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J Appenzeller

Publications and source records attributed to J Appenzeller.

17 recordsLinked to original sources
PubMed2004

Band-to-band tunneling in carbon nanotube field-effect transistors.

A detailed study on the mechanism of band-to-band tunneling in carbon nanotube field-effect transistors (CNFETs) is presented. Through a dual-gated CNFET structure tunneling currents from the valence into the conduction band and vice versa can be enabled or disabled by changing the gate potential. Different from a conventional device where the Fermi distribution ultimately limits the gate voltage range for switching the device on or off, current flow is controlled here by the valence and conduction band edges in a bandpass-filter-like arrangement. We discuss how the structure of the nanotube is the key enabler of this particular one-dimensional tunneling effect.

Journal Article↗
PubMed2004

Multimode transport in Schottky-barrier carbon-nanotube field-effect transistors.

We present a detailed study on the impact of multimode transport in carbon nanotube field-effect transistors. Under certain field conditions electrical characteristics of tube devices are a result of the contributions of more than one one-dimensional subband. Through potassium doping of the nanotube the impact of the different bands is made visible. We discuss the importance of scattering for a stepwise change of current as a function of gate voltage and explain the implications of our observations for the performance of nanotube transistors.

Journal Article↗
PubMed2004

Tunneling versus thermionic emission in one-dimensional semiconductors.

This Letter focuses on the role of contacts and the influence of Schottky barriers on the switching in nanotransistors. Specifically, we discuss (i) the mechanism for injection from a three-dimensional metal into a low-dimensional semiconductor, i.e., the competition between thermionic emission and thermally assisted tunneling, (ii) the factors that affect tunneling probability with emphasis on the importance of the effective mass for transistor applications, and (iii) a novel approach that enables determination of barrier presence and its actual height.

Journal Article↗
PubMed2003

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.

Journal Article↗
PubMed2002

Field-modulated carrier transport in carbon nanotube transistors.

We have investigated the electrical transport properties of carbon nanotube field-effect transistors as a function of channel length, gate dielectric film thickness, and dielectric material. Our experiments show that the bulk properties of the semiconducting carbon nanotubes do not limit the current flow through the metal/nanotube/metal system. Instead, our results can be understood in the framework of gate and source-drain field induced modulation of the nanotube band structure at the source contact. The existence of one-dimensional Schottky barriers at the metal/nanotube interface determines the device performance and results in an unexpected scaling behavior.

Journal Article↗
PubMed2002

Carbon nanotubes as schottky barrier transistors.

We show that carbon nanotube transistors operate as unconventional "Schottky barrier transistors," in which transistor action occurs primarily by varying the contact resistance rather than the channel conductance. Transistor characteristics are calculated for both idealized and realistic geometries, and scaling behavior is demonstrated. Our results explain a variety of experimental observations, including the quite different effects of doping and adsorbed gases. The electrode geometry is shown to be crucial for good device performance.

Journal Article↗
PubMed2001

Ambipolar electrical transport in semiconducting single-wall carbon nanotubes.

Ambipolar electrical transport is reported in single-wall carbon nanotube (SWNT) field-effect transistors. In particular, the properties of SWNT junctions to TiC are discussed in detail. The carbide-nanotube junctions are abrupt and robust. In contrast to planar junctions, these contacts present low resistance for the injection of both p- and n-type carriers--the apparent barrier height of the junction is modified by the gate field. Thus SWNTs offer the novel possibility of ambipolar Ohmic contacts.

Journal Article↗
PubMed2000

Intertube coupling in ropes of single-wall carbon nanotubes

We investigate the coupling between individual tubes in a rope of single-wall carbon nanotubes using four probe resistance measurements. By introducing defects through the controlled sputtering of the rope we generate a strong nonmonotonic temperature dependence of the four terminal resistance. This behavior reflects the interplay between localization in the intentionally damaged tubes and coupling to undamaged tubes in the same rope. Using a simple model we obtain the coherence length and the coupling resistance. The coupling mechanism is argued to involve direct tunneling between tubes.

Journal Article↗
PubMed1996

Kikuchi's histiocytic necrotizing lymphadenitis associated with ruptured silicone breast implant.

OBJECTIVE: In this report we explore the relationship between Kikuchi's necrotizing lymphadenitis (Kikuchi-Fujimoto disease, KD) and a leaky silicone breast implant. PATIENT: The simultaneous occurrence of KD and silicone lymphadenopathy in an axillary lymph node of a patient with a leaking silicone breast implant is reported. Since both KD and silicone implants have been implicated in autoimmune diseases, including systemic lupus erythematosus, serologic tests for antinuclear antibodies and rheumatoid factor were performed. RESULTS: Axillary lymph nodes showed both silicone lymphadenopathy, as well as classic morphologic and immunophenotypic features of KD. Screening tests for systemic autoimmune disorders were within normal range, suggesting that the unusual Kikuchi's-like immune reaction in one axillary lymph node was localized. The patient has no evidence of progressive immunologic disorders 3 years later. Subsequent lymph node biopsies showed silicone adenopathy with no evidence of KD. CONCLUSIONS: Our findings indicate that silicone compounds may be associated with transient abnormal immune reactions and lend further support to the hypothesis that KD represents an exuberant T-cell-mediated immune response to a variety of nonspecific stimuli.

Adult↗