Spin-polarized current generated by magneto-electrical gating

We theoretically study spin-polarized current through a single electron tunneling transistor (SETT), in which a quantum dot (QD) is coupled to non-magnetic source and drain electrodes via tunnel junctions, and gated by a ferromagnetic (FM) electrode. The I–V characteristics of the device are investigated for both spin and charge currents, based on the non-equilibrium Green's function formalism. The FM electrode generates a magnetic field, which causes a Zeeman spin-splitting of the energy levels in the QD. By tuning the size of the Zeeman splitting and the source–drain bias, a fully spin-polarized current is generated. Additionally, by modulating the electrical gate bias, one can effect a complete switch of the polarization of the tunneling current from spin-up to spin-down current, or vice versa.     

Symmetric tunnel field-effect transistor (S-TFET)

A novel heterojunction symmetric tunnel field-effect transistor (S-TFET) has been proposed and investigated, for the first time, in order to address the inborn technical challenges of the conventional p-i-n TFET (i.e., asymmetric TFET). With a band-to-band tunneling process between the germanium source/drain region and the silicon channel region, the theoretical limit of the subthreshold slope (SS) can be overcome (i.e., SS ∼ 45 mV/decade). The bidirectional current flow in the S-TFET is implemented with a p-n-p structure. And better performance in the S-TFET is achieved with a thin silicon-pad layer below the source/drain regions. The effects of source/drain/channel doping concentration and thickness on the performance of the device are investigated in order to create an S-TFET design guideline. In the future, the S-TFET will be one of the promising device structures for ultra-low-power applications, especially in integrated circuits that operate with a half-volt power supply voltage.     

Design and simulation of double-lightly doped MOSCNT using non-equilibrium Green’s function

In this paper, we propose a new ohmic-structure of ballistic carbon nanotube field-effect transistors (CNTFETs) in which the source and drain regions are doped stepwise and the device acts as MOSFET like CNTFET (MOSCNT). The number of lightly doped regions and their doping concentrations are optimized to obtain the lowest OFF current. To study the device characteristics, the Poisson–Schrödinger equations are solved self-consistently using the Nonequilibrium Green’s Function (NEGF) formalism in the mode space approach. To find the Hamiltonian matrix, the tight-binding approximation with only p _z orbital is used. The obtained results show that the stepwise regions lead to barrier widening due to the reduction in potential gradient. Therefore, the band-to-band tunneling (BTBT) and the ambipolar behavior of the device decrease due to band engineering. This causes to the superior reduction of OFF current and dissipative power. In addition, the device performance shows lower subthreshold swing (SS), smaller drain induced barrier lowering (DIBL), and larger current ratio than that of the previous structures.     

Tunneling in 2-D quantum dots via quantum adiabatic switching route

We explore the phenomenon of tunneling in single carrier 2-D quantum dot by quantum adiabatic switching route. The confinement in the y-direction is kept harmonic which ensures that tunneling is allowed only along the x-direction. The harmonic confinement potential is kept fixed and a constant external magnetic field is applied along the z-direction. The growth of probability density in the classically forbidden zones and tunneling current are monitored critically which reveals how tunneling significantly depends on the barrier parameters. The efficacy of the switching function in enforcing adiabaticity of the evolution is demonstrated. The effective mass, barrier width, and height emerge as important control parameters.     

Lateral scaling in carbon-nanotube field-effect transistors

We have fabricated carbon-nanotube (CN) field-effect transistors with multiple, individually addressable gate segments. The devices exhibit markedly different transistor characteristics when switched using gate segments controlling the device interior versus those near the source and drain. We ascribe this difference to a change from Schottky-barrier modulation at the contacts to bulk switching. We also find that the current through the bulk portion is independent of gate length for any gate voltage, offering direct evidence for ballistic transport in semiconducting carbon nanotubes over at least a few hundred nanometers, even for relatively small carrier velocities.     

Quantum transport andI–Vcharacteristics of quantum size field effect transistor

Quantum electron transport is expected to occur in nanometer-size field effect transistors. We show that the amplitude of the transmitted wave equals 1 only when the electric field in the conducting channel is zero. By reducing the dimension of the quantum transport from bulk to a two-dimensional electron gas system, and further to a one-dimensional quantum wire, the current-bias relation is not affected while the gate control over the drain current weakens. Starting from the Poisson and Schrödinger equations, we have studied numerically the quantum wave transport through the conduction channel where scattering processes are neglected, theI–Vcharacteristic of a typical heterojunction high electron mobility transistor shows a linear relationship between drain current and voltage at low drain bias, but the drain current decreases with increasing drain voltage at a high bias.     

Modelling of contact effects in microcrystalline silicon thin-film transistors

Hydrogenated microcrystalline silicon has recently emerged as a promising material system for large-area electronic applications such as thin-film transistors and solar cells. In this paper, thin-film transistors based on microcrystalline silicon were realized with charge carrier mobilities exceeding 40 cm²/Vs. The electrical characteristics of the microcrystalline silicon thin-film transistors are limited by the influence of contact effects. The influence of the contact effects on the charge carrier mobility was investigated for transistors with different dimensions of the drain and source contacts. The experimental results were compared to an electrical model which describes the influence of the drain and source contact dimension on the transistor parameters. Furthermore, the Transmission Line Method was applied to investigate the contact effects of the thin-film transistors with different drain and source contact dimensions. Finally, optimized device geometries like the channel length of the transistor and dimension of the drain and source contacts were derived for the microcrystalline transistors based on the electrical model.     

