Conductivity Of Semiconductors Research Articles

Electrical and Switching Properties of TlBiSe2 Chalcogenide Compounds

The electrical conductivity of TlBiSe2 narrow gap semiconductors prepared by the Bridgman–Stockbarger method was investigated. The temperature dependence of the electrical conductivity was measured to establish the dominant conductivity mechanism in a temperature range between 293 K and 373 K. The conduction activation energy has a single value indicating the existence of one type of conduction mechanism in the investigated temperature range. The electrical conductivity of the sample is controlled by a thermally activated mechanism. It was also found that these samples exhibit a current-controlled negative resistance and threshold switching. The value of the threshold voltage decreases exponentially with increasing temperature. The calculated ratio of the threshold energy to the activation energy is one half, and is derived from the electro-thermal model for the switching process. Therefore, the electrical switching in the investigated samples can be explained in terms of the electro-thermal model. A possible conduction mechanism in the pre-switching state of the sample associated with the space charge limited current is described.

Conductivity in organic semiconductors hybridized with the vacuum field.

Much effort over the past decades has been focused on improving carrier mobility in organic thin-film transistors by optimizing the organization of the material or the device architecture. Here we take a different path to solving this problem, by injecting carriers into states that are hybridized to the vacuum electromagnetic field. To test this idea, organic semiconductors were strongly coupled to plasmonic modes to form coherent states that can extend over as many as 10(5) molecules and should thereby favour conductivity. Experiments show that indeed the current does increase by an order of magnitude at resonance in the coupled state, reflecting mostly a change in field-effect mobility. A theoretical quantum model confirms the delocalization of the wavefunctions of the hybridized states and its effect on the conductivity. Our findings illustrate the potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials.

A Hybrid Modular Multilevel Converter With Novel Three-Level Cells for DC Fault Blocking Capability

A novel hybrid, modular multilevel converter is presented that utilizes a combination of half-bridge and novel three-level cells where the three-level cells utilize a clamp circuit which, under dc side faults, is capable of blocking fault current thereby avoiding overcurrents in the freewheel diodes. This dc fault blocking capability is demonstrated through simulation and is shown to be as good as the modular multilevel converter which utilizes full-bridge cells but with the added benefits of: lower conduction losses; fewer diode and semiconductor switching devices, and; fewer shoot-through modes. The semiconductor count and conduction loss of the proposed converter are reduced to around 66.5% and 72% of that of modular multilevel converter based on the full-bridge cells respectively, yielding lower semiconductor cost and improved efficiency. Dc fault ride-through operation is realized without exposing the semiconductors to significant fault currents and overvoltages due to the full dc fault blocking capability of the converter.

Multifunctionality of Giant and Long-Lasting Persistent Photoconductivity: Semiconductor–Conductor Transition in Graphene Nanosheets and Amorphous InGaZnO Hybrids

Composite materials can play a decisive role to reveal novel physical properties and enable to advance new generation technologies. Here, we discover that phototransistors based on the integration of two-dimensional graphene nanosheets (GNSs) and amorphous indium–gallium–zinc–oxide (a-IGZO) semiconductors exhibit a giant photo-to-dark current ratio and long-lasting persistent photoconductivity (PPC). Under the illumination of UV light (350 nm) at 50 mW/cm2, a photo-to-dark current ratio up to 2.0 × 107 was obtained, which is about 3 orders of magnitude higher than its pure a-IGZO device counterpart. Moreover, the GNSs/a-IGZO phototransistor possesses an enduring lifetime up to years for the recovery of the transfer characteristics after switching off the UV light. The giant and long-lasting PPC leads GNSs/a-IGZO to become an excellent conductor with conductivity much better than indium tin oxide. The observed unique features represent a semiconductor–conductor transition. In addition to next generation flat, flexible, and display, it can open up a wide variety of application, such as transparent electrodes for optoelectronic devices, optical memory, and light harvesting for energy storage. As an example, we demonstrated the operation of optical memory devices, which may lead to the novel application of holographic storage. Our results shown here therefore provide an outstanding new route for the future development of solution-processable semiconducting optoelectronic devices.

