Schottky-barrier Transistors Research Articles

Analysis of Sensing Mechanisms in a Gold-Decorated SWNT Network DNA Biosensor

We show that carbon nanotube sensors with gold particles on the single-walled carbon nanotube (SWNT) network operate as Schottky barrier transistors, in which transistor action occurs primarily by varying the resistance of Au-SWNT junction rather than the channel conductance modulation. Transistor characteristics are calculated for the statistically simplified geometries, and the sensing mechanisms are analyzed by comparing the simulation results of the MOSFET model and Schottky junction model with the experimental data. We demonstrated that the semiconductor MOSFET effect cannot explain the experimental phenomena such as the very low limit of detection (LOD) and the logarithmic dependence of sensitivity to the DNA concentration. By building an asymmetric oncentricelectrode model which consists of serially-connected segments of CNTFETs and Schottky diodes, we found that for a proper explanation of the experimental data, the work function shifts should be ~ 0.1 eV for 100 pM DNA concentration and ~ 0.4 eV for 100 μM.

Vacuum filtration based formation of liquid crystal films of semiconducting carbon nanotubes and high performance transistor devices

In this paper, we report ultra-thin liquid crystal films of semiconducting carbon nanotubes using a simple vacuum filtration process. Vacuum filtration of nanotubes in aqueous surfactant solution formed nematic domains on the filter membrane surface and exhibited local ordering. A 2D fast Fourier transform was used to calculate the order parameters from scanning electron microscopy images. The order parameter was observed to be sensitive to the filtration time demonstrating different regions of transformation namely nucleation of nematic domains, nanotube accumulation and large domain growth.Transmittance versus sheet resistance measurements of such films resulted in optical to dc conductivity of σopt/σdc = 9.01 indicative of purely semiconducting nanotube liquid crystal network.Thin films of nanotube liquid crystals with order parameters ranging from S = 0.1–0.5 were patterned into conducting channels of transistor devices which showed high Ion/Ioff ratios from 10–19 800 and electron mobility values μe = 0.3–78.8 cm2 (V-s)−1, hole mobility values μh = 0.4–287 cm2 (V-s)−1. High Ion/Ioff ratios were observed at low order parameters and film mass. A Schottky barrier transistor model is consistent with the observed transistor characteristics. Electron and hole mobilities were seen to increase with order parameters and carbon nanotube mass fractions. A fundamental tradeoff between decreasing on/off ratio and increasing mobility with increasing nanotube film mass and order parameter is therefore concluded. Increase in order parameters of nanotubes liquid crystals improved the electronic transport properties as witnessed by the increase in σdc/σopt values on macroscopic films and high mobilities in microscopic transistors. Liquid crystal networks of semiconducting nanotubes as demonstrated here are simple to fabricate, transparent, scalable and could find wide ranging device applications.

Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: An Insight into Current Flow across Schottky Barriers

In this article, we study the properties of metal contacts to single-layer molybdenum disulfide (MoS2) crystals, revealing the nature of switching mechanism in MoS2 transistors. On investigating transistor behavior as contact length changes, we find that the contact resistivity for metal/MoS2 junctions is defined by contact area instead of contact width. The minimum gate dependent transfer length is ∼0.63 μm in the on-state for metal (Ti) contacted single-layer MoS2. These results reveal that MoS2 transistors are Schottky barrier transistors, where the on/off states are switched by the tuning of the Schottky barriers at contacts. The effective barrier heights for source and drain barriers are primarily controlled by gate and drain biases, respectively. We discuss the drain induced barrier narrowing effect for short channel devices, which may reduce the influence of large contact resistance for MoS2 Schottky barrier transistors at the channel length scaling limit.

Schottky barrier height reduction for metal/n-InP by inserting ultra-thin atomic layer deposited high-k dielectrics

Fermi level pinning at metal/n-InP interface and effective Schottky barrier height (ФB,eff) were optimized by inserting ultrathin dielectrics in this work. Comparing the inserted monolayer and bilayer high-k dielectrics, we demonstrated that the introduction of bilayer dielectrics can further reduce ФB,eff (from 0.49 eV to 0.22 eV) than the monolayer dielectric (from 0.49 eV to 0.32 eV) even though the overall dielectric thickness was thicker. The additional dipole formed at high-k/high-k interfaces could be used to expound the mechanism. This work proposed an effective solution to reduce resistance contacts for InP based transistors and Schottky barrier transistors.