对称三材料双栅应变硅金属氧化物半导体场效应晶体管二维解析模型

提出了对称三材料双栅应变硅金属氧化物半导体场效应晶体管器件结构,为该器件结构建立了全耗尽条件下的表面势模型、表面场强和阈值电压解析模型,并分析了应变对表面势、表面场强和阈值电压的影响,讨论了三栅长度比率对阈值电压和漏致势垒降低效应的影响,对该结构器件与单材料双栅结构器件的性能进行了对比研究.结果表明,该结构能进一步提高载流子的输运速率,更好地抑制漏致势垒降低效应.适当优化三材料栅的栅长比率,可以增强器件对短沟道效应和漏致势垒降低效应的抑制能力.     

Analytical modeling of electrical characteristics of coaxial nanowire FETs

In this paper, an analytical approach based on ballistic current transport is presented to investigate the electrical characteristics of the coaxial nanowire field effect transistor (CNWFET). The potential distribution along the nanowire is derived analytically by applying Laplace equation. In addition to application of WKB approximation and ballistic transport, tunneling process and quantum state of energy are implemented to determine the amount of electron transport along the nanowire from the source to the drain terminals. To consider the tunneling phenomena, WKB approximation is used and the transmission coefficients on both sides of the channel are obtained separately. In ballistic regime, an expression for channel current in terms of the bias voltages and Schottky barrier height (SBH) is derived. The results confirm a close correlation between the current equation of this work and the results presented via other approaches.     

Electrical and optical properties of light-emitting field-effect transistors based on MEH-PPV polymer composite films with ZnO nanoparticles

The optical and electrical properties of light-emitting field-effect transistor structures with an active layer based on nanocomposite films containing zinc oxide (ZnO) nanoparticles dispersed in the matrix of the soluble conjugated polymer MEH-PPV have been investigated. It has been found that the current-voltage characteristics of the field-effect transistor based on MEH-PPV: ZnO films with a composite component ratio of 2: 1 have an ambipolar character, and the mobilities of electrons and holes in these structures at a temperature of 300 K reach high values up to ～1.2 and ～1.4 cm²/V s, respectively, which are close to the mobilities in fieldeffect transistors based on ZnO films. It has been shown that the ambipolar field-effect transistor based on MEH-PPV: ZnO films emits light at both positive and negative gate bias voltages. The mechanisms of injection, charge carrier transport, and radiative recombination in the studied structures have been discussed.     

Resonant tunneling field-effect transistors

We report an experimental project to incorporate double-barrier tunnel structures into three-terminal devices. These devices have the negative-differential-resistance (NDR) features of the double barrier, and the added flexibility of a third controlling electrode. One way to make such a device involves the series combination of a double-barrier tunnel structure with a field-effect transistor. We have realized this concept in two types of devices, using samples grown by metalorganic chemical vapor deposition. The devices consist of a GaAsAl_xGa_1−xAs double-barrier tunneling heterostructure, the current through which is controlled by either an integrated vertical field-effect transistor or a planar metal-semiconductor field effect transistor. The voltage location and peak-to-valley current ratio of the NDR present in the source-drain circuit can be modulated with gate voltage. Experimental results for four samples are presented.     

Hybrid semiconductor/metal nanostructures with two-dimensional electron systems

A brief account of our recent work on InAs/metal hybrids is given. In particular we discuss Josephson field-effect transistors with niobium source and drain contacts, first spin transistor structures with permalloy electrodes, and devices with gold shunts exhibiting the extraordinary magnetoresistance effect.     

The current-voltage characteristics of field-effect transistors with short channels

The current-voltage characteristics of MOS field-effect transistors is investigated theoretically and experimentally in the region of extremely high drain electric fields E_D using the electron temperature concept in the classical three-dimensional theory. It is found that the drain field at which the drain current becomes non-ohmic on the basis of hot electron effects is related to the surface field E_zs by E_D～√E_zs. Other expressions for the field dependent mobility are also given which allow the construction of the current-voltage characteristics.     

Laser printing of polythiophene for organic electronics

We report on the development of hybrid organic/inorganic thin-film transistors using regioregular poly-3-hexylthiophene (P3HT) semiconductor material deposited by means of the solid-phase Laser Induced Forward Transfer (LIFT) technique. P3HT pixels were LIFT-printed onto Au/Ti source and drain electrodes formed on silicon dioxide/p⁺-type Si substrate. Deposition of the P3HT pixels was investigated as a function of the laser fluence using donor substrates with and without a dynamic release layer. Device electrical characterization reveals efficient field-effect action of the bottom gate on the organic channel. The transfer I_DS-V_GS characteristics exhibit well-defined sub-threshold, linear and saturation regimes designating LIFT as a promising technique for hybrid organic/inorganic transistor technology.     