Linear correlation between Debye temperature and lattice thermal conductivity in II-VI and III-V semiconductors

A simple linear empirical relationship between high intrinsic lattice thermal conductivity K, and Debye temperature θa is suggested from data on high intrinsic lattice thermal conductivity K, and Debye temperature θa for some selected II-VI and III-V cubic zincblende type, and I-VII and II-VI rock-salt type binary semiconductors. A good linear correlation between Debye temperature and the high intrinsic lattice thermal conductivity was obtained. The minimum average percentage deviations in the present approach reveal that our simple model prove its identity and soundness compared to those of other author relations.

Magnetic polarons in doped La0.7Ca0.3MnO3, La0.7Ba0.3MnO3, and La0.7Sr0.3MnO3 manganites

The characteristics of the paramagnetic state of doped manganites containing 30% Mn4+ ions are studied. It is shown that the ferromagnetic Curie temperature for the materials with chemical composition La0.7A0.3MnO3 (A = Ca and Ba) characterized by semiconductor conductivity at T > T C can be determined from the measured χ–1(T) dependence. In the conducting La0.7Sr0.3MnO3 compound, the nonlinear behavior of χ–1 (T) is interpreted as a result of summation of different contributions related to individual Mn ions, as well as to the paramagnetic and ferromagnetic polarons with enhanced magnetic moments. The order (first or second) of the magnetic transition and the crystal lattice symmetry do not affect the formation of correlated polarons in the paramagnetic range.

Conductivity-limiting bipolar thermal conductivity in semiconductors.

Intriguing experimental results raised the question about the fundamental mechanisms governing the electron-hole coupling induced bipolar thermal conduction in semiconductors. Our combined theoretical analysis and experimental measurements show that in semiconductors bipolar thermal transport is in general a “conductivity-limiting” phenomenon, and it is thus controlled by the carrier mobility ratio and by the minority carrier partial electrical conductivity for the intrinsic and extrinsic cases, respectively. Our numerical method quantifies the role of electronic band structure and carrier scattering mechanisms. We have successfully demonstrated bipolar thermal conductivity reduction in doped semiconductors via electronic band structure modulation and/or preferential minority carrier scatterings. We expect this study to be beneficial to the current interests in optimizing thermoelectric properties of narrow gap semiconductors.

Cyclotron Frequency Oscillations in Semiconductor Plasma in Modulated Magnetic Field

Radio frequency magnetoplasmic waves known as helicons will propagate in solid-state plasma of semiconductors when a strong magnetic field is applied. Helicons have an exact analogy with a electromagnetic whistler wave which is frequently propagated in the rarefied plasma of the Earth’s ionosphere. In our experiments the modulated magnetic field is being used for excitation of helicons. Physically it means that the semiconductor conductivity and effective dielectrical constant are modulated with the same cyclotron frequency of infrared range. The semiconductor sample is in the form of plae and magnetic field is perpendicular to the surface of the plate. It is shown that in the case of modulated field along with the RF helicon waves the transient cyclotron frequency oscillations exist in the semiconductor plasma. For observation of the cyclotron radiation frequency c the modulation depth about one percent is sufficient. The measurement of c provides an opportunity to determine the masses of electrons and holes in solid-state plasma of semiconductors. The proposed method can be used in the cases of pulse and sinusoidal modulation of the magnetic field. Bibl. 5 (in English; summaries in English, Russian and Lithuanian).

Calculation of electronic coupling matrix elements in 4,4′-bis(9-carbazolyl)-biphenyl amorphous material

The electronic coupling matrix element responsible for positive charge carrier (hole) transport has been calculated for a variety of 4,4′-bis(9-carbazolyl)-biphenyl dimers corresponding to a disordered solid matrix. A procedure applying the computational scheme to a real system (organic molecules with a large molecular mass) is proposed. The consistency of the results with the existing model concept of hopping conductivity in organic semiconductors is discussed.