Formation of contacts between doped carbon nanotubes and aluminum electrodes

A theoretical study of the a semiconducting carbon nanotube (CNT) bonding to an aluminum electrode is presented using density functional theory to determine the electronic structure, and charge transport across the junction is studied using non-equilibrium Green's functions. The properties of CNT-metal junctions are of interest for optimizing metal-semiconductor junctions for Schottky barrier transistors and for the formation of Ohmic contacts for nanoelectronics. We first consider the properties of an undoped (16,0) CNT bonded to an aluminum electrode, including an analysis of metal induced gap states and examination of the surface dipole. The junction is then modified by introduction of substitutional dopants into the CNT using nitrogen and boron to form n- and p-type semiconductors, respectively, and the resulting impact of the doping on current transport across the junctions is calculated. As an alternative doping strategy, tetrathiafulvalene is introduced endohedrally and found to act as an n-type do...

Characteristics of n-Type Asymmetric Schottky-Barrier Transistors with Silicided Schottky-Barrier Source and Heavily n-Type Doped Channel and Drain

Characteristics of n-Type Asymmetric Schottky-Barrier Transistors with Silicided Schottky-Barrier Source and Heavily n-Type Doped Channel and Drain

Multifunctional Devices and Logic Gates With Undoped Silicon Nanowires

We report on the electronic transport properties of multiple-gate devices fabricated from undoped silicon nanowires. Understanding and control of the relevant transport mechanisms was achieved by means of local electrostatic gating and temperature-dependent measurements. The roles of the source/drain contacts and of the silicon channel could be independently evaluated and tuned. Wrap gates surrounding the silicide-silicon contact interfaces were proved to be effective in inducing a full suppression of the contact Schottky barriers, thereby enabling carrier injection down to liquid helium temperature. By independently tuning the effective Schottky barrier heights, a variety of reconfigurable device functionalities could be obtained. In particular, the same nanowire device could be configured to work as a Schottky barrier transistor, a Schottky diode, or a p-n diode with tunable polarities. This versatility was eventually exploited to realize a NAND logic gate with gain well above one.

Characteristics of n-Type Asymmetric Schottky-Barrier Transistors with Silicided Schottky-Barrier Source and Heavily n-Type Doped Channel and Drain

In this study, we explore the operation of operation a novel asymmetric Schottky-barrier transistor (ASSBT) through using technology computer aided design (TCAD). The new ASSBT features a silicided Schottky-barrier (SB) source, with the channel and drain made of heavily n-doped silicon. By eliminating the SB drain junction contained in conventional symmetrical-type SB metal–oxide–semiconductor field-effect transistors (MOSFETs), a larger on-state current is achievable. Moreover, combined with the adoption of fully depleted thin-film channel, the off-state leakage current can be efficiently suppressed as well. In addition, we also comprehensively analyze the transport mechanisms dominating in different operational regions of this new ASSBT. A pseudo-subthreshold region that shows worse subthreshold swing (SS) than the subthreshold region is identified. A decrease in channel and/or gate oxide thicknesses can contribute to the improvement of the SS of this region. A modified form of scaling length (λ) is also introduced to describe the impacts of structural parameters and gate configurations on the SS characteristics of this new ASSBT.

Computational study of dopant segregated nanoscale Schottky barrier MOSFETs for steep slope, low SD-resistance and high on-current gate-modulated resonant tunneling FETs

We study here, using non-equilibrium Green’s function quantum simulations, the impact of dopant segregation (DS) on Schottky barrier (SB) nanoscale transistors for the implementation of ultimate CMOS with low series resistance and steep slope. Owing to their adequate multi-barrier structure, DS–SB transistor can present a gate modulated barrier resonant tunneling (MBRT) effect that allows them to have improved ION/IOFF ratio, even breach the kT/q subthreshold slope limit of classical MOSFET and therefore pave a way towards steep slope, low S/D resistance electronics. However, as shown here, these improved characteristics rely in a quite narrow window of dopant segregation parameters. Also sub-kT/q slopes are very sensitive to scattering. However, on the contrary to a standard MOSFET, electron–phonon scattering should improve on-current and ION/IOFF performances of SB-FETs and provide them with on-current levels comparable to standard MOSFETs.

