Narrow Results

We report on two-dimensional isothermal simulations of recessed gate and field-plated AlGaN-GaN HFETs with submicron gates. The optimization of the recessed gate shape allows us to reduce the electric field at the drain-side gate edge by approximately 30%. Our simulations reveal a dramatic increase of the effective gate length with increasing drain-to-source bias with a commensurate decrease of the cutoff frequency (up to 40% decrease for 50V). To improve the cutoff frequency for the high drain-to-source bias, we suggest using the second field plate connected to the drain with a small gap between the two field plates. In this design, the electric field in the gap between the gate and the drain field plate is higher leading to a significant reduction of the effective gate length and, as a consequence, to an increase in the cutoff frequency at high drain-to-source biases (compared to the conventional design).

This paper provides an overview of recent work and future directions in Gallium Nitride transistor research. We discuss the present status of Ga-polar AlGaN/GaN HEMTs and the innovations that have led to record RF power performance. We describe the development of N-polar AlGaN/GaN HEMTs with microwave power performance comparable with state-of-art Ga-polar AlGaN/GaN HEMTs. Finally we will discuss how GaN-based field effect transistors could be promising for a less obvious application: low-power high-speed digital circuits.

We present a novel approach to achieve terahertz-range cutoff frequencies and maximum frequencies of operation of GaN based heterostructure field-effect transistors (HFETs) at relatively high drain voltages. Strong short-channel effects limit the frequency of operation and output power in conventional geometry GaN HFETs. In this work, we propose a novel device with two additional independently biased electrodes controlling the electric field and space-charge close to the gate edges. As a result, the effective gate length extension due to short channel effects is diminished and electron velocity in the device channel is increased. Our simulations show that the proposed five-terminal HFET allows achieving f_T=1.28 THz and f_max= 0.815 THz at the drain voltages as high as 12 V. Hence, this device opens up a new approach to designing THz transistor sources.

We investigated transient amplitude and phase response of GaN-on-sapphire SAW delay-line device subjected to pulsed sub-band UV illumination. We correlated these results with photoluminescence measurements in two GaN samples with the same emission spectra but different carrier lifetime. The SAW response measurements showed that under pulse illumination the sample with shorter lifetime exhibited gradual rise in the phase and amplitude change in millisecond range. Under similar conditions, the sample with a longer lifetime responded faster. We attribute this change to the presence of defect related transitions of the photoexcited carriers and their interaction with surface acoustic waves. Our results demonstrate the possibility of characterization of compensated GaN samples by measuring the phase and amplitude variations in the SAW transmission mode under pulsed UV excitation.

III-Nitride heterostructure field-effect transistors (HFETs) demonstrated a new paradigm in microwave switching and control applications due to unique combination of extremely low channel resistance (leading to low loss), very high RF power, low off-state capacitance, broad range of operating temperatures, chemical inertness and robustness. The paper reviews novel approaches and recent advances in III-Nitride technology for RF switching devices leading to higher operating frequencies and even lower insertion loss.

Power semiconductor devices are important for numerous applications with power conversion being an important one. Wide energy gap semiconductors SiC and GaN have properties that make them attractive for such applications. Among these properties are high thermal conductivity, high breakdown electric field, wide energy gap, low intrinsic carrier concentration, high thermal stability, high saturation velocity and chemical inertness. These lead to low on-resistance, high breakdown voltage, high frequencies, small volume, and small passive inductors and capacitors. These desirable properties are offset by the higher material costs and higher defect densities. Although wide energy gap devices have been in development for many years, only recently have they become available commercially. Their main competition is silicon power devices with breakdown voltages up to 8000 V and very high surge current capacity. However, silicon power devices are approaching their material limits and wide energy gap devices are beginning to have an impact in the power electronics space. SiC has the advantage of substrates with diameters approaching 150 mm and the ability to grow thermal SiO₂. GaN has the heterojunction advantage, but no viable substrate technology. In fact, a large portion of SiC production is used for GaN substrates. GaN material development has also benefited significantly from the development of optical devices, e.g., light-emitting diodes and lasers.

Integrated optics is a promising optical platform both for its enabling role in optical interconnects and applications in on-chip optical signal processing. In this paper, we discuss the use of group III-nitride (GaN, AlN) as a new material system for integrated photonics compatible with silicon substrates. Exploiting their inherent second-order nonlinearity we demonstrate and second, third harmonic generation in GaN nanophotonic circuits and high-speed electro-optic modulation in AlN nanophotonic circuits.

