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In this paper, we present a highly robust GaN-based X-band low-noise amplifier (LNA) showing promising small-signal and noise performance as well as good linearity. The LNA is fabricated using in-house 0.15 μm AlGaN/GaN on a SiC HEMT process. Owing to the optimum choice of HEMT topologies and simultaneous matching technique, LNA achieves a noise figure better than 2 dB, output power at 1 dB gain compression higher than 19 dB, input and output reflection coefficients better than −9 and −11 dB, respectively. The small-signal gain of LNA is more than 19 dB for the whole band, and NF has a minimum of 1.74 dB at 10.2 GHz. LNA obtains an OIP3 up to 34.2 dBm and survives input power as high as 42 dBm. Survivability is investigated in terms of gain compression and forward gate current. Reverse recovery time (RRT), a crucial parameter for radar front-ends, is explored with respect to the RC time constant and trap phenomenon. The analysis shows that the significant contribution in RRT is due to traps while the RC time constant is in the nanoseconds range. Moreover, this study also addresses the requirement and choice of a DC gate feed resistor for the subsequent stages in a multi-stage design. The size of the designed LNA chip is 3 mm (Formula presented.) 1.2 mm only.

Metal-organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN-based materials as well-known. GaN-based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 x 10(8) cm(-2) for Sample B with a lattice-matched In0.17Al0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4-nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high-electron mobility transistors (HEMTs).

GaAs and SiGe technologies take an edge over GaN-based devices in terms of better noise figure (NF). In this article, we present HEMT topologies and design techniques to achieve a sub-1.2 dB NF for a GaN-based X-band low-noise amplifier (LNA). This NF is comparable with state-of-the-art reported works in competitive GaAs and SiGe technologies. Moreover, this is the best reported NF in X-band using GaN technology to date. Two LNAs are fabricated using in-house 0.15 μm AlGaN/GaN on the SiC HEMT process. LNA-1 has inductive source degenerated (ISD) HEMTs at both stages, while LNA-2 has ISD HEMT at the first and common source at the second stage. The significance of ISD HEMT, for the first or subsequent stages in a multi-stage design, towards NF improvement is addressed. The criticality of stability networks towards NF contribution and its design is discussed in detail. Furthermore, even-mode stability of each HEMT after complete LNA design is assured using the S-probe method in Pathwave Advanced Design Systems.

The replacement of traditional ruthenium-based photosensitizers with low-cost and abundant iron analogs is a key step for the advancement of scalable and sustainable dye-sensitized water splitting cells. In this proof-of-concept study, a pyridinium ligand coordinated pentacyanoferrate(II) chromophore is used to construct a cyanide-based CoFe extended bulk framework, in which the iron photosensitizer units are connected to cobalt water oxidation catalytic sites through cyanide linkers. The iron-sensitized photoanode exhibits exceptional stability for at least 5 h at pH 7 and features its photosensitizing ability with an incident photon-to-current conversion capacity up to 500 nm with nanosecond scale excited state lifetime. Ultrafast transient absorption and computational studies reveal that iron and cobalt sites mutually support each other for charge separation via short bridging cyanide groups and for injection to the semiconductor in our proof-of-concept photoelectrochemical device. The reorganization of the excited states due to the mixing of electronic states of metal-based orbitals subsequently tailor the electron transfer cascade during the photoelectrochemical process. This breakthrough in chromophore-catalyst assemblies will spark interest in dye-sensitization with robust bulk systems for photoconversion applications.

Materials with a tunable optical response that can be controllably tailored using external stimuli excitation have undergone considerable research effort for the development of active optical devices, such as thermo-optical modulators. Although bismuth (Bi) nanodots, embedded into glass matrices, have been proven to have a thermo-optical response, the recyclability of the structure in solid-liquid phase transitions is a major challenge. Herein, a facile and lithography-free fabrication method is proposed to realize densely packed stand-alone Bi nanorods (NRs), with deep subwavelength gaps and a resonance at the midinfrared range (lambda approximately equal to 4.462 mu m). Owing to these ultrasmall gaps that support lossy Mie-like resonances, strong field confinement is achieved, and the resonance wavelength exhibits great sensitivity to temperature, as the thermal sensitivity reaches as high as 1.0316 nm degrees C-1. This operation is conducted in the moderate temperature interval of 25-85 degrees C, which is far from the melting point of Bi. Overall, our simple, robust, and high-performance device is highly promising for realizing optical switches, thermo-optic modulators, and infrared camouflage.

