Analytical modeling of field-induced interband tunneling-effect transistors and its application

In the room-temperature I-V characteristics of field-induced interband tunneling-effect transistors (FITETs), negative-differential conductance (NDC) characteristics as well as negative-differential transconductance (NDT) characteristics have been observed. The key operation principle of this quantum-tunneling device is the field-induced interband tunneling. To include the effect of interband tunneling, we have developed an analytical equation of interband tunneling current. Due to the inherent SOI-MOSFET structure of the FITET, the current equation of MOSFET has also been included in the analytical equation of the FITET. By comparing the calculated data from these two current components with the measured data, an additional excess tunneling current component has been introduced in the final analytical equation of the FITET. SPICE simulation results with this analytical model have shown good agreements with the experimental results. Also, this analytical model has been applied to verify the functionality of a simple digital logic gate such as XOR and four-level parity checker made by one FITET.     

A dual-gate-controlled single-electron transistor using self-aligned polysilicon sidewall spacer gates on silicon-on-insulator nanowire

A dual-gate-controlled single-electron transistor was fabricated by using self-aligned polysilicon sidewall spacer gates on a silicon-on-insulator nanowire. The quantum dot formed by the electric field effect of the dual-gate structure was miniaturized to smaller than the state-of-the-art feature size, through a combination of electron beam lithography, oxidation, and polysilicon sidewall spacer gate formation processes. The device shows typical MOSFET I-V characteristics at room temperature. However, the Coulomb gap and Coulomb oscillations are clearly observed at 4 K.     

Vertical spinal electronic device with large room temperature magnetoresistance

We report experimental transport measurements of a vertical hybrid ferromagnetic (FM)/III-V semiconductor (SC)/ferromagnetic(FM) type structure, i.e., Cr(20ML)/Co(15ML)/GaAs(50 nm, n-type)/Al/sub 0.3/Ga/sub 0.7/As(200 nm, n-type)/FeNi(30 nm). The current-voltage (I-V) characteristics reveal Schottky/tunneling type behavior in the direction of FeNi/Semiconductor/Co and observed to be dependent on external magnetic field. The magnetoresistance (MR) behavior shows a strong dependence on the measured current and field. At low fields no significant change in MR has been observed with increasing current. However, at high fields the MR initially increases with increasing current and becomes stable beyond a critical current of 10 /spl mu/A. A maximum of 12% change in the MR has been observed at room temperature, which is far larger than that of the conventional AMR effect. This property of the device could be utilized as field sensors or magnetic logic devices.     

Photodegradation of organic light-emitting devices observed in nitrogen-filled environment

Degradation of organic light-emitting devices (OLEDs) upon ultraviolet (UV) irradiation has been studied by measuring luminance-voltage (L-V) and current-voltage (I-V) characteristics of the devices in a nitrogen-filled glove-box. Photo-oxidation or reaction is no longer the main origin of the degradation for the devices protected by nitrogen. Conventional double-layer OLEDs with tris(8-hydroxyquinoline) aluminum (Alq₃) as the electron transport material and single-layer devices containing Alq₃ as the only organic material exhibit different degradation behaviors: both L-V and I-V characteristics degrade severely for the irradiated double-layer devices, whereas whether I-V degrades or not in a single-layer device is closely related to the species of the charge carriers flowing in the device. By comparing electroluminescent and photoluminescent degradation behaviors of the single-Alq₃-layer devices, we conclude that lowered fluorescent quantum efficiency and hole current after UV irradiation are two origins of the degraded characteristics of the devices isolated from the moist environment.     

Hybrid white-light emitting-LED based on luminescent polyfluorene polymer and quantum dots

We report the fabrication and electroluminescence (EL) characterization of a white-emitting hybrid organic/inorganic light emitting device (LED) by integrating core-shell CdSe/ZnS quantum dots (QDs) acting as a yellow emitter and polyfluorenes as the blue emitter in a multilayered structure. The hybrid white-emitting device was fabricated by spin-coating QDs in varied concentrations onto the layer of polyfluorenes with different functional groups (i.e., sulfur-containing PF1 and PFO). Depending on the QDs concentrations and the design of the hybrid LED, our device is able to exhibit either electroluminescence from QDs or a combination of both PF and QD. The EL spectra at different driving voltages, I-V characteristics, and EL chromaticity of the LEDs based on PF1/QD and PFO/QD LEDs have been measured, investigated, and compared, respectively.     

