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The effect of carbon doping on the structural and electrical properties of GaN buffer layer of AlGaN/GaN high electron mobility transistor (HEMT) structures has been studied. In the undoped HEMT structures, oxygen was identified as the dominant impurity using secondary ion mass spectroscopy and photoluminescence (PL) measurements. In addition, a notable parallel conduction channel was identified in the GaN buffer at the interface. The AlGaN/GaN HEMT structures with carbon doped GaN buffer using a CBr4 beam equivalent pressure of 1.86×10-7 mTorr showed a reduction in the buffer leakage current by two orders of magnitude. Carbon doped GaN buffers also exhibited a slight increase in the crystalline tilt with some pits on the growth surface. PL and Raman measurements indicated only a partial compensation of donor states with carbon acceptors. However, AlGaN/GaN HEMT structures with carbon doped GaN buffer with 200 nm thick undoped GaN near the channel exhibited good 2DEG characteristics. [ABSTRACT FROM AUTHOR]

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrödinger equations. From simulations, an AlGaN buffer with "Al" mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (~1013cm2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510cm2/V.s and a sheet carrier concentration (ns) of 0.97×1013cm2 for the DH-HEMT structure, while they are 1310cm2/V.s and 1.09×1013cm2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of 2DEG in the DH-HEMT heterostructure, which helped in the enhancement of its room temperature mobility. DH-HEMT showed 3 times higher buffer break-down voltage compared to SH-HEMT, while both devices showed almost similar drain current density. Small signal RF measurements on the DH-HEMT showed a unity currentgain cut-off frequency (fT) and maximum oscillation frequency (fmax) of 22 and 25 GHz, respectively. Thus, overall, DH-HEMT heterostructure was found to be advantageous due to its higher buffer break-down voltages compared to SH-HEMT heterostructure. [ABSTRACT FROM AUTHOR]

The calculated electron Hall Mobility and Hall scattering factor in n-type 6H-SiC based on numerical solutions to the Boltzmann transport equation are presented. These results were obtained by solving the Boltzmann equation exactly for the electron Hall mobility using the contraction mapping principle and the electron drift mobility with Rode’s iterative method. The relative importance of the various scattering mechanisms for these calculations is discussed, and a new set of values for the acoustic deformation potential, intervalley deformation potential, and intervalley phonon energy is found, which simultaneously fit experimental Hall mobility and Hall scattering factor data. The calculated Hall mobility and Hall scattering factor are in good agreement with experimental results. In addition, predicted values of the Hall scattering factor for various temperatures and doping concentrations are given. [ABSTRACT FROM AUTHOR]

Vertical Schottky barrier diodes (SBD) with different drift-layer thicknesses (DLT) of GaN up to 30 μm grown by metalorganic chemical vapour deposition (MOCVD) were fabricated on free-standing GaN grown by hydride vapour phase epitaxy (HVPE). At room temperature, SBD's exhibited average barrier heights (ΦB) in the range of 0.73 eV to 0.81 eV. The effective barrier heights (ΦBeff) of SBDs with different DLT also exhibited a similar range of ΦB measured at room temperature. The measured reverse breakdown voltages (VBD) of SBDs increased from 562 V to 2400 V with an increase in DLT. The observation of high VBD of SBDs could be due to the lower effective donor concentration (7.6×1014 /cm3), which was measured from SIMS analysis. The measured VBD of 2400 V is the highest value ever reported for a 30 μm DLT vertical GaN SBD without additional edge termination or field plate (FP). [ABSTRACT FROM AUTHOR]

Presents information on a study which discussed the growth of pseudomorphic indium[subx]gallium[sub1-x]arsenic/indium[sub0.52]aluminum[sub0.48]arsenic modulation-doped heterostructures by molecular beam epitaxy under carefully controlled growth conditions. Low-field transport; Mobility-field characteristics obtained from pulsed Hall measurements; Calculation of electron velocity from the output current density.

Investigates the dark-line defect growth suppression in InxGa1-xAs/GaAs strained heterostructures based on vacancy unsaturation. Degradation modes in semiconductor heterostructure lasers; Osmotic force on threading dislocation; Force on dislocation line; Factors influencing vacancy concentration in materials.

