Design and Analysis of InGaN-GaN Modulation Doped Field-Effect Transistors (MODFETs) for Over 60 GHz Operation

A Modulation-Doped Field-Effect Transistor (MODFET) structure realized in InGaN-GaN material system is presented for the first time. An analytical model predicting the transport characteristics of the proposed MODFET structure is given in detail. Electron energy levels inside and outside the quantum well channel of the MODFET are evaluated. The two-dimensional electron gas (2DEG) density in the channel is calculated by self-consistently solving Schrödinger and Poisson's equations simultaneously. Analytical results of the current-voltage and transconductance characteristics are presented. The unity-current gain cutoff frequency (f _T) of the proposed device is computed as a function of the gate voltage V _G . The results are compared well with experimental f _T value of a GaN/AlGaN HFET device. By scaling the gate length down to 0.25 μm the proposed InGaN-GaN MODFET can be operated up to about 80GHz. It is shown in this paper that InGaN-GaN system has small degradation in f _T as the operating temperature is increased from 300°K to 400°K.     

Effect of Temperature Variation on the Characteristics of Microwave Power AlGaN/GaN MODFET

The prime motivation for developing the proposed model of AlGaN/GaN microwave power device is to demonstrate its inherent ability to operate at much higher temperature. An investigation of temperature model of a 1 μm gate AlGaN/GaN enhancement mode n-type modulation-doped field effect transistor (MODFET) is presented. An analytical temperature model based on modified charge control equations is developed. The proposed model handles higher voltages and show stable operation at higher temperatures. The investigated temperature range is from 100 °K–600 °K. The critical parameters of the proposed device are the maximum drain current (I_Dmax), the threshold voltage (V_th), the peak dc trans-conductance (g_m), and unity current gain cut-off frequency (f_T). The calculated values of f_T (10–70 GHz) at elevated temperature suggest that the operation of the proposed device has sufficiently high current handling capacity. The temperature effect on saturation current, cutoff frequency, and trans-conductance behavior predict the device behavior at elevated temperatures. The analysis and simulation results on the transport characteristics of the MODFET structure is compared with the previously measured experimental data at room temperature. The calculated critical parameters suggest that the proposed device could survive in extreme environments.     

Traveling wave amplifier using “VP×B” effect

The “v _p ×B” effect is introduced into a traveling wave amplifier trying to increase its output power and efficiency. The numerical calculating results show that by introducing an tapered static magnetic field into a traditional traveling wave amplifier, its output power and efficiency can be increased obviously comparing with an ordinary traveling wave amplifier, because of the effective interactive duration of electron bunches and a rf field is extended by the “v _p ×B” effect.     

Donor–Acceptor–Donor Modular Small Organic Molecules Based on the Naphthalene Diimide Acceptor Unit for Solution-Processable Photovoltaic Devices

Two novel solution-processable small organic molecules, 4,9-bis(4-(diphenylamino)phenyl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (S6) and 4,9-bis(benzo[b]thiophen-2-yl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8 (2H,7H)-tetraone (S7), have been successfully designed, synthesized, characterized, and applied in solution-processable photovoltaic devices. S6 and S7 contain a common electron-accepting moiety, naphthalene diimide (NDI), with different electron-donating moieties, triphenylamine (S6) and benzothiophene (S7), and are based on a donor–acceptor–donor structure. S7 was isolated as black, rod-shaped crystals. Its triclinic structure was determined by single crystal x-ray diffraction (XRD): space group \(P\bar{1}\) , Z = 2, a = 9.434(5) Å, b = 14.460(7) Å, c = 15.359(8) Å, α = 67.256(9) degrees, β = 80.356(11) degrees, γ = 76.618(10) degrees, at 150 Kelvin (K), R = 0.073. Ultraviolet–visible absorption spectra revealed that use of triphenylamine donor functionality with the NDI acceptor unit resulted in an enhanced intramolecular charge transfer (ICT) transition and reduction of the optical band gap compared with the benzothiophene analogue. Solution-processable inverted bulk heterojunction devices with the structure indium tin oxide/zinc oxide (30 nm)/active layer/molybdenum trioxide (10 nm)/silver (100 nm) were fabricated with S6 and S7 as donors and (6,6)-phenyl C₇₀-butyric acid methyl ester (PC₇₀BM) as acceptor. Power conversion efficiencies of 0.22% for S6/PC₇₀BM and 0.10% for S7/PC₇₀BM were achieved for the preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm^?2). This paper reports donor–acceptor–donor modular small organic molecules, with NDI as central accepting unit, that have been screened for use in solution-processable inverted photovoltaic devices.     