Performance Investigation on p-Type Si-, Ge-, and Ge–Si Core–Shell Nanowire Schottky Barrier Transistors

In this paper, we investigate the working mechanism of p-channel Schottky barrier Ge–Si core–shell nanowire transistors and then study the impact factors on the device performance of Schottky barrier transistors using Si-, Ge-, and Ge–Si core–shell nanowires as channels. For Ge–Si core–shell channel devices, most holes tunnel at the source near the heterojuction and transport in the Ge core region. Ge channel devices can provide the largest drive current and core–shell devices have the smallest sub-threshold slope among above three types of transistors. It is also found that core–shell device's conductive currents vary a little when fixing Ge core radius and changing Si shell thickness, and core–shell devices' normalized drain current can be greatly enhanced by reducing nanowires' radius or increasing core radius. Moreover, the drivability of core–shell devices is insensitive to both silicide/channel and germanide/channel barrier heights, which will further relax the requirement for contact materials.

Contact mechanisms and design principles for (Schottky and Ohmic) metal contacts to semiconductor nanowires

Contact mechanisms and design principles for (Ohmic and Schottky) metal (M) contacts to semiconductor nanowires (NWs) have been studied. The NWs have been assumed to be cylindrical. A unified model has been developed for the contacts. The model takes into consideration the amorphicity of the M/NW interface structure, the diameter dependence of the energy band gap, the barrier height modulation, and the fluctuations in both the barrier height and the applied bias. While the fluctuations in the barrier height are assumed to involve band tails, the fluctuations in applied bias are assumed to involve tiny Gaussian peaks. Several different features of the Ohmic and the Schottky contacts have been addressed. These include temperature and dimension dependencies of the current-voltage characteristics, the influence of the M/NW interface on the contact characteristics, the relationship between the barrier height and the ideality factor, and the barrier height reduction as a function of temperature. The model appears to be very general. It seems to explain all experimental results available to date in the literature. The calculated results are almost always in good correspondence with the experimental results. The study seemingly demonstrates an alternative to the doping dependence of the Ohmic contacts. It elucidates the fundamental physics underlying M/NW contacts. It highlights means to yield low-resistivity Ohmic contacts by thermionic emission. It describes design criteria for both Ohmic and Schottky contacts. The design criteria for Ohmic contacts tend to address the long-term reliability concerns for devices. They explain also the behavior of both good and bad Ohmic contacts. All these may be the most striking attributes of the study. These attributes explain why Schottky contacts to NWs, with proper gate modulation, may act also as Schottky barrier transistors.

Unipolar p-type single-walled carbon nanotube field-effect transistors using TTF–TCNQ asthe contact material

We demonstrate herein that organic metal tetrathiafulvalene–tetracyanoquinodimethane(TTF–TCNQ) can serve as an ideal material for source and drain electrodesto build unipolar p-type single-walled carbon nanotube (SWNTs) field-effecttransistors (FETs). SWNTs were synthesized by the chemical vapor deposition(CVD) method on silicon wafer and then TTF–TCNQ was deposited by thermalevaporation through a shadow mask to form the source and drain contacts. AnSiO2 layer served as the gate dielectric and Si was used as the backgate. Transfer characteristicsshow that these TTF–TCNQ contacted devices are Schottky barrier transistors just likeconventional metal contacted SWNT-FETs. The most interesting characteristic of theseSWNT transistors is that all devices demonstrate the unipolar p-type transport behavior.This behavior originates from the unique crystal structure and physical properties ofTTF–TCNQ and this device may have potential applications in carbon nanotubeelectronics.

Dopant-Segregated Schottky Silicon-Nanowire MOSFETs With Gate-All-Around Channels

In this letter, we demonstrated dopant-segregated Schottky (DSS) p-MOSFET with gate-all-around silicon-nanowire (SiNW) channel of 10 nm in diameter. The DSS transistor shows improved performance as compared to a reference Schottky barrier (SB) transistor without dopant segregation. The DSS transistor shows I ON of 319 muA/mum at a low gate overdrive of -0.6 V, high I ON/I OFF ratio (~105), and short-channel performance with subthreshold slope ~90 mV/dec down to 100-nm gate length with relatively thick (6 nm) deposited gate oxide. The DSS transistor also shows significant reduction (~40times lower) in the series resistance as compared to the SB transistor. The origin of the improved performance of the DSS is the thin dopant layer segregated at the nickel monosilicide/SiNW point contact which results in the enhanced hole injection at the source side and the suppressed electron injection at the drain side.