This paper presents a self-consistent analytic model to describe the current-voltage (I-V) and charge-voltage (Q-V) behavior of quasi-ballistic III-nitride transistors. We focus on two types of transistor geometries: (i) high electron mobility transistors (HEMTs) suitable for radio frequency (RF) applications and (ii) nanowire field-effect transistors (FETs) for digital applications. Our core model is based on Landauer transport theory which is combined with the calculation of charge density and velocity of charges at the top-of-the-barrier in the transistor. The effect of extrinsic device features, such as the nonlinearity of access regions and Joule heating at high currents, are included in the static I-V model. In the case of the dynamic Q-V model, we calculate intrinsic terminal charges by approximating the solution of the 2D Poisson equation in the channel over a broad bias range. The effect of fringing capacitances, prominently inner-fringing capacitance that varies nonlinearly with the gate bias, is included in our Q-V model. We amend the model electrostatics and the description of source/drain rectifying contacts in our core model to represent the I-V characteristics of III-nitride nanowire FETs. The model shows excellent match against experimentally and numerically measured characteristics of GaN transistors with gate lengths ranging from 42 nm to 274 nm. With only 38 input parameters, most of which are extracted based on straightforward device characterization, our model can be used for device-circuit co-design and optimization using a standard hierarchical circuit simulator.

Mg-doped GaN films have been successfully prepared on Si(111) substrate by metal-organic chemical vapor deposition (MOCVD). Upon rapid thermal annealing (RTA) treatment, the films showed p-type conductivity with a carrier density of 7.84 cm^-3, a mobility of 5.54 cm²V^-1s^-1, and a resistivity of about 0.144 Ωcm, which were much better than that of the films without rapid thermal annealing (RTA) treatment. It was found that the surface morphology and crystal quality of the obtained p-type GaN films were greatly improved by RTA treatment, while the residual stress and dislocations in these films were decreased.

Luminescence experiments in magnetic fields up to 28 T on a freestanding gallium nitride (GaN) sample and a heteroepitaxial GaN layer lightly doped with silicon are presented. In these samples, the principal D⁰X recombination channel is accompanied by two electron satellites (TES), which involve the excited donor states as well as by the longitudinal-optic (LO) phonon replica of the principal transitions. When the internal donor excitations are magnetically tuned into resonance with the LO-phonon excitation, the intensity of TES is strongly enhanced and clear avoided-crossings between the LO-phonon replica of the principal D⁰X transition and TES involving highly excited states of oxygen and silicon are resolved. The observed behavior is explained in terms of resonant interaction between LO-phonons and donor-bound electrons. It is found that the resonant magnetopolaron interaction is stronger for the oxygen as compared to the silicon donor.

Theoretical as well as experimental studies in the literature suggest that defect sites associated with the threading dislocation lines within $n$-type gallium nitride (GaN) act to trap free electrons from the bulk of this semiconductor material. As a result, the core of the threading dislocation lines become negatively charged. The charge accumulated along the core of a threading dislocation line should be screened by a charge of opposite polarity and equal in absolute value per unit length along the dislocation line. In the present work, we model this screened charge buildup along the threading dislocation lines by two concentric space-charge cylinders. Quantum mechanical theory of scattering in cylindrical coordinates is then employed in order to numerically compute the electron mobility limited by scattering from the charged threading dislocation lines. The dependence of the computed electron mobility on the dislocation line density and on the amount of charge accumulated per unit length along the core of the dislocation lines is also investigated in this work. Our computed electron mobility results are compared with results from existing calculations of the GaN dislocation scattering limited electron mobility in the literature.

Optical and electronic properties of ZB, RS and WZ structures of gallium nitride (GaN) are studied in equilibrium and under pressure using the first-principles calculation in the density functional theory (DFT) framework to obtain quantities like dielectric function, loss function, reflectance and absorption spectra, refractive index and their relation parameters. The electronic properties are studied using EV-GGA and GGA approximations and the results calculated by EV-GGA approximation were found to be much closer to the experimental results. The interband electron transitions are studied using the band structure and electron transition peaks in the imaginary part of the dielectric function; these transitions occur in three structures from N-2p orbital to Ga-4s and Ga-4p orbitals in the conduction band. Different optical properties of WZ structure were calculated in two polarization directions of (100) and (001) and the results were close to each other. Plasmon energy corresponding to the main peak of the energy-loss function in RS with the value of 26 eV was the highest one, which increased under pressure. In general, RS shows more different properties than WZ and ZB.

P-type GaN was successfully achieved by vertically implanting positive monovalent cations of oxygen (O${}^{+}$) into undoped (native n-type with an electron concentration of $8.7\times 10^{16}$cm${}^{-3}$) (0001) monocrystalline GaN. This implantation was carried out with an energy of 200keV and a dose of $5\times 5\times 10^{15}$ ions/cm². In the absence of subsequent rapid thermal annealing (RTA) or when exposed to RTA at 950°C for 10s in a nitrogen ambient environment, temperature-dependent Hall measurements in a vacuum consistently indicated stable p-type conductivity. For the sample that underwent subsequent RTA, the room-temperature Hall hole concentration measured $8.03\times 10^{17}$cm${}^{-3}$, the Hall resistivity was 0.44$\mathrm{\Omega }$⋅cm, the Hall hole mobility reached 17.81cm²⋅V${}^{-1}$⋅s${}^{-1}$, and the acceptor ionization energy was determined to be 0.08eV. The doping efficiency was calculated at 5.5%. O${}^{+}$ ions effectively serve as acceptors, whether annealed or not. The p-type conductivity induced by O${}^{+}$ implantation in GaN is notably advantageous and holds practical significance for the ongoing development of future device technology.