Presents information on a study which examined the growth and device characteristics of pseudomorphic modulation-doped field-effect transistors on indium phosphorus substrates. Methods; Results; Discussion.

Atrial fibrillation (AF) is regarded as a complex arrhythmia, with one or more co-existing mechanisms, resulting in an intricate structure of atrial activations. Fractionated atrial electrograms (AEGs) were thought to represent arrhythmogenic tissue and hence have been suggested as targets for radiofrequency ablation. However, current methods for ablation target identification have resulted in suboptimal outcomes for persistent AF (persAF) treatment, possibly due to the complex spatiotemporal dynamics of these mechanisms. In the present work, we sought to characterize the dynamics of atrial tissue activations from AEGs collected during persAF using recurrence plots (RPs) and recurrence quantification analysis (RQA). 797 bipolar AEGs were collected from 18 persAF patients undergoing pulmonary vein isolation (PVI). Automated AEG classification (normal vs. fractionated) was performed using the CARTO criteria (Biosense Webster). For each AEG, RPs were evaluated in a phase space estimated following Takens' theorem. Seven RQA variables were obtained from the RPs: recurrence rate; determinism; average diagonal line length; Shannon entropy of diagonal length distribution; laminarity; trapping time; and Shannon entropy of vertical length distribution. The results show that the RQA variables were significantly affected by PVI, and that the variables were effective in discriminating normal vs. fractionated AEGs. Additionally, diagonal structures associated with deterministic behavior were still present in the RPs from fractionated AEGs, leading to a high residual determinism, which could be related to unstable periodic orbits and suggesting a possible chaotic behavior. Therefore, these results contribute to a nonlinear perspective of the spatiotemporal dynamics of persAF. [ABSTRACT FROM AUTHOR]

In this paper, a bistable optical-driven silicon-nanowire memory is demonstrated, which employs ring resonator to generate optical gradient force over a doubly clamped silicon-nanowire. Two stable deformation positions of a doubly clamped silicon-nanowire represent two memory states ("0" and "1") and can be set/reset by modulating the light intensity (<3 mW) based on the optical force induced bistability. The time response of the optical-driven memory is less than 250 ns. It has applications in the fields of all optical communication, quantum computing, and optomechanical circuits. [ABSTRACT FROM AUTHOR]

The influence of electric field (EF) on the dynamic ON-resistance (dyn-R DS[ON]) and threshold-voltage shift (ΔVth) of AlGaN/GaN high electron mobility transistors on Si has been investigated using pulsed current-voltage (IDS-VDS) and drain current (ID) transients. Different EF was realized with devices of different gate-drain spacing (Lgd) under the same OFF-state stress. Under high-EF (Lgd = 2 µm), the devices exhibited higher dyn-R DS[ON] degradation but a small ΔVth (~120 mV). However, at low-EF (Lgd = 5 µm), smaller dyn-R DS[ON] degradation but a larger ΔVth (~380 mV) was observed. Our analysis shows that under OFF-state stress, the gate electrons are injected and trapped in the AlGaN barrier by tunnelling-assisted Poole-Frenkel conduction mechanism. Under high-EF, trapping spreads towards the gate-drain access region of the AlGaN barrier causing dyn-R DS[ON] degradation, whereas under low-EF, trapping is mostly confined under the gate causing ΔVth. A trap with activation energy 0.33 eV was identified in the AlGaN barrier by ID-transient measurements. The influence of EF on trapping was also verified by Silvaco TCAD simulations. [ABSTRACT FROM AUTHOR]