Toxicity Testing in Vitro and Biocompatibility Scaffold for Transplantation of Stem Cells in Spinal Cord Injury

Introduction Spinal cord injury occurs mainly in young healthy individuals, causing motor impairment and temporary or sensory loss and leading to chronic disability. An attractive possibility of treatment for TRM is the use of stem cells for restoration of neural cells in the spinal cord. Support Matrices for use in cell therapy must be able to provide structural and microenvironmental support to ensure the viability of the transplanted cells and to facilitate integration in the target tissue. Materials and Methods Biocompatible polymeric films composed of polylactic acid-co-acid (PLGA) polymer nanofiber polypyrrole (PPy) were used polypyrrole nanofibers and films made containing 10% (W/W) PLGA in PPy by solvent evaporation method were synthesized. The PPy nanofibers showed electrical conductivity of semiconductor and the dispersion of the nanofibers in PLGA resulted in PPy films with fibrous topographical morphology with suitable for use as scaffolds for cell growth driven by PPy nanofiber voids. Cell toxicity test method of extraction was performed. Samples of the polymers were maintained in DMEM culture medium obeying the proportions established by ISO 10993 (2013) by 24, 48, 72 hours, 7 and 14 days. After each time period, NIH3T3 fibroblasts were cultured for 24 hours with the film exposed to the environment, and then was held cellular toxicity test by MTT assay. Wistar rats were anesthetized and underwent laminectomy of the eighth thoracic vertebra. On the local display of the spinal cord was placed a cut polymer approximately 3 mm wide and 8 mm long. The motor recovery of the animals 24 hours and 7, 14, and 28 days after injury was assessed. To assess the degree of inflammation generated by the polymer in tissues other than the spinal cord, a 1 cm incision in the flank and placement polymer was performed. In the region against lateral incision was performed only. Histological evaluation of the spinal cord and the fabric flank was held. Results The electrical conductivity of PPy fibers showed values of 8.10 to 4 S/cm, lying within the range of semiconductors. The molecular structure of PPy was evaluated in the range of 400 to 4,000/cm and all spectra were similar. There was no change in cell viability after exposure of the cells to the conditioned medium with the polymer in both seasons. Cell viability was maintained at 100% when compared with the control. The motor functional assessment of mice showed no alteration in any evaluation period are because of the surgical procedure or a possible effect of inflammatory irritation of the polymer on the spinal nerve tissue. Histological evaluation showed no changes suggestive of inflammation or infection of the marrow tissue adjacent to the polymer or the flank of the animal tissues. Conclusion PLGA/PPy polymer has conductivity expected for semiconductors and structural heterogeneity, without any type of cell toxicity in periods. The polymer is safe to be used as a scaffold in the TRM model because it does not cause any functional, structural, or inflammatory change.

Arsenic related defect states resonant with the semiconductor conduction band at the In0.53Ga0.47As/oxide interface: A density functional study

An arsenic related defect at the In0.53Ga0.47As/oxide interface is studied through density functional theory. This defect corresponds to an As–As dimer which transforms into two doubly occupied As dangling bonds upon capture of two electrons. The defect charge transition level is found to be resonant with the bottom of the In0.53Ga0.47As conduction band and weakly sensitive to its local chemical environment. Hence, this defect is a possible candidate for the observed defect density in the conduction band of In0.53Ga0.47As.

