands were found to be shorter and the rings thinner after treatment. Besides, Ti, Al and Ni were found to be etched by BOE and caused Au to peel off from the Ohmic metals. The etching of Ti, Al and Ni was believed to be the reason of degradation.

Proton irradiations from the backside of the AlGaN/GaN high electron mobility transistors (HEMTs) were employed to enhance its off-state drain breakdown voltage. Via holes were fabricated directly under the active area of the HEMTs by etching through the Si substrate for the backside irradiations. The steep drop at the end of proton energy loss profile allows radiation-induced defects to be placed at precise locations within the AlGaN/GaN HEMT structure. Neither degradation of drain current nor enhancement of off-state drain voltage breakdown voltage was observed for the HEMTs irradiated with the proton energy of 225 or 275 keV, for which the defects were put in the GaN buffer. HEMTs with defects placed in the two dimensional electron gas (2DEG) channel region and the AlGaN barrier, using 330 or 340 keV irradiations, not only showed degradations on drain current and extrinsic transconductance but also demonstrated improvements of the off-state drain breakdown voltage.

We investigated the traps affecting 2DEG density in AlGaN/GaN HEMTs by sub-bandgap optical pump. Based on the fact that the band bending can alter due to the variation in the density of trapped charge at heterostructure, new transition is proposed to explain the phenomenon. The density change in the trapped charges before and after semi-on state stress at VDS=20V and VGS=-2V with the ambient temperature of 80 can also be estimated.

We model the effect of radiation damage on AlGaN/GaN HEMTs in FLOODS. While excellent fits are obtained for threshold voltage shift, the models do not fully explain the increased reliability observed experimentally. In short, the addition of negatively-charged traps in the GaN buffer layer does not significantly change the electric field at the gate edges at radiation fluence levels seen in this study. We propose that negative trapped charge at the nitride/AlGaN interface actually produces the virtual-gate effect that results in decreasing the magnitude of the electric field at the gate edges and thus the increase in critical voltage. Simulation results including nitride interface charge show significant changes in electric field profiles while the I-V device characteristics do not change.

The effect of 5 MeV proton irradiation on DC performance and reliability of circular-shaped AlGaN/GaN high electron mobility transistors were investigated. The drain saturation current (IDSS) reduced 12.5% and the sheet resistance increased 9.2% after irradiation with a dose of 2x1014 1/cm2. Threshold voltage shifted more positively with a dose of 2x1014 1/cm2 compared to 2x1013 /cm2. Besides, the breakdown voltage improved 39.8% after irradiation with dose of 2x1014 1/cm2. Simulation was done and the peak electric field at the drain side of the gate edge was found to be reduced after irradiation. The defects generated after irradiation at the AlGaN/GaN interface create a virtual gate and thus reduce the electric field. Gate lag measurement was performed to characterize the defects. The current dispersion of DC to high frequency indicates the defects had been generated after irradiation, which is in agreement with the simulation data.

The effect of low dose gamma irradiation on DC performance of circular-shaped AlGaN/GaN high electron mobility transistors weas investigated. The drain saturation current (IDS) increased 11.44% after irradiation with a dose of 700 Gy. Sheet resistance (Rs) was measured from transfer line method and it decreased 3.6% after irradiation. By extracting the resistance between source and drain in the drain I-V curve and combining with TLM data, mobility was found to increase 34.53% after irradiation. The mobility increase may come from the donor-type defects or the strain relaxation effect. Gate lag measurement was also performed and 5% current dispersion was found after irradiation with the dose of 700 Gy, indicating there were more defects generated after irradiation.

Defects produced from the application of electrical stress to AlGaN/GaN high electron mobility transistors were studied. The defects observed include micron-length cracking of the AlGaN and GaN layers, a reaction of the native oxide interfacial layer between the gate contact and AlGaN epilayer resulting in a band defect, and an electrochemical pitting reaction between the gate contact and AlGaN epilayer. These defects were characterized using a combination of surface etching chemical deprocessing, top-down scanning electron microscopy, and cross sectional transmission electron microscopy. The cracks in the AlGaN and GaN layers are shown to lengthen under application of electrical stress. The band defects appear to coincide with a large increase in gate leakage current associated with the critical voltage during electrical stressing. However, continued electrical stressing past the critical voltage results in pitting of the AlGaN epilayer under and near the gate contact. Simulations of the electric field lines from these pitted devices match the defect morphology across a range of different stress conditions and device structures and geometries.

Recent progress in development of GaN-based transistors for gas and bio-sensing applications and amorphous IGZO layers for use thin film transistors (TFTs) on flexible substrates, including paper, is presented. For the detection of gases such as hydrogen, the gateless GaN transistors are typically coated with a catalyst metal such as Pd to increase the detection sensitivity at room temperature. Functionalizing the surface with oxides, polymers and nitrides enhances the detection sensitivity for gases and ionic solutions. The use of enzymes or antibody layers on the surface leads to highly specific detection of a range of antigens of interest in the medical and security fields. We give examples showing sensitive detection of glucose, prostate cancer and breast cancer markers and the integration of the sensors with wireless data transmission systems. The amorphous transparent conducting oxide InZnGaO4 (IGZO) is attracting attention because of its high electron mobility, high transparency in the visible region and its ability to be deposited with a wide range of conductivities. This raises the possibility of making low-cost electronics on a range of arbitrary surfaces, including paper and plastics. Remaining obstacles to use of IGZO TFTs are presented.