Improved carrier injection in gate-all-around Schottky barrier silicon nanowire field-effect transistors

This letter presents the performance improvement of Schottky barrier metal-oxide-semiconductor field-effect transistor by employing gate-all-around (GAA) Si-nanowire (SiNW) structure. Without employing any barrier lowering technique, the mid-band-gap Ni-silicide Schottky barrier transistors demonstrated excellent performance and achieved subthreshold slope of ∼86 mV/decade and on-current of 19 μA/μm on a 12.5 nm SiNW, and subthreshold slope of ∼79 mV/decade and on-current of 207 μA/μm on a 4 nm diameter SiNW. Assisted with simulation, we show that this improvement can be attributed to the strong reduction in the Schottky barrier thickness as a result of the better gate control of GAA SiNW structure.

Performance Comparison of Graphene Nanoribbon FETs With Schottky Contacts and Doped Reservoirs

We present an atomistic 3D simulation study of the performance of graphene nanoribbon (GNR) Schottky barrier (SB) FETs and transistors with doped reservoirs (MOSFETs) by means of the self-consistent solution of the Poisson and Schrodinger equations within the non-equilibrium Green's function (NEGF) formalism. Ideal MOSFETs show slightly better electrical performance, for both digital and THz applications. The impact of non-idealities on device performance has been investigated, taking into account the presence of single vacancy, edge roughness and ionized impurities along the channel. In general, MOSFETs show more robust characteristics than SBFETs. Edge roughness and single vacancy defect largely affect performance of both device types.

Single nanoparticle semiconductor devices

Using a new technique in forming the cubic single-crystal silicon nanoparticles that are about 40 nm on a side, the authors have demonstrated a vertical-flow surround-gate Schottky-barrier transistor. This approach allows the use of well-known approaches to surface passivation and contact formation within the context of deposited single-crystal materials for device applications. It opens the door to the novel three-dimensional integrated circuits and new approaches to hyper integration. The fabrication process involves successive deposition and planarization and does not require nonoptical lithography. Device characteristics show reasonable turn-off characteristics and on-current densities of more than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>

Theoretical investigation on photoconductivity of single intrinsic carbon nanotubes

The photoconductivity of carbon nanotube (CNT) Schottky barrier transistors is studied by solving the nonequilibrium Green’s function transport equation. The model provides a detailed and coherent picture of electron-photon coupling and quantum transport effects. The photocurrent shows peaks at photon energies near the subband gaps, which can be engineered by controlling the CNT diameter. Electron-phonon coupling (i) slightly broadens the peaks, (ii) leads to phonon-assisted photocurrent at certain energy ranges, and (iii) changes the energy-resolved photocurrent. We also show that the metal-CNT barrier height has a much smaller effect on the photocurrent than on the dark current.

Resistance Evaluation and Growth of Carbon Nanotubes

We have investigated electronic transport in single-wall carbon nanotubes attached to multiple electrodes. Resistance measurement using a pair of electrodes with different gaps enabled separate evaluation of nanotube resistivity and contact resistance. We found that the resistivity depends on nanotube diameter. Electrodes with gold or palladium exhibit similar contact resistance with an order of 10kΩ. Contact resistance is insensitive to back gate voltage, contrary to the Schottky-barrier transistor model. We also demonstrated the top-down control of diameter and position of Fe catalysts by means of “lithographically anchored nanoparticle synthesis (LANS)” for nanotube growth. Chemical vapor deposition by using these patterned particles successfully produced single-wall carbon nanotubes.

Effective Schottky barrier lowering in silicon-on-insulator Schottky-barrier metal-oxide-semiconductor field-effect transistors using dopant segregation

We present an investigation of the use of dopant segregation in Schottky-barrier metal-oxide-semiconductor field-effect transistors on silicon-on-insulator. Experimental results on devices with fully nickel silicided source and drain contacts show that arsenic segregation during silicidation leads to strongly improved device characteristics due to a strong conduction/valence band bending at the contact interface induced by a very thin, highly doped silicon layer formed during the silicidation. With simulations, we study the effect of varying silicon-on-insulator and gate oxide thicknesses on the performance of Schottky-barrier devices with dopant segregation. It is shown that due to the improved electrostatic gate control, a combination of both ultrathin silicon bodies and gate oxides with dopant segregation yields even further improved device characteristics greatly relaxing the need for low Schottky barrier materials in order to realize high-performance Schottky-barrier transistors.

Carbon nanotube transistor optimization by chemical control of the nanotube–metal interface

Most carbon nanotube transistors work as Schottky barrier transistors. We show that chemical treatment of operational p-type nanotube transistors by trifluoro-acetic acid (TFA) leads to the drastic improvement of all the key device parameters. This effect is due to the highly polar nature of the TFA molecule which, once adsorbed at the metal–nanotube interface, lowers the Schottky barrier for the holes and thus favors their injection.