Electrical characteristics of ZrN metallised metal-oxide-semiconductor and metal-insulator-metal devices

The electrical properties of DC reactive sputtered zirconium-nitride metallized metal-oxide-semiconductor (MOS) and metal-insulator-metal (MIM) devices on TiO₂/p-Si and TiO₂/ZrN films were studied using capacitance-voltage (C-V) and current-voltage (I-V) measurements at room temperature. Capacitances of the ZrN/TiO₂/p-Si MOS device were measured in accumulation mode and inversion mode, from which flat band capacitance was found to be 2.86pF, which corresponds to flat band voltage of −1.7 V. Fixed oxide charged density and interface state density was found to be 1.63× 10¹⁰ cm⁻² and 6.3× 10¹¹ cm⁻² eV⁻¹. I-V characteristics revealed that the leakage current density was of 0.5 mA/cm² in accumulation mode and 2 mA/cm² in inversion mode at a field of 0.12 MV/cm, respectively. Dielectric breakdown of ZrN/TiO₂/p-Si device was found to be 0.12 MV/cm in accumulation mode. Based on the C-V and I-V characteristics, the ZrN/TiO₂/ZrN structure showed no variation in the capacitance value as the bias voltage was changed.     

Localized oxidation influence from conductive atomic force microscope measurement on nano-scale I-V characterization of silicon thin film solar cells

Nano-scale current-voltage (I-V) characteristics of hydrogenated microcrystalline silicon (μc-Si:H) prepared by Hot-Wire CVD (HWCVD) technique have been studied by Conductive Atomic Force Microscope (Conductive-AFM) under atmospheric conditions. It is demonstrated that a local modification is caused by the current, detected as a dramatic decrease in the forward biased current of I-V characteristics with the number of repeated scans. On the other hand, smaller change of reverse biased current is observed after the repeated scans. On the base of these results, we discuss and demonstrate the validity of our proposed new junction characterization method at the nanometer scale; that is, simultaneous nano-scale Topographical and Current-Voltage Imaging (TCVI) for Silicon (Si) thin film solar cells.     

The use of I-V characteristics for the investigation of selected contacts for spray-deposited CdS:In thin films

An investigation of selected contacts for indium doped cadmium sulphide (CdS:In) thin films was performed through the analysis of the I-V characteristics in the dark and room light at room temperature. Indium, aluminum and silver were selected as contacts where two strips of each metal were vacuum- evaporated on the surface of the film. All of these metals could form ohmic contacts, but indium had shown the best characteristics then aluminum. Films with indium contacts gave the best electrical properties and they are slightly affected by light. Doping and annealing were found to improve the contacts as seen in the I-V plots because they are expected to reduce the contact potential and annealing helps in the formation of an alloy with the semiconductor which forms an ohmic contact.     

Piezoelectric-potential-controlled polarity-reversible Schottky diodes and switches of ZnO wires

Using a two-end bonded ZnO piezoelectric-fine-wire (PFW) (nanowire, microwire) on a flexible polymer substrate, the strain-induced change in I-V transport characteristic from symmetric to diode-type has been observed. This phenomenon is attributed to the asymmetric change in Schottky-barrier heights at both source and drain electrodes as caused by the strain-induced piezoelectric potential-drop along the PFW, which have been quantified using the thermionic emission-diffusion theory. A new piezotronic switch device with an "on" and "off" ratio of approximately 120 has been demonstrated. This work demonstrates a novel approach for fabricating diodes and switches that rely on a strain governed piezoelectric-semiconductor coupling process.     

Resistive-switching behavior in polycrystalline CaCu(3)Ti(4)O(12) nanorods

Highly aligned CaCu(3)Ti(4)O(12) nanorod arrays were grown on Si/SiO(2)/Ti/Pt substrates by radio-frequency sputtering at a low deposition temperature of 300 °C and room temperature. Structural and morphological studies have shown that the nanostructures have a polycrystalline nature and are oriented perpendicular to the substrate. The high density of grain boundaries in the nanorods is responsible for the nonlinear current behavior observed in these arrays. The current-voltage (I-V) characteristics observed in nanorods were attributed to the resistive memory phenomenon. The electrical resistance of microcapacitors composed of CaCu(3)Ti(4)O(12) nanorods could be reversibly switched between two stable resistance states by varying the applied electric field. In order to explain this switching mechanism, a model based on the increase/decrease of electrical conduction controlled by grain boundary polarization has been proposed.     

a-SiCx:H p-i结的电致发光

在以ITO薄膜为正极的透明导电玻璃上，利用PECVD方法进行B掺杂制备出具有p-i结的a-SiCx：H薄膜，然后在其上溅Al作为负电极形成“三明治”器件结构，并对它的I-V特性和发光特性进行了研究。结果表明，器件具有整流效应，正反向电压分别为8V和12V；在正向电压高于8V时，观测到了电致发光。最后，根据我们提出的能带模型很好地解释了实验结果。     