Mobile ambipolar domain in carbon-nanotube infrared emitters

We spatially resolve the infrared light emission from ambipolar carbon-nanotube field-effect transistors with long-channel lengths. Electrons and holes are injected from opposite contacts into a single nanotube molecule. The ambipolar domain, where electron and hole currents overlap, forms a microscopic light emitter within the carbon nanotube. We can control its location by varying gate and drain voltages. At high electric fields, additional stationary spots appear due to defect-assisted Zener tunneling or impact ionization. The laterally resolved measurement provides valuable insight into the transistor behavior, complementary to electronic device characteristics.     

Theoretic analysis on electric conductance of nano-wire transistors

By employing the commercial software nanoMos and Vienna ab Initio Simulation Package (VASP), the performance of nano-wire field-effect transistors is investigated. In this paper, the Density-Gradient Model (DG Model) is used to describe the carrier transport behavior of the nano-wire transistor under quantum effects. The analysis of the drain current with respect to channel length, body dielectric constant and gate contact work function is presented. In addition, Fermi energy and DOS (Density of State) are introduced to explore the relative stability of carrier transport and electrical conductance for the silicon crystal with dopants. Finally, how the roughness of the surface of the silicon-based crystal is affected by dopants and their allocation can be illuminated by a few broken bonds between atoms near the skin of the crystal.     

Influence of high energy electron irradiation on the characteristics of polysilicon thin film transistors

The influence of high energy electron (23 MeV) irradiation on the electrical characteristics of p-channel polysilicon thin film transistors (PSTFTs) was studied. The channel 220 nm thick LPCVD (low pressure chemical vapor deposition) deposited polysilicon layer was phosphorus doped by ion implantation. A 45 nm thick, thermally grown, SiO2 layer served as gate dielectric. A self-alignment technology for boron doping of the source and drain regions was used. 200 nm thick polysilicon film was deposited as a gate electrode. The obtained p-channel PSTFTs were irradiated with different high energy electron doses. Leakage currents through the gate oxide and transfer characteristics of the transistors were measured. A software model describing the field enhancement and the non-uniform current distribution at textured polysilicon/oxide interface was developed. In order to assess the irradiation-stimulated changes of gate oxide parameters the gate oxide tunneling conduction and transistor characteristics were studied. At MeV dose of 6×10¹³ el/cm², a negligible degradation of the transistor properties was found. A significant deterioration of the electrical properties of PSTFTs at MeV irradiation dose of 3×10¹⁴ el/cm² was observed.     

Simulation study on effect of drain underlap in gate-all-around tunneling field-effect transistors

In this work, the effects of underlapping drain junction on the performances of gate-all-around (GAA) tunneling field-effect transistors (TFETs) have been studied in terms of direct-current (DC) characteristics including on-current (I_on), off-current (I_off), subthreshold swing (S), and I_on/I_off ratio. In addition, the dependences of intrinsic delay time (τ) and radio-frequency (RF) performances including cut-off frequency (f_T) and maximum oscillation frequency (f_max) on gate–drain capacitance (C_gd) with the underlapping were investigated as the gate length (L_gate) is scaled. A GAA TFET with asymmetric junctions, with an underlap at the drain side, demonstrated DC and RF performances superior to those of a device with symmetric junctions.     

A study on the performance of metal-oxide-semiconductor-field-effect-transistors with asymmetric junction doping structure

Diode currents of MOSFET were studied and characterized in detail for the ion implanted pn junction of short channel MOSFETs with shallow drain junction doping structure. The diode current in MOSFET junctions was analyzed on the point of view of the gate-induced-drain leakage (GIDL) current. We could found the GIDL current is generated by the band-to-band tunneling (BTBT) of electrons through the reverse biased channel-to-drain junction and had good agreement with BTBT equation. The effect of the lateral electric field on the GIDL current according to the body bias voltage is characterized and discussed. We measured the electrical doping profiling of MOSFETs with a short gate length, ultra thin oxide thickness and asymmetric doped drain structure and checked the profile had good agreement with simulation result. An accurate effective mobility of an asymmetric source–drain junction transistor was successfully extracted by using the split C–V technique.     

Design and analysis of Si-based arch-shaped gate-all-around (GAA) tunneling field-effect transistor (TFET)

In this work, a Si-based arch-shaped gate-all-around (GAA) tunneling field-effect transistor (TFET) has been designed and analyzed. Various studies on III–V compound semiconductor materials for applications in TFET devices have been made and we adopt one of them to perform a physical design for boosting the tunneling probability. The GAA structure has a partially open region for extending the tunneling area and the channel is under the GAA region, which makes it an arch-shaped GAA structure. We have performed the design optimization with variables of epitaxy channel thickness (t_epi) and height of source region (H_source) in the Si-based TFET. The designed arch-shaped GAA TFET based on Si platform demonstrates excellent performances for low-power (LP) applications including on-state current (I_on) of 694 μA/μm, subthreshold swing (S) of 7.8 mV/dec, threshold voltage (V_t) of 0.1 V, current gain cut-off frequency (f_T) of 12 GHz, and maximum oscillation frequency (f_max) of 283 GHz.