A stress engineered three dimensional (3D) Triple T-gate (TT-gate) on lattice matched In0.17Al0.83N/GaN nano-channel (NC) Fin-High-Electron-Mobility Transistor (Fin-HEMT) with significantly enhanced device performance was achieved that is promising for high-speed device applications. The Fin-HEMT with 200-nm effective fin-width (Weff) exhibited a very high IDmax of 3940 mA/mm and a highest gm of 1417 mS/mm. This dramatic increase of ID and gm in the 3D TTgate In0.17Al0.83N/GaN NC Fin-HEMT translated to an extracted highest electron velocity (ve) of 6.0 x 107cm/s, which is ~1.89 x higher than that of the conventional In0.17Al0.83N/GaN HEMT (3.17 x 107cm/s). The ve in the conventional III-nitride transistors are typically limited by highly efficient optical-phonon emission. However, the unusually high ve at 300 K in the 3D TT-gate In0.17Al0.83N/GaN NC Fin-HEMT is attributed to the increase of in-plane tensile stress component by SiN passivation in the formed NC which is also verified by micro-photoluminescence (0.47 ° 0.02 GPa) and micro-Raman spectroscopy (0.39 ° 0.12 GPa) measurements. The ability to reach the ve = 6 x 107cm/s at 300 K by a stress engineered 3D TT-gate lattice-matched In0.17Al0.83N/GaN NC Fin-HEMTs shows they are promising for next-generation ultra-scaled highspeed device applications. [ABSTRACT FROM AUTHOR]

In this Letter, an optical gradient force driven Nanoelectromechanical Systems (NEMS) actuator, which is controlled by the Q-factor attenuation of micro-ring resonator, is demonstrated. The actuator consists of a tunable actuation ring resonator, a sensing ring resonator, and a mechanical actuation arc. The actuation displacement can reach up to 14 nm with a measured resolution of 0.8 nm, when the Q-factor of the ring resonator is tuned from 15 × 103 to 6 × 103. The potential applications of the NEMS actuator include single molecule manipulation, nano-manipulation, and high sensitivity sensors. [ABSTRACT FROM AUTHOR]

This Letter reports on the epitaxial growth, characterization, and device characteristics of crack-free AlGaN/GaN heterostructures on a 200 mm diameter Si(111) substrate. The total nitride stack thickness of the sample grown by the metal-organic chemical vapor deposition technique is about 3.3 ± 0.1 μm. The structural and optical properties of these layers are studied by cross-sectional scanning transmission electron microscopy, high-resolution x-ray diffraction, photoluminescence, and micro-Raman spectroscopy techniques. The top AlGaN/GaN heterointerfaces reveal the formation of a two-dimensional electron gas with average Hall mobility values in the range of 1800 to 1900 cm2/Vs across such 200 mm diameter GaN on Si(111) samples. The fabricated 1.5 μm-gate AlGaN/GaN high-electron-mobility transistors exhibited the drain current density of 660 mA/mm and extrinsic transconductance of 210 mS/mm. These experimental results show immense potential of 200-mm diameter GaN-on-silicon technology for electronic device applications. [ABSTRACT FROM AUTHOR]

In this Letter, the device performance of AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) on silicon substrate using 10-nm-thick atomic-layer-deposited ZrO2 as gate dielectrics is reported. The ZrO2 AlGaN/GaN MISHEMTs showed improved maximum drain current density (Idmax) with high peak transconductance (gmmax) as comparison to Schottky-barrier-gate HEMTs (SB-HEMTs). Also compared to SB-HEMTs, reverse gate leakage current was four orders of magnitude lower and forward gate bias extended to +7.4 V. At energy from -0.29 eV to -0.36 eV, low interface trap state density evaluated by AC conductance and 'Hi-Lo frequency' methods indicates good quality of atomic-layer-deposited ZrO2 dielectric layer. [ABSTRACT FROM AUTHOR]

The mechanisms of the temperature-dependent forward gate current transport in the atomic-layer-deposited Al2O3/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MISHEMT) were investigated. In contrast to the conventional Schottky-gate AlGaN/GaN HEMT, thermionic field emission was found not to be the dominant transport mechanism for the Al2O3/AlGaN/GaN MISHEMT. Fowler-Nordheim tunneling was found to be dominant at low temperature (T<0 °C) and high electrical filed, whereas trap assistant tunneling was found to be dominant at medium electrical field and high temperature (T>0 °C). [ABSTRACT FROM AUTHOR]