Synthesis and characterization of two dimensional graphene lamellae based PAn nanocomposites

Novel room temperature synthesis method for graphene lamellae (GL) has been developed by using wet electrochemical route. Cyclic voltammetry and current transient studies were performed to explore the possibility of periodic precursor intercalation in the graphite. The synthesis parameters have been optimized and discussed. The GL sample quality and its mono-layered nature has been examined by Raman spectroscopy and scanning tunneling microscopy. Polyaniline (PAn) nanocomposite was prepared by in-situ doping/mixing of the two dimensional GL material through a novel electrochemical oxidative polymerization method. I-V characteristics of as prepared GL-PAn nanocomposite exhibits low resistance per unit length. The as prepared GL-PAn nanocomposites further investigated using optical absorption spectroscopy show GL dependent bulk heterostructure charge transfer properties. Prospects of GL-PAn nanocomposites in photovoltaic conversion have also been discussed.     

Coulomb blockade effects in silicon nanoparticles embedded in thin silicon-rich oxide films

Silicon nanoparticles (Si-nps) embedded in silicon oxide matrix were created using silicon-rich oxide (SRO) films deposited by low pressure chemical vapour deposition (LPCVD) followed by a thermal annealing at 1100?°C. The electrical properties were studied using metal-oxide-semiconductor (MOS) structures with the SRO films as the active layers. Capacitance versus voltage (C-V) exhibited downward and upward peaks in the accumulation region related to charge trapping and de-trapping effects of Si-nps, respectively. Current versus voltage (I-V) measurements showed fluctuations in the form of spike-like peaks and a clear staircase at room temperature. These effects have been related to the Coulomb blockade (CB) effect in the silicon nanoparticles embedded in SRO films. The observed quantum effects are due to 1?nm nanoparticles.     

Negative Differential Transconductance and Nonreciprocal Effects in a Y-Branch Nanojunction: High-Frequency Analysis

We report on negative differential transconductance (NDT) effect in Y-branch nanojunctions which leads to small-signal nonreciprocity of the device for certain biasing schemes. We present the dc analysis of the device from which we identify the bias regions where NDT effect occurs. We compare experimental dc measurements with results of Monte Carlo simulations, which yields very good qualitative and quantitative agreement. We perform a high-frequency analysis of the NDT effect up to 110 GHz which shows that the effect discussed is still present at such high frequencies     

Highly Sensitive Humidity Sensor Using Pd/Porous GaAs Schottky Contact

This paper investigates the suitability of porous GaAs as a semiconductor material for sensing humidity. The authors have developed two types of sensors based on Pd/porous GaAs and Pd/GaAs Schottky contacts for humidity measurements. It was found that the porosity on GaAs wafer promoted the sensing properties of the contact used as highly sensitive humidity sensor toward different amounts of relative humidity operated at room temperature. On the contrary, the Pd/GaAs sample operated at room temperature exhibited negligible sensitivity to relative humidity. The advantages of using porous GaAs for Schottky humidity sensor are the following: high sensitivity, low response time, and insignificant dependence on temperature. Current-voltage (I-V) characteristics of the Pd/porous GaAs Schottky humidity sensor exhibited a saturation current value of 8.5times10^-10 A under dry condition (5% relative humidity). This was increased to 7.0times10^-9 A when submitted to a relative humidity of 25%. The saturation current was further increased considerably to 3.0times10^-7 A as the relative humidity was increased to 95%. This is more than two orders of magnitude increase in saturation current compared to dry condition. A parameter called humidity sensitivity was defined using the current value at a fixed forward voltage of 0.2 V to present the sensitivity of the sensor. Response times are reported to discuss the adsorption and desorption characteristics of the device. Pd/porous GaAs sensor operated at room temperature showed a fast response time of 2 s and a sensitivity value of 93.5% in the presence of 25% relative humidity. Furthermore, the influence of increase in relative humidity as well as heating effects on the responsivity of the sensor is described. Scanning electron microscopy analysis of the Pd/porous GaAs sample exhibited highly porous structures     

Electrical characterization of carbon monoxide sensitive high temperature sensor diode based on catalytic metal gate-insulator-silicon carbide structure

