Field-effect Transistor Geometry Research Articles

Formation of the Coulomb gap in a Coulomb glass

We have studied relaxation in the Coulomb glass from a nonequilibrium state arising due to an abrupt change of the electron density. Our results are of relevance to some recent experiments on doped and amorphous semiconductors using metal-oxide-semiconductor field-effect transistor geometry, in which the relaxation of conductivity has been studied under changes of the gate voltage. By solving the time-dependent equation for the density of states, we have obtained some important features of how the Coulomb gap develops under these conditions. We also present the implications of our results on the relaxation of conductivity.

Field effect conductance of conducting polymer nanofibers

AbstractWe report on the electrical conductance of nanofibers of regioregular poly(3‐hexylthiophene) (RRP3HT) as a function of gate‐induced charge. Nanofibers of RRP3HT were deposited onto SiO2/Si substrates by casting from dilute p‐xylene solutions. An analysis of the nanofibers by atomic force microscopy revealed fiber lengths of 0.2–5 μm, heights of 3–7 nm, and widths of approximately 15 nm. A field effect transistor geometry was used to probe the conductance of webs of nanofibers and single nanofibers; in these measurements, gold electrodes served as source and drain contacts, and the doped SiO2/Si substrate served as the gate. Temperature‐dependent transport studies on webs of nanofibers revealed an activation energy of 108 meV at a gate‐induced hole density of 3.8 × 1012 charges/cm2. Pretreating SiO2 with a hydrophobic hexamethyldisilazane (HMDS) layer reduced the activation energy to 65 meV at the same charge density. The turn‐on gate voltage on treated and untreated substrates increased in magnitude with decreasing temperature. Conductance measurements on single nanofibers on HMDS‐treated SiO2 yielded hole mobilities as high as 0.06 cm2/Vs with on/off current ratios greater than 103. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2674–2680, 2003

Effect of field-effect transistor geometry on charge ordering of transition-metal oxides

We examine the effect of an FET geometry on the charge ordering phase diagram of transition metal oxides using numerical simulations of a semiclassical model including long-range Coulomb fields, resulting in nanoscale pattern formation. We find that the phase diagram is unchanged for insulating layers thicker than approximately twice the magnetic correlation length. For very thin insulating layers, the onset of a charge clump phase is shifted to lower values of the strength of the magnetic dipolar interaction, and intermediate diagonal stripe and geometric phases can be suppressed. Our results indicate that, for sufficiently thick insulating layers, charge injection in an FET geometry can be used to experimentally probe the intrinsic charge ordering phases in these materials.

Alcohol Vapor Sensors Based on Single-Walled Carbon Nanotube Field Effect Transistors

We have measured conductance of single-walled semiconducting carbon nanotubes in field-effect transistor (FET) geometry and investigated the device response to alcoholic vapors. We observe significant changes in FET drain current when the device is exposed to various kinds of alcoholic vapors. These responses are reversible and reproducible over many cycles of vapor exposure. Our experiments demonstrate that carbon nanotube FETs are sensitive to a wide range of alcoholic vapors.

Resistivity of dilute 2D electrons in an undoped GaAs heterostructure.

We report resistivity measurements from 0.03 to 10 K in a dilute high mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a gated field-effect transistor geometry, a wide range of densities, 0.16 x 10(10) to 7.5 x 10(10) cm(-2), are explored. For high densities, the results are quantitatively shown to be due to scattering by acoustic phonons and impurities. In an intermediate range of densities, a peak in the resistivity is observed for temperatures below 1 K. This nonmonotonic resistivity can be understood by considering the known scattering mechanisms of phonons, bulk, and interface ionized impurities. Still lower densities appear insulating to the lowest temperature measured.

High-temperature superconductivity in lattice-expanded C60.

C60 single crystals have been intercalated with CHCl3 and CHBr3 in order to expand the lattice. High densities of electrons and holes have been induced by gate doping in a field-effect transistor geometry. At low temperatures, the material turns superconducting with a maximum transition temperature of 117 K in hole-doped C60/CHBr3. The increasing spacing between the C60 molecules follows the general trend of alkali metal-doped C60 and suggests routes to even higher transition temperatures.

A new type of terahertz frequency range instability of DC current in "thick" ballistic field-effect transistors

A new type of dc current instability in a ballistic field-effect transistor (FET) is proposed, which emerges due to the finite thickness of the 2D current-carrying channel. The physical origin of the instability in “thick” ballistic FETs is the nonlocality of the relation between the surface electron density and field potential. The instability arises at wavelengths of the order of the characteristic scale length of this nonlocality, which is determined by the FET geometry and vanishes for infinitesimal thickness of the current-carrying channel. The dispersion equation is derived and the domain of parameters under which the system becomes unstable is determined. Estimates show that this new type of instability is promising for driving a high-gain source of THz radiation.

A Novel Procedure for Evaluating Design Scalability Based on Device Performance Linked to Photolithography

We propose a novel procedure for evaluating design scalability based on printed (silicon) pattern simulation and experimental data followed by the extraction of metal oxide semiconductor field effect transistor (MOSFET) geometry and the distribution of device electrical parameters. We demonstrate the procedure on a six-transistor dual word line static random access memory (SRAM) cell, to optimize photolithography process for a technology shrink from 0.16 to 0.13 µm.

Ballistic metal-oxide-semiconductor field effect transistor

Experiments on ultra-small metal-oxide-semiconductor field effect transistors (MOSFETs) less than 100 nm have been widely reported recently. The frequency of carrier scattering events in these ultra-small devices is diminished, so that further suppression of carrier scattering may bring these devices close to the regime of ballistic transport. Carrier scattering is suppressed by constructing their channel regions with intrinsic Si and also by low temperature operation. This article proposes the ballistic transport of carriers in MOSFETs, and presents the current-voltage characteristics of the ballistic n-channel MOSFET. The current is expressed with the elementary parameters without depending on the carrier mobility. It is independent of the channel length and is proportional to the channel width. The current value saturates as the drain voltage is increased and the triode and the pentode operation are specified as in the conventional MOSFET. Similar current-voltage characteristics in the ballistic transport regime are also investigated for the p-channel MOSFET, the dual gate ultra-thin silicon on insulator MOSFET, and the high electron mobility transistor device. The obtained current gives the maximum current limitation of each field effect transistor geometry. The current control mechanism of ballistic MOSFETs is discussed. The current value is governed by the product of the carrier density near the source edge in the channel, and the velocity with which carriers are injected from the source into the channel. Influence of optical phonon emission to the transport is discussed. It is suggested that if the device is operated with relatively low carrier density at low temperatures, and if the scattering processes other than the optical phonon emission are suppressed so as to attain the ballistic transport, the optical phonon emission is also suppressed and ballistic transport is sustained. A convenient figure of merit to show the ballisticity of carrier transport in an experimental MOSFET is proposed. Its value is estimated for some examples of the recent ultra-small MOSFET experiment. The proposed current voltage characteristics are evaluated for a dual gate silicon on insulator MOSFET geometry. The result is compared with the recently reported elaborate Monte Carlo simulation with satisfactory agreement.