We have performed a first principles study of the structural, elastic and electronic properties of orthorhombic SnS and GeS compounds using the density functional theory within the local density approximation. The second-order elastic constants have been calculated, and the other related quantities such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocities, Debye temperature, and hardness have also been estimated in the present work. All of the calculated modulus and Poisson's ratio for SnS were less than the same parameters for GeS. Our calculations have discovered the large anisotropy of elastic parameters in the (100) and (010)-planes for both compounds. The band structures of orthorhombic SnS and GeS have been calculated along high symmetry directions in the first Brillouin zone (BZ). The calculation results for the band gap of Sn(Ge)S gave Eg = 0.256 eV (0.852 eV) and has an indirect character for an interband transition. The real and imaginary parts of dielectric functions and (by using these results) the optical constant such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant were calculated. All of the principal features and singularities of the dielectric functions for both compounds were found in the energy region between 2 eV and 20 eV. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report the effect of a thin GaN (2 nm) interlayer on the magnetotransport properties of AlInN/AlN/GaN-based heterostructures. Two samples were prepared (Sample A: AlInN/AlN/GaN and sample B: AlInN/GaN/AlN/GaN). Van der Pauw and Hall measurements were performed in the 1.9–300 K temperature range. While the Hall mobilities were similar at room temperature (RT), sample B had nearly twice as large Hall mobility as sample A at the lowest temperature; 679 and 889 cm2/Vs at RT and 1460 and 3082 cm2/Vs at 1.9 K for samples A and B. At 1.9–10 K, the longitudinal magnetoresistance was measured up to 9 T, in turn revealing Shubnikov–de Haas (SdH) oscillations. The carrier concentration, effective mass and quantum mobility of the two-dimensional electron gas (2DEG) were determined from SdH oscillations. At 1.9 K, the 2DEG concentration of sample B was nearly seven times larger than of sample A (1.67 × 1013/cm2 vs. 0.24 × 1013/cm2). On the contrary, the quantum mobility was changed adversely nearly three times (sample B 2500 cm2/Vs and sample A 970 cm2/Vs). The increase of the 2DEG concentration was attributed to the existence of the GaN interlayer, which has strengthened the spontaneous polarization difference between the AlInN and GaN layers of the heterostructure. Hence, the stronger electric field at the 2DEG region bent the conduction band profile downwards and consequently the quantum mobility decreased due to the increased interface roughness scattering.

The electron transport properties in Al{sub 0.25}Ga{sub 0.75}N/AlN/GaN/In{sub x}Ga{sub 1-x}N/GaN double heterostructures with various indium compositions and GaN channel thicknesses were investigated. Samples were grown on c-plane sapphire substrates by MOCVD and evaluated using variable temperature Hall effect measurements. In order to understand the observed transport properties, various scattering mechanisms, such as acoustic phonon, optical phonon, interface roughness, background impurity, and alloy disorder, were included in the theoretical model that was applied to the temperature-dependent mobility data. It was found that low temperature (T 160 K), optical phonon scattering is the dominant scattering mechanism for AlGaN/AlN/GaN/InGaN/GaN heterostructures. The higher mobility of the structures with InGaN back barriers was attributed to the large conduction band discontinuity obtained at the channel/buffer interface, which leads to better electron confinement. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

The strain–stress analysis of AlxGa1−xN/GaN (x = 0.3) heterostructures with and without a high-temperature HT-AlN interlayer (IL) grown on sapphire (Al2O3) substrates and AlN buffer/Al2O3 templates via metal organic chemical vapour deposition (MOCVD) was carried out based on the precise measurement of the lattice parameters by using high-resolution X-ray diffraction (HRXRD). The a- and c-lattice parameters were measured from the peak positions that were obtained by rocking the theta axis at the vicinity of the symmetric and asymmetric plane reflection angles, followed by the in-plane and out-of-plane strains. Then, the biaxial and hydrostatic components were extracted from the total strain values that were obtained and were then discussed in the present study as the effect of the HT-AlN buffer and IL. The AlN buffer layer (BL) affects the strain values of the AlGaN ternary layer (TL). A further effect was realized by inserting an AlN IL between GaN BL and AlGaN TL. However, the experimental results also show that an AlN IL changes the strain behaviour in the a- and c-directions of the AlGaN TL from the tensile to compressive and the compressive to tensile type, respectively. These similar behaviours were observed in hydrostatic strain, biaxial strain and stress. Their reasons are explained with an effective a-lattice parameter, post-growth cooling and lattice and thermal mismatches.

In this present article, we studied the transmission, refraction and focusing properties of one, two and three dimensional labyrinth based left-handed metamaterials. We demonstrated that the proper periodic arrangement of labyrinth structures and wires results in left-handed transmission. By using a two dimensional labyrinth based left-handed slab, it is shown that it is possible to image the field emitted from source in sub-wavelength detail. Moreover, we demonstrated the focusing and negative refraction of electromagnetic waves by a three dimensional labyrinth based left-handed slab.

We studied the far field radiation properties of a new resonator antenna composed of split ring resonators (SRRs) and a monopole. It is shown that the antenna size at the operation frequency (3.52 GHz) is approximately one tenth of the free space wavelength (λ/10). Moreover, increasing the number of SRRs yields steerability properties. These achievements provide a way to create rather small steerable resonant antennas.

The authors investigate the resonances of split-ring resonator (SRR) structures experimentally and theoretically. The transmission measurements were performed on a single SRR unit cell and periodic arrays of SRRs. The magnetic response of the SRR structure was demonstrated by comparing the SRRs' transmission spectra of with that of closed ring resonators (CRR). The surface currents along the SRR surfaces at the magnetic and electrical resonances were simulated and a circular surface current was observed at the magnetic resonance frequency. The transmission characteristics of SRR arrays with two different orientations were also presented by varying the incident angles of the plane wave.