The detonation of carbon- or nitrogen-containing explosives not only produces powerful shock waves but also provides elemental building blocks and a unique high-pressure and high-temperature physical environment for the construction of various nanostructures. This review highlights the situation and key progresses in the detonation approach towards diamond nanoparticles, graphitic carbon nanotubes, fullerene molecules, and gallium nitride nanocrystals. Further extension of the peaceful-use detonation applications and rational reactor design are also proposed.

Recent advances in silicon technology have pushed the silicon properties to its theoretical limits. Therefore, wide band gap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN) have been considered as a replacement for silicon. The discovery of these wide band gap semiconductors have given the new generation power devices a magnificent prospect of surviving under high temperature and hostile environments. The primary focuses of this review are the properties of GaN, the alternative substrates that can be used to deposit GaN and the substitution of SiO₂ gate dielectric with high dielectric constant (k) film. The future perspectives of AlGaN/GaN heterostructures are also discussed, providing that these structures are able to further enhance the performance of high power devices.

In this paper, we propose a micro-ring resonator model based on gallium nitride (GaN) and graphene, which exhibits tunable properties of nonlinearity. It provides a great bandwidth covering from visible to telecommunication band. Especially, based on the characteristic of GaN, it has unique advantages in shorter wavelength, which is used for demonstrating the ultrafast signal processing including wavelength conversion, temporal amplification and pulse compression. Moreover, the tunable signal processing is achieved via the method of applying additional bias voltage to graphene without changing the geometric dimension of the device. These results have significant potential applications of nonlinear optics and optical communications.

Excellent properties of gallium nitride (GaN) make it an ideal material for realizing gas sensors, especially for ammonia (NH${}_3)$ detection. Although many researchers have pursued to describe the characteristics of GaN-based NH₃ gas sensors by different approaches, few models have been reported. In this paper, with the consideration of the exponential distribution of interfacial states, a model for ammonia concentration detection of GaN gas sensors has been presented. The Poisson equation is applied to model the effect of defect states on the potential. By taking advantage of the current-voltage characteristics, the value of Schottky barrier height can be obtained. The concentration of the adsorbed NH₃ gas is derived by exploiting the surface potential. It indicates that densities of acceptor interfacial trap states are in the order of 10${}^{11}\sim 10^{12}$cm${}^{-2}$eV${}^{-1}$. The current increases with the NH₃ concentration at the same applied voltage. In addition, detailed investigations of physical mechanisms and the analysis of the sensitivity have been depicted. It shows that the sensitivity followed an approximately exponential dependence on NH₃ density. Results compared well with experimental data that verify the proposed model and simulation method.

A critical evaluation of high-power electronics switching in semiconductor materials is made from the standpoint of performance, reliability, and commercial viability. This study takes into account recent experimental results obtained from the field-reliability study of silicon power MOSFETs in high-density power supplies where residual material defects present in the space charge region of the device were found to generate local micro plasma that eventually caused power MOSFETs to fail. Based on these results and commercial progress made to date in wide bandgap semiconductor technologies, it is suggested that silicon carbide (SiC) promises to be the preferred material for high-power electronics switching from cost, performance and reliability considerations — this assessment is further strengthened by the near-term potential for developing large-area, low-cost, and defect-free SiC bulk substrates and epitaxial layers. This conclusion is also supported by the feasibility and the need for vertical, MOS-controlled, bipolar power switches in compact and efficient megaWatt-level power converters in order to make transformational changes in the 21st century electrical transmission and distribution infrastructure.

A quantitative structural determination of the Ga-polar 1×1 (0001) surface of GaN is performed by quantitative low energy electron diffraction (LEED). The global best-fit structure is obtained by a new frozen LEED approach connected to a simulated annealing algorithm. The global minimization frozen (GMF) LEED search finds that the ordered structure consists of 1 ML of Ga adatoms at atop sites above Ga-terminated bilayers. The Ga adatoms are bonded with a Ga–Ga bond length of 2.51 Å. The spacings within surface bilayers show a weak oscillatory trend, with the outmost bilayer thickness expanding to 0.72 Å and the next bilayer thickness contracting to 0.64 Å, compared to the bulk thickness of 0.65 Å. The interlayer spacing between the first and second bilayers is 1.89 Å, while the next interlayer spacing is 1.94 Å, compared to the bulk value of 1.95 Å. These results are compared with data from other theoretical and experimental studies.

The growth and stability of hafnium films on $n$-GaN(0001) surface with native oxide was investigated with X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS). It is shown that hafnium creates a continuous and stable layer on GaN substrate. Thermal treatment at $850^{\circ }$C of Hf/GaN system causes decomposition of GaN and reaction of hafnium with atomic nitrogen from the substrate. XPS spectra demonstrate the reaction by a strong shift of the N 1s and Hf 4f lines. An attempt for bringing on the same reaction with molecular nitrogen under pressure of $1.2\times 10^{-6}$ mbar was not successful. UPS spectra show a metallic character of the hafnium adlayer in such instances.