Impact of Gate Metal on the Performance of p-GaN/AlGaN/GaN High Electron Mobility Transistors

For conventional GaN-based high electron mobility transistors (HEMTs), the work function of gate metal is critical to electrical parameters, such as OFF-state leakage current, forward operating current, and threshold voltage. A high work function is thus required to maintain Schottky gate contact. In this letter, an enhancement-mode HEMT composed of p-type GaN/AlGaN/GaN was fabricated. Unlike typical HEMTs that the Schottky barrier height is determined by the energy difference between gate metal work function and semiconductor (AlGaN, or GaN) conduction band, the insertion of the p-GaN relieves the constraint of gate metal. In addition, the gate Schottky barrier now correlates to the valence band of the semiconductor. Here we compare the HEMT performance of different gate metals-Ni/Au, Ti/Au, and Mo/Ti/Au. The results reveal that a tradeoff between V TH and output drain current.

Density functional theory study of new azo dyes with different π-spacers for dye-sensitized solar cells

Some of new azo-based metal-free dyes with different π-conjugation spacers, such as carbazole, fluorene, pyrrole, thiophene, furan and thiazole, have been investigated with density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Theoretical calculations allow us to quantify factors such as light harvesting efficiency (LHE), electron injection driving force (ΔGinject) and the weight of the LUMO orbital on the carboxylic group (QLUMO) related to the short-circuit photocurrent density (Jsc), and to evaluate both charge recombination between the semiconductor conduction band electrons and the oxidized dyes and/or electrolyte, and also the shift of the conduction band of the semiconductor as a result of the adsorption of the dyes onto the semiconductor surface, associated with the open-circuit photovoltage (Voc). According to the results, we could predict that how the π-conjugation spacers influence the Jsc as well as the Voc of DSSCs. Among these dyes, the carbazole and fluorene-based dyes (dyes 1 and 2) show the highest LHE, ΔGinject, QLUMO, and the slowest recombination rate. Consequently, the obtained results show that the carbazole and fluorene-based dyes could have the better Jsc and Voc compared to the other dyes.

Future Prospects of Widebandgap (WBG) Semiconductor Power Switching Devices

Electrical power switching devices based on widebandgap (WBG) semiconductors have the potential for transformative impact on a wide range of energy conversion applications. Significantly improved electrical and thermal conductivities of WBG semiconductors compared with the semiconductor silicon have the potential for more efficient, compact, and robust power conversion systems. However, to offset inherently higher manufacturing cost of WBG power devices and obtain system-level benefits, power converters need to be operated at higher semiconductor chip junction temperatures and/or at higher switching frequencies. This paper discusses the future prospects of WBG-based power electronics by considering the current state of the art of WBG chip manufacturing, packaging, and thermal management technologies.

Anomalous migration of polarons in disordered organic semiconductors and its manifestation in magnetic-field effects

The specific features of the anomalous (dispersive) migration of polarons P (electrons and holes) in disordered organic semiconductors and manifestations of these features in the magnetic field effects on the conductivity of semiconductors are theoretically examined. According to the results, the most important reason for the influence of weak magnetic fields on the photoconductivity is their impact on PP-recombination (recombination of electrons and holes), a spin-selective process. Within the framework of an approach based on the continuous time random walk (CTRW) model, it is shown that anomalous (dispersive) migration, which is described in the CTRW model by a heavy-tailed hop waiting time distribution ψ(t)∼t-(1+α), clearly manifests itself in the magnetic field dependence of the PP-recombination yield and, hence, the conductivity. The specific features of this influence are illustrated by the example of the dependence of the recombination yield on the magnetic induction, which is predicted by the Δg-mechanism of magnetic sensitivity.

Determining the Efficiency of Fast Ultrahigh-density Writing of Low-Conductivity Patterns on Semiconducting Polymers

ABSTRACTWe present a nano-patterning process for semiconducting polymeric composites that could potentially be utilized for the development of polymer-based data storage devices. Nano-patterning (writing) operates on the basis of the mechanical interaction between the electrically unbiased tip of an atomic force microscope and the surface of polymeric composite films. Via friction forces, the tip/sample interaction produces a local increase of molecular disorder in the polymer matrix, inducing a localized lowering in the conductivity of the organic semiconductor. Herein we suggest a figure of merit for quantifying the efficiency of pattern formation and we address the dependence of the writing process on the thermal annealing temperature of the composite film. Control experiments on composite films deposited on substrates with different roughness suggest that the writing effect is invariant to the roughness of the substrate. The potential storage density of the writing process depends on the tip curvature.