Photovoltaic properties of low band gap copolymers based on phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units

Two novel copolymers P1 and P2 having phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units, were synthesized by heck coupling and employed as electron donor along with PCBM or modified PCBM (F) as electron acceptor for the fabrication of bulk heterojunction (BHJ) photovoltaic devices. These copolymers P1 and P2 showed broad band absorption around 640-700 nm and optical band gap of 1.60 eV and 1.72 eV, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) estimated from cyclic voltammetry measurement reveals that these values are well suitable for the use of these copolymers as electron donor along with PCBM derivatives as electron acceptor for BHJ active layer. The suitable LUMO off set allows efficient photo-induced charge transfer at the donor/acceptor interfaces in the BHJ photovoltaic device and resulting power conversion efficiency (PCE) of 2.8% and 3.29% for P1 and P2, respectively, when PCBM is used as acceptor. This value has been improved up to 3.52% and 4.36% for the devices based on P1 and P2 when F is used as electron acceptor, instead of PCBM. We have also investigated the effect of solvent annealing on the photovoltaic performance of device based on P1: F and P2: F blends and found that the over all PCE of the devices is 4.36% and 4.88%, respectively. The increase in PCE is mainly due to the improvement in the J_sc, which is due to the increased charge transport in the annealed device as compared to as cast device.     

Specific features of the charge carrier mobility in nanowires in transverse electric and magnetic fields

The effect of an electric field E orthogonal to the quantum-wire axis and a magnetic field H (H ⊥ E, H ‖ E) on conductivity is studied within the context of the parabolic potential model. It is shown that, if the interaction of charge carriers with the rough surface of the nanostructure is taken into account, the charge-carrier mobility μ as a function of increasing E is described by an unsteady oscillating curve. A physical interpretation of such behavior of μ with E is proposed. The specific features of mobility in a transverse magnetic field are discussed.     

Nonmonotonic behavior of magnetophotoconductivity in p-CdHgTe

Magnetophotoconductivity of p-Cd_xHg_1?x Te, measured in the Hall configuration with illumination along the magnetic field (k ∥ B ⊥ E), exhibits nonmonotonic behavior with magnetic field in the temperature range in which the material has a mixed conduction. The effect manifests itself as a peak at B ≠ 0; it is caused by a significant magnetoresistance associated with equilibrium charge carriers. The peak appears if the conductivity of equilibrium electrons is greater than or equal to half the conductivity of equilibrium heavy holes; in Cd _x Hg_1?x Te with x = 0.22, this condition is satisfied at temperatures of 140–150 K.     

Optical properties and electronic structure of mercury iodide

The spectra of sets of optical functions of a HgI₂ crystal for the polarization E ⊥ c are determined in the energy range of 0–20 eV. The ?₂ and ?Im?^?1 spectra are decomposed into elementary components with establishment of their principal parameters. The calculations are carried out based on the experimental reflection spectrum at 100 K for E ⊥ c and computer programs composed with the help of integral Kramers-Kronig relations, analytical formulas, and the method of combined Argand diagrams. The principal features of the ?₂ and ?Im?^?1 spectra of optical functions and parameters of components of the decomposition bands are established.     

Effect of nonstoichiometry and doping on the photoconductivity spectra of GeSe layered crystals

The polarization photoconductivity spectra of Bi-doped nonstoichiometric GeSe layered crystals grown by static sublimation were investigated. Two strongly polarized maxima at the photon energies hν_max = 1.35 eV (E ∥ a) and 1.44 eV (E∥ b) due to the V ₁ ^V → V ₁ ^c and Δ ₂ ^v → Δ ₁ ^c optical transitions, respectively, were found in the spectra of nominally undoped GeSe crystals near the intrinsic absorption edge at 293 K. In the low-temperature region, an exciton photoconductivity band peaked at hν_max=1.32 eV, which is due to exciton dissociation at the cation vacancies, was revealed. With an increase in excess Se in crystals, a sharp increase in the intensity of the exciton peak in the photoconductivity spectra was observed. It is shown that doping of GeSe crystals with donor Bi impurity is an effective tool of the control of their electrical and photoelectric properties. Although introduction of Bi into germanium monoselenide does not lead to the conductivity conversion from the p to n type, a sharp increase in the resistivity is observed, the crystals become photosensitive, and a strong impurity band peaked at 1.11 eV arises in the photoconductivity spectra.     