The surface leakage currents and the surface trapping effects of the AlGaN/GaN high electron mobility transistors (HEMTs) on silicon with different passivation schemes, namely, a 120 nm plasma enhanced chemical vapor deposited SiN, a 10 nm atomic layer deposited (ALD) Al2O3 and a bilayer of SiN/Al2O3 (120/10 nm) have been investigated. After SiN passivation, the surface leakage current of the GaN HEMT was found to increase by about six orders; while it only increased by three orders after the insertion of Al2O3 between SiN and AlGaN/GaN. The surface conduction mechanism is believed to be the two-dimensional variable range hopping for all the samples. The leakage current in the etched GaN buffer layer with SiN/Al2O3 bilayer passivation was also much smaller than that with only SiN passivation. The pulse measurement shows that the bilayer of SiN/Al2O3 passivation scheme can effectively reduce the surface states and suppress the trapping effects. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high-electron-mobility transistor structures grown on 100 mm high-resistivity Si(111) substrates using plasma-assisted molecular beam epitaxy are reported. The two-dimensional electron gas (2DEG) formation in the heterostructures was realized by the growth optimization of two-step low temperature and high temperature AlN layers and GaN buffer layer. High-electron mobility of 1100 cm2/V s with a sheet carrier density of 9×1012 cm-2 was achieved. The presence of 2DEG in the AlGaN/GaN interface was confirmed by temperature dependent Hall measurements and capacitance-voltage carrier profiling. The fabricated 1.5 μm gate length high electron mobility transistor exhibited a maximum drain current density of 530 mA/mm and a peak extrinsic transconductance of 156 mS/mm. [ABSTRACT FROM AUTHOR]

The effects of Al2O3 gate insulator grown by atomic layer deposition (ALD) system on the two-dimensional electron gas (2DEG) transport characteristics in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MISHEMT) were investigated. The shape of the electron lateral distribution in the quantum well at AlGaN/GaN interface was found to be slightly influenced by the Al2O3 thin layer. The drift mobility (μd) of the electrons in ALD-Al2O3/AlGaN/GaN MISHEMT is increased due to the surface passivation effects of the included dielectric layer. The higher dynamic channel current of the MISHEMT indicates that the electron saturation velocity (vsat) is also increased. These results show the improvement of the transport characteristics of 2DEG in Al2O3/AlGaN/GaN MISHEMT by the excellent properties of the Al2O3 grown by ALD. [ABSTRACT FROM AUTHOR]

The temperature dependent characteristics of Ti/Al/Ni/Au Ohmic contact for AlGaN/GaN high electron mobility transistors (HEMTs) on high-resistivity Si and sapphire substrates were investigated over the temperature range from -50 to 200 °C. The sheet resistances (Rsh) of AlGaN/GaN HEMTs on Si and sapphire substrates were found to increase with the temperature according to the power-law equation with power indices of -2.7 and -2.3, respectively. The specific contact resistivity (ρC) decreased with the temperature, and it was observed to be determined by the electron thermionic field emission (TFE) from the lowered conduction band edge in the modified AlGaN region with high-density N-vacancies to the quantum well at the AlGaN/GaN heterojunction with two dimensional electron gas (2DEG). Three important parameters in the TFE are extracted, that is, the barrier height ([lowercase_phi_synonym]B) of around 0.5 eV, the energy difference between the effective conductive-band edge and the Fermi level in 2DEG (En) of around 0.1 eV, and the 2DEG electron density (ND) with the order of magnitude of 1019 cm-3. [ABSTRACT FROM AUTHOR]

The AlGaN/GaN heterostructures (HSs) with high two dimensional electron gas mobility (1070 cm2/V s) were grown on nonpolar a-plane (1120) sapphire substrate by metal organic chemical vapor deposition. High crystalline quality (reduced screw, edge, and mixed dislocations) with small root mean square surface roughness and small peak-valley distance values were observed in a-plane sapphire grown AlGaN/GaN HSs. Hexagonal-GaN phase was also observed on a-plane sapphire grown AlGaN/GaN HSs. An average increase of ns up to 22% for a plane and 26% for c plane were found after Si3N4 passivation. The product of ns and μH also increases for both a-plane (19%) and c-plane (28%) sapphire grown AlGaN/GaN HSs in a wide range of measurement temperature (87–400 K). The enhancement of sheet carrier density by Si3N4 passivation for nonpolar (1120) sapphire grown AlGaN/GaN HSs is a useful result for the design of future GaN high-electron-mobility transistors. [ABSTRACT FROM AUTHOR]