Interface Charge Density Measurement for Ultra Thin ZrO2 Material Based MOS Devices Using Conductance Method

Abstract The metal oxide semiconductor (MOS) conductance technique has been a key tool for the understanding of electrically active interface states since it was first developed by Nicollian and Brews in the mid 1960's. In this letter the interface charge densities (Dit) are analyzed for ultra thin dielectric material based MOS devices using high-k dielectric material ZrO2 at p-type doping level of 1x1017 cm-3. The Dit have been calculated by a novel approach using conductance method and it indicates that by reducing the thickness of the oxide, the Dit increases and similar increase is also found by replacing SiO2 with ZrO2. Numerical calculations and analytical solutions are performed by MATLAB and device simulation is done by ATLAS, a TCAD tool from Silvaco.

Synthesis and Investigation of Pseudo Binary System CaTiSiO<sub>5</sub>-YFeSnO<sub>5</sub>

The research is devoted to the multicomponent system CaTiSiO5-YFeSnO5. The synthesis of solid solutions Ca1-xYxTi1-xSnxSi1-xFexO5 (x = 0 - 1.0, Δx = 0.1) was conducted in low-temperature plasma of hydrogen-oxygen flame. It was found that ions Ca2+, Ti4+ and Si4+ in the molecule of titanit may be substituted with t ions Y3+, Fe3+ and Sn4+. In this case, the system produces two phases of variable composition with broad regions of homogeneity. There were defined the boundaries of formed phases, crystallographic and electrical parameters of the solid solutions. All solid solutions have a semiconductor conductivity type, whose value is linearly dependent on the temperature and com- position of the sample.

History of Agrophysics: The Creative Activity of A. F. Chudnovsky

A. F. Chudnovsky was a prominent agrophysicist and a pioneer in the heat physics of soil. His primary research interests were thermo-physics of soils, thermo-physical properties of dispersed materials, the thermal conductivity of semiconductors, engineering and physical aspects of thermal soil melioration, and energy and mass transfer in the soil-plant-air system. He also conducted research on physical and technical methods (electronics) in agrophysics, remote sensing of agricultural fields, and agrophysical basics of crop programming. Among these areas of agrophysics, he completed 20 scientific monographs. A. F. Chudnovsky was a talented scholar, a great mentor, and under his leadership, more than 50 Masters of Science and 20 Doctoral Theses were completed. This paper is written to commemorate Dr. A. F. Chudnovsky’s dedicated contributions to agrophysical research and development.

Analytical model of gate leakage current through bilayer oxide stack in advanced MOSFET

A compact model for gate tunneling current in advanced nano-scale MOSFET has been developed on the basis of both direct and Fowler–Nordheim tunneling through dual layer Silicon oxide–Hafnium oxide stack used as gate dielectric. Calculation includes the effect of different subbands of the semiconductor conduction band those arise due to quantum confinement of charge carriers in the oxide–substrate interface. Effect of charge trapping in the bulk of the oxides and at the localized energy levels at different interfaces of the oxides has also been taken into consideration. Tunneling probability as a function of gate bias has been determined considering Wentzel–Kramers–Brillouin (WKB) approximation to account for varying potential profile. Probability amplitude of an electron for tunneling has been calculated by solving Schrodinger equations at different regions in the effective mass approximation model of class I crystal interface. Tunneling current as a function of effective oxide thickness and gate bias estimated in this model shows substantial reduction in gate leakage current if HfO2 as high-k dielectric is used along with 1nm thick SiO2 with almost negligible change in threshold voltage.