A crystalline D-π-A organic small molecule with naphtho[1,2-b:5,6-b′]dithiophene-core for solution processed organic solar cells

In this work, we have designed and synthesized a new naphtho[1,2-b:5,6-b′]dithiophene-containing enlarged π-conjugated donor–acceptor (D–A) small molecule, NDT(TTz)₂, for use in solution-processed organic photovoltaics. NDT(TTz)₂, which contains a thiophene-bridged naphtho[1,2-b:5,6-b′]dithiophene as the central fused core and triphenylamine-flanked thiophene thiazolothiazole as a spacer, was synthesized via sequential Suzuki and Stille coupling reactions. The thermal, physiochemical, and electrochemical properties of NDT(TTz)₂ have been evaluated by differential scanning calorimetry, thermogravimetry, UV–Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction, and cyclic voltammetry. As desired for photovoltaic applications, NDT(TTz)₂ possesses good solubility, thermal stability, and a well-ordered, π–π stacked, crystallinity. The optical band gap and HOMO level of NDT(TTz)₂ were determined to be 2.0 eV and −5.23 eV, respectively. In addition to organic thin film transistor studies, application of NDT(TTz)₂ to preliminary photovoltaic devices has also been investigated by fabricating solution-processed bulk heterojunction solar cells together with PC₇₁BM in a typical layered device structure, ITO/PEDOT:PSS/NDT(TTz)₂:PC₇₁BM/LiF/Al. Without extensive optimization of the device, NDT(TTz)₂ in these devices shows a maximum power conversion efficiency of 1.44% under AM 1.5 illumination at a 100 mW/cm² intensity.     

Terahertz Switching Focuser Based on Thin Film Vanadium Dioxide Zone Plate

In this paper, we propose a switchable focuser device based on a Fresnel zone plate (FZP) structure for terahertz (THz) applications. Each FZP contains seven rings, etched in thin VO₂ film with the designed focal lengths of 50 and 100 mm for 3.7-THz frequency. Temperature-induced VO₂ phase transition leads to the change in dielectric susceptibility of the material, which allows one to switch on and off the focusing properties of the device. The devices were tested with radiation of 3.1 and 3.7 THz emitted by quantum cascade lasers. Experimental results were compared with numerical simulations. In this article, we compare the FZP based on VO₂ films with different properties and show that a thicker VO₂ film reveals higher focusing efficiency, while a thinner one reveals a higher modulation ratio for the peak intensity at the focal point of FZP. We demonstrate experimentally the near-diffraction-limited size of the beam in the focal point of the device. Switching between two phase states of the VO₂ films results in up to the 38-fold change of intensity in the focal point.     

The impurity optical absorption and conduction band structure in 6H-SiC

Absorption spectra of nitrogen-doped n-type 6H-SiC crystals were studied for two orientations of the light wave electric field (E) relative to the optical axis (C), E ∥ C and E ⊥ C, within the range from the near-infrared region to the fundamental band. For E ∥ C, a weak absorption band peak at 2.85 eV was investigated for the first time. All the absorption bands observed are attributed to donor (nitrogen) photoionization, as a result of which electrons are transferred to the conduction-band upper minima that correspond to different critical points of the Brillouin zone. A combined analysis of the new data obtained in this study, the previous experimental results concerning photoionization of nitrogen, and theoretical data on 6H-SiC conduction band structure made it possible to refine the arrangement and symmetry of the additional conduction-band extrema in the Brillouin zone.     

Biosensors based on a method for determining the conductance matrix of multiterminal semiconductor nanostructures

A method for determining the conductance matrix is analyzed to study the properties of silicon nanostructures fabricated within Hall geometry on an n-type Si(100) surface as ultra-narrow p-type silicon quantum wells bounded by δ barriers heavily doped with boron. Within the proposed approach, the total current flowing through the multiterminal silicon nanostructure is written in the matrix form as I = G · V, where I and V are the columns of currents and voltages for each of the N terminals, G is the N × N conductance matrix uniquely describing the conductance of the structure under study, taking into account the contribution of contact-area resistances. The high sensitivity of matrix elements to changes in the state of the silicon nanostructure surface under conditions of the precipitation of sodium-acetate solution containing single-strand synthetic oligonucleotides is demonstrated. The prospects of practical application of the results obtained in developing modern biosensors based on determining the conductance matrix of multiterminal semiconductor nanostructures are discussed.     