Impedance matching at the surface of left-handed materials (LHM) is required for certain applications including a perfect lens. In this study, we present the experimental and theoretical verification of an impedance-matched LHM to free space. Reflection characteristics of both one-dimensional and two-dimensional LHM were investigated. The reflection was observed to be very low at a narrow frequency range. FDTD simulations and retrieval procedures were used to theoretically verify impedance matching. By varying the number of layers along the propagation direction, the ultralow reflection at specific frequencies was shown to be independent of the sample thickness. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2548–2552, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22003

Conference name: Proceedings of the 5th International Conference on Nitride Semiconductors (ICNS‐5) Solar-blind AlGaN metal-semiconductor-metal (MSM) photodetectors with fast pulse response have been demonstrated. The devices were fabricated on MOCVD-grown epitaxial Al0.38Ga0.62N layers, using a microwave compatible fabrication process. The photodiode samples exhibited low leakage with dark current densities below 1 × 10-6 A/cm 2 at 40 V reverse bias. Photoconductive gain-assisted photoresponse was observed with a peak responsivity of 1.26 A/W at 264 nm. A visible rejection of ∼3 orders of magnitude at 350 nm was demonstrated. Temporal high-speed measurements at 267 nm resulted in fast pulse responses with 3-dB bandwidths as high as 5.4 GHz. This corresponds to a record high-speed performance for solar-blind detectors. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA.

AES has been supported by National Science Centre (NCN), Poland under the project MetaSel (DEC-2015/17/B/ST3/00118). AL thanks the Charles Godfrey Binder Endowment at the Pennsylvania State University for ongoing support of his research activities. Contribution of EO has been supported by DPT (HAMIT), TUBITAK (113E331, 114E374, 115F560), and Turkish Academy of Sciences.

20th International Conference on Ternary and Multinary Compounds (ICTMC) -- SEP 05-09, 2016 -- Halle, GERMANY, WOS: 000405267800024, In present work, our research is mainly focused on the electronic structures, optical, and magnetic properties of XMnSb (X = Ni, Cu, Pd), Co(2)YZ (Y = Ti; Z = Si, Ge, Sn), and Co(2)YZ (Y = Mn; Z = Al, Ga, Si) Heusler compounds by using ab initio calculations within the generalized gradient approximation. The calculations are performed by using the Vienna ab initio simulation package based on the density functional theory. The band structure of these Heusler alloys for majority spin and minority spin were calculated and the majority spin states cross the Fermi level and thus have the metallic character, while the minority spin states open the band gaps around the Fermi level and thus have the narrow-band semiconducting nature. We also find that these Heuslercompounds have the indirect band gaps in the minority spin channel. The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant for XMnSb and Co2YZ compounds were also calculated. In addition, we also show the variations of the total magnetic moment per f.u. and minority spin gap width of these compounds with optimized lattice constants: minority spin gap width decreases with increasing the lattice constants.

In the present study, we first demonstrate the polarization and orientation dependent transmission response of split ring resonators at the infrared and visible band. Then, we numerically demonstrate a configuration for the enhancement of power passing through an electrically small hole. By using deep subwavelength optical split ring resonator antennas and couplers we achieved a 400-fold enhanced transmission from a subwavelength aperture area of the electrical size λ2/25. The power was transmitted to the far field with 3.9 dBi directivity at 300 THz. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

(Ni/Au)AlxGa1-xN/AlN/GaN(x = 0.22) heterostructures with and without a passivation layer of the SiNx were fabricated in order to see the effect of the insulator layer on the main electrical parameters such as zero-bias barrier height (BH) (Phi(B0)), ideality factor (n), series resistance (R-s) of the structure, and the interface state density (N-ss). Some of these parameters were determined from both I-V and admittance (C-V and G/omega-V) measurements at room temperature and at 1 MHz and were compared. The experimental results show that the value of N-ss in a Schottky contact without passivation is nearly 1 order of magnitude larger than that in a Schottky contact with SiNx passivation layers. Also, the values of R-s increase with the increasing thickness of the passivation layer. In the forward bias region, the negative values of capacitance are an attractive result of this study. This negative capacitance disappears in presence of the passivation layer. Copyright (C) 2010 John Wiley & Sons, Ltd.

In this work, we suggest and demonstrate a robust method to tune the plasma frequencies of wire mediums. The method we suggest involves the use of two or more wire arrangements in the unit cell. By incorporating the method we suggested it is possible to tune the plasma frequencies of wire mediums effectively by use of lower metal densities. In addition, we study the effective permittivities and permeabilities of labyrinth based metamaterials. Our results show that the effective permeability of the labyrinth based metamaterial medium is negative above a certain frequency. The results of the effective permittivity calculations for the labyrinth based metamaterial medium reveal that the labyrinth structure exhibits a strong dielectric response near the magnetic resonance frequency. Finally, we design labyrinth based left-handed mediums that have several desired properties such as simultaneous μ, ϵ = −1 and μ, ϵ = 0. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2611–2615, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21988