Field-effect gas sensors based on catalytic metal-insulator-silicon carbide (MISiC) devices are investigated. For the evaluation of the barrier height, the temperature dependence of the current-voltage (I-V) and the capacitance-voltage (C-V) characteristics of MISiC Schottky diodes were investigated in CO and O/sub 2/ atmospheres. Four methods were used to evaluate how a change in gas ambient influences the barrier height of the diode: a change of the intersection current at zero voltage in the forward direction of the I-V curve, a change of the temperature dependence in the forward direction and the reverse direction, respectively, of the I-V curve, and a change of the intersection voltage of 1/C/sup 2/ versus V plot. The four methods gave similar changes in the barrier height for the device in 8000 ppm CO and 4000 ppm O/sub 2/. The values of barrier height obtained from the I-V curves were here normalized by the ideality factor calculated from I-V measurements. The correlation between the barrier height change obtained from the I-V and the C-V measurements, respectively, is discussed regarding the ideality factor.     

GaAs three-terminal double delta-doped triangular barrier switch prepared by molecular beam epitaxy

A novel S-shaped negative differential resistance (NDR) switching device, prepared by molecular beam epitaxy (MBE), has been successfully developed in a GaAs double delta-doped triangular barrier (TB) structure. All symmetrical S-shaped NDR characteristics, either unidirectional or bidirectional bistability phenomena, are observed experimentally. The bidirectional current-voltage (I–V) characteristics exhibit a new type of NDR caused by an avalanche multiplication process in the reverse-biased base-collector region and by barrier redistribution. Under a base current injection with respect to the cathode, the device behaves as a conventional transistor with a current gain of 1.2 at room temperature. Experimentally, electrical results can be easily understood by an equivalent circuit and a proposed model. In addition, a new optoelectronic switching device is also proposed which may have the potential for bidirectional wavelength emission. On the basis of the first utilization of a double delta-doped TB structure, a conceptual understanding of such a set of results enhances our understanding of the physics of double delta-doped TB devices in general.     

Junction properties of Schottky diode based on composite organic semiconductors

Composite of polyaniline with polyvinyl chloride (PANI-PVC) has been used for the fabrication of Schottky diode with configuration Pt/PANI-PVC/In. Current-voltage (I-V) plots were non-linear and capacitance-voltage (C-V) plots were almost linear in reverse bias indicating rectification behavior. The observed current-voltage characteristics can be satisfactorily fitted using the modified Schottky equation. Various junction parameters were calculated from the temperature dependent I-V and C-V data and discussed. It has been observation that ideality factor decreases as the temperature increases. Impedance studies shows the R(RC)(RC) equivalent circuit for the fabricated Schottky diode.     

3-D Nonequilibrium Green's Function Simulation of Nonperturbative Scattering From Discrete Dopants in the Source and Drain of a Silicon Nanowire Transistor

As these As transistors are scaled to nanometer dimensions, the discreteness of the dopants becomes increasingly important. Transistors of 3 times 3 nm² cross section contain, on average, approximately one dopant atom per nanometer of length, making any self-averaging impossible. The individual random dopants act as localized scatterers whose distribution, and therefore, impact on the electron transport, varies from device to device. This is complemented by electrostatic variation in the potential that controls the threshold voltage and the dominant current paths. The current density is greatly influenced by resonances associated with the attractive potential of the donors and screening effects. In this paper, for the first time, a full 3-D nonequilibrium Green's function (NEGF) simulation in the effective mass approximation has been used to study the influence of individual discrete donors in the source/drain on the I-V characteristics of a narrow n-channel Si nanowire transistor. We have compared devices with microscopically different configuration of dopants. The simulated variations in the I-V curves are analyzed with reference to the behavior of the transmission coefficients. We have highlighted the importance of resonance states when solving the NEGF and Poisson equations self-consistently.     

Probing electrical transport in individual carbon nanotubes and junctions

The electrical transport properties of individual carbon nanotubes (CNTs) and multi-terminal junctions of CNTs are investigated with a quadraprobe scanning tunneling microscope. The CNTs used in this study are made of stacked herringbone-type conical graphite sheets with a cone angle of ～20° to the tube axis, and the CNT junctions have no catalytic particles in the junction areas. The CNTs have a significantly higher resistivity than conventional CNTs with concentric walls. The straight CNTs display linear current-voltage (I-V) characteristics, indicating diffusive transport rather than ballistic transport. The structural deformation in CNTs with bends substantially increases the resistivity in comparison with that for the straight segments on the same CNTs, and the I-V curve departs slightly from linearity in curved segments. The junction area of the CNT junctions behaves like an ohmic-type scattering center with linear I-V characteristics. In addition, a gating effect has not been observed, in contrast to the case for conventional multi-walled CNT junctions. These unusual transport properties can be attributed to the enhanced inter-layer interaction in the herringbone-type CNTs.