On the photopleochroism coefficient and its temperature dynamics in native oxide-p-InSe heterojunctions

The temperature dependence of the photopleochroism coefficient for a native oxide-p-InSe heterojunction is studied. Different temperature dependences of the shift of the photocurrent long-wavelength edge are recorded for two polarization orientations E ‖ C and E ⊥ C.     

Bifunctional organic materials for OLEDs based on Tröger’s base: Subtle structural changes and significant differences in electroluminescence

Three bifunctional organic materials based on Tröger’s base (TB) scaffold have been designed, synthesized and their functional utility in OLEDs demonstrated through fabrication of devices. The diarylamino-functionalized TBs, i.e., A-TB, PA-TB and P-TB, are all amorphous with considerably high T_gs and permit modulation of electroluminescence with variation of one of the N-aryl rings, e.g., anthryl to 10-phenylanthryl to pyrenyl. Subtle structural modifications are found to manifest in rare pure yellow EL emission from PA-TB.     

Fundamental spectra of optical functions for indium bromide in the energy range of 2–30 eV at 4.2 K

The spectra of sets of optical fundamental functions are determined for an indium-bromide crystal in the range of 0–30 eV at 4.2 K for the polarizations E ∥ a and E ∥ c. The calculations are carried out using experimental reflection spectra R(E) and several software packages. Their basic features are established.     

An EAODV routing approach based on DARED and integrated metric

Wireless Mesh Network (WMN) is a new high-capacity and high-speed distributed broadband access network. Because of supporting multi-band devices, WMN is suitable for different application in the Internet of Things. In order to meet the performance requirements of multimedia traffic transmission, the routing approach designed for WMN must be taken into account the combination of load balance and anti-interference. We propose an extended Ad hoc on demand distance vector (AODV, which is a peer-to-peer the reactive routing approach) routing approach which is based on distributed adaptive random early detection and integrated metric for WMN. We firstly introduce several typical WMN routing approaches, and then based on the optimization theorem and cross-layer design proposes an integrated metrics to improve AODV routing approach. The steps and example using convex programming method are presented to solve routing optimization problem. Finally the feasibility of the proposed algorithm is verified with Method simulation. Simulation and analysis results show that the proposed EAODV routing approach improves the performance of WMN significantly.     

Relationship between HOMO energy level and open circuit voltage of polymer solar cells

A series of π-conjugated polymers (PDHF-BT and PDHF-TBT) with 4-(3,4-ethylenedioxythienyl)-2,1,3-benzothiadiazole (BT), 4,7-bis(3,4-ethylenedioxythienyl)-2,1,3-benzothiadiazole (TBT), and 9,9′-dihexylfluorene were synthesized by the Suzuki coupling reaction. The HOMO energy level of PDHF-BT was −5.47 eV, which was lower than that of PDHF-TBT (−5.22 eV), while the LUMO energy level of PDHF-BT (−3.45 eV) was very similar to that of PDHF-TBT (−3.42 eV). These energy levels of PDHF-BT and PDHF-TBT were also supported by a DFT calculation. The power conversion efficiency (PCE) of the polymer solar cell (PSC) with a structure of ITO/PEDOT:PSS/PDHF-BT:PCBM (1:1)/Al was determined as 0.34% and it was larger than that of the device based on PDHF-TBT (0.22%). Correspondingly, the V_oc of the PSC based on PDHF-BT (0.71 V) was much larger than that of the device based on PDHF-TBT (0.40 V). The results support that the V_oc of polymer based PSCs is strongly related to the HOMO energy level of the active polymers.     

On the stability of real exponential polynomials with interval-valued delays

We consider the family ? of exponential polynomials. $$f_T (z): = f_0 (z) + f_1 (z)e^{ - zT_1 } + f_2 (z)e^{ - T_2 } + \cdots + f_n (z)e^{ - zT_n } ,T : = T_1 \times T_2 \times \cdots \times T_n ,$$ where thef _k(z),k=0(1)n, are real polynomials under the degree restriction deg(f ₀)>deg(f _k),k=1(1)n, and theT _k,k=1(1)n, are intervals inR ₊. The functions in ? are characteristic functions of linear, retarded dynamical systems with constant coefficients and finitely many interval-valued discrete delays. A stability criterion for ? is expounded; ? is stable if (a) ? contains a stable member and (b) a certain functionalT:S(y) →; {0, 1} vanishes fory in a compact interval inR ₊. Here,S(y) is the boundary of a circular are regionS(y) in the complex plane derived fromf _T. Tools needed for a computer implementation are compiled.