High temperature power performance of AlGaN/GaN high-electron-mobility transistor (HEMT) on high-resistivity-(HR) Si for different microwave frequencies of 3, 6, and 8 GHz was studied. The device output power density (Pout) reduction with temperature for HR-Si-based AlGaN/GaN HEMTs is almost equivalent to the Pout reduction with temperature for semi-insulating–SiC substrate based AlGaN/GaN HEMTs. After high temperature stress, very small degradation was observed in the power performance at 3 GHz. Moreover, the rate of Pout decrease with the temperature is not much affected by measurement frequencies. The conduction mechanism of trap assisted impact ionization and temperature assisted tunneling were identified on Si3N4 passivated AlGaN/GaN HEMTs. The AlGaN/GaN HEMTs on HR-Si can also be used for high-power and high-frequency applications at elevated temperatures. [ABSTRACT FROM AUTHOR]

The effects of gate-source (GS) and gate-drain (GD) region passivation on the drain current (ID) collapse in AlGaN/GaN high-electron-mobility transistors (HEMTs) were studied. Electrical stresses were applied to the GS and GD regions of HEMTs, (1) unpassivated (device A), (2) fully passivated (device B), (3) GD passivated only (device C), and (4) device C with additional full passivation (device D). An increase of ID density and the extrinsic transconductance was observed in devices with full Si3N4 passivation. Due to the increase of gate leakage current by Si3N4 passivation, breakdown voltage decreases. Device A exhibited severe (96%) ID collapse when it was electrically stressed in both the GS and GD regions. Devices C and D have suppressed only 14% of ID collapse after passivation. However, HEMTs with full Si3N4 passivation (device B) have suppressed more than 80% of the ID collapse. Full passivation is required to suppress the ID collapse effectively for AlGaN/GaN HEMTs. The remaining ID collapse (19%) of device B is affected primarily by bulk/buffer related traps rather than surface related traps. [ABSTRACT FROM AUTHOR]

The enhancement of both direct-current (dc) and microwave characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) were demonstrated by conventional furnace annealing at 400 °C for 5 min. Due to the improvement in Ni/Au Schottky contact properties by furnace annealing, about 17%, 34%, 23%, and 25% of enhancements in maximum drain current density, maximum extrinsic transconductance (gm max), cutoff frequency and maximum oscillation frequency were observed, respectively. A positive threshold voltage shift and the increase in gm max can also be correlated to the improved Schottky parameters such as ideality factor and barrier height. The annealed devices exhibited low reverse gate-leakage-current by more than three orders of magnitude and low drain-leakage-current by two orders of magnitude. Correspondingly, the devices exhibited 55% of higher breakdown voltage after annealing. The furnace annealing is an effective and viable means to enhance both dc and microwave characteristics of AlGaN/GaN HEMTs. [ABSTRACT FROM AUTHOR]

A study of the properties of In0.52Al0.48As/In0.53+xGa0.47-xAs high electron mobility transistors is carried out for 0%, 7%, and 12% excess In values in the channel. Theoretical analysis shows that the enhanced In causes a biaxial compressive strain of 0.49% to 0.84% in the channel, increases the band-edge discontinuity from 0.437 to 0.500 eV, and reduces the carrier mass by 6%. Experimental characterizations support the theoretical predictions by demonstrating an increase of mobility from 9900 to 11 200 cm2/V s at 300 K, and a transconductance enhancement from 160 to at least 230 mS/mm. [ABSTRACT FROM AUTHOR]

Investigates the inhomogeneous cyclotron emission source for the five branch mode conversion problem. Spatial profile of the emission source distribution from a cyclotron mode conversion layer; Principle of maximum blackbody radiation.

InP/InGaAs double-heterojunction bipolar transistor (HBT) structures were grown metamorphically on GaAs substrates by solid-source molecular-beam epitaxy. A linearly graded In[sub x]Ga[sub 1-x]P (x varying from 0.48 to 1) buffer layer was used to accommodate the strain relaxation. The crystallinity of the buffer layer and the HBT structure was examined by x-ray diffractometry. Devices with 5x5 μm[sup 2] emitter area showed a typical peak current gain of 40, a common-emitter breakdown voltage (BV[sub CEO]) higher than 9 V, a current gain cut-off frequency (f[sub T]) of 46 GHz, and a maximum oscillation frequency (f[sub max]) of 40 GHz. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]