A single-atom transistor

The ability to control matter at the atomic scale and build devices with atomic precision is central to nanotechnology. The scanning tunnelling microscope can manipulate individual atoms and molecules on surfaces, but the manipulation of silicon to make atomic-scale logic circuits has been hampered by the covalent nature of its bonds. Resist-based strategies have allowed the formation of atomic-scale structures on silicon surfaces, but the fabrication of working devices-such as transistors with extremely short gate lengths, spin-based quantum computers and solitary dopant optoelectronic devices-requires the ability to position individual atoms in a silicon crystal with atomic precision. Here, we use a combination of scanning tunnelling microscopy and hydrogen-resist lithography to demonstrate a single-atom transistor in which an individual phosphorus dopant atom has been deterministically placed within an epitaxial silicon device architecture with a spatial accuracy of one lattice site. The transistor operates at liquid helium temperatures, and millikelvin electron transport measurements confirm the presence of discrete quantum levels in the energy spectrum of the phosphorus atom. We find a charging energy that is close to the bulk value, previously only observed by optical spectroscopy.     

Multi-basin avalanche simulation: A model

A process oriented avalanche prediction model for wide-area use was developed using five winters' data from Berthoud Pass, Colorado. The model was successfully tested on independent data from the Colorado Front Range and San Juan mountains. This research model is driven by temperature, precipitation, wind, and radiation; it simulates snow transport and deposition in starting zones and the development of a layered snow cover in forest sheltered clearings. Probabilities of avalanche occurrence are estimated according to recent loading and simulated regional snowpack stratigraphy.     

A heteroepitaxial perovskite metal-base transistor

'More than Moore' captures a concept for overcoming limitations in silicon electronics by incorporating new functionalities in the constituent materials. Perovskite oxides are candidates because of their vast array of physical properties in a common structure. They also enable new electronic devices based on strongly-correlated electrons. The field effect transistor and its derivatives have been the principal oxide devices investigated thus far, but another option is available in a different geometry: if the current is perpendicular to the interface, the strong internal electric fields generated at back-to-back heterojunctions can be used for oxide electronics, analogous to bipolar transistors. Here we demonstrate a perovskite heteroepitaxial metal-base transistor operating at room temperature, enabled by interface dipole engineering. Analysis of many devices quantifies the evolution from hot-electron to permeable-base behaviour. This device provides a platform for incorporating the exotic ground states of perovskite oxides, as well as novel electronic phases at their interfaces.     

A 30-nm-gate field-effect transistor

The fabrication technology is developed for and characteristics are investigated of a GaAs Schottky-barrier field-effect transistor (SBFET) with an effective gate length of 30 nm. The SBFET power gain cutoff frequency is 150 GHz. The noise factor at 12–37 GHz is comparable with that of two-dimensional electron gas transistors. The theoretical electron transit time under the gate is below 0.1 ps.     

A dielectric-modulated field-effect transistor for biosensing

Interest in biosensors based on field-effect transistors (FETs), where an electrically operated gate controls the flow of charge through a semiconducting channel, is driven by the prospect of integrating biodetection capabilities into existing semiconductor technology. In a number of proposed FET biosensors, surface interactions with biomolecules in solution affect the operation of the gate or the channel. However, these devices often have limited sensitivity. We show here that a FET biosensor with a vertical gap is sensitive to the specific binding of streptavidin to biotin. The binding of the streptavidin changes the dielectric constant (and capacitance) of the gate, resulting in a large shift in the threshold voltage for operating the FET. The vertical gap is fabricated using simple thin-film deposition and wet-etching techniques. This may be an advantage over planar nanogap FETs, which require lithographic processing. We believe that the dielectric-modulated FET (DMFET) provides a useful approach towards biomolecular detection that could be extended to a number of other systems.     

A two-fluid model for avalanche and debris flows

Geophysical mass flows--debris flows, avalanches, landslides--can contain O(10(6)-10(10)) m(3) or more of material, often a mixture of soil and rocks with a significant quantity of interstitial fluid. These flows can be tens of meters in depth and hundreds of meters in length. The range of scales and the rheology of this mixture presents significant modelling and computational challenges. This paper describes a depth-averaged 'thin layer' model of geophysical mass flows containing a mixture of solid material and fluid. The model is derived from a 'two-phase' or 'two-fluid' system of equations commonly used in engineering research. Phenomenological modelling and depth averaging combine to yield a tractable set of equations, a hyperbolic system that describes the motion of the two constituent phases. If the fluid inertia is small, a reduced model system that is easier to solve may be derived.     

A hierarchy of avalanche models on arbitrary topography

We use the non-Cartesian, topography-based equations of mass and momentum balance for gravity driven frictional flows of Luca et al. (Math. Mod. Meth. Appl. Sci. 19, 127–171 (2009)) to motivate a study on various approximations of avalanche models for single-phase granular materials. By introducing scaling approximations we develop a hierarchy of model equations which differ by degrees in shallowness, basal curvature, peculiarity of constitutive formulation (non-Newtonian viscous fluids, Savage–Hutter model) and velocity profile parametrization. An interesting result is that differences due to the constitutive behaviour are largely eliminated by scaling approximations. Emphasis is on avalanche flows; however, most equations presented here can be used in the dynamics of other thin films on arbitrary surfaces.     

A nanoscale memory and transistor using backside trapping

We report results on a new structure that provides a scalable memory cell and a scalable transistor simultaneously in the same structure. The operational distinction is achieved through a difference in the bias range. The device employs a modified silicon-on-insulator substrate where charge is stored in a defected region underneath a thin single-crystal silicon layer employed for the formation of the transistor channel. At low voltages (below 1.5 V), the device operates as a transistor making use of the front silicon interface (preferred form), or the back interface, or both. The memory operation is obtained by use of high voltages, which allow injection of charge into the defected region in a stack of insulating films underneath the thin silicon channel, as well as the removal of the charge. The transistors are scalable because of the thin silicon technology and the memories are highly scalable because they allow efficient coupling between the carriers and storage region. The structure provides for a very useful decoupling of the memory read and transistor operation from the memory electrical storage operation. The experimental operation of the devices is described.     

A microchannel avalanche photodiode with a fast recovery time of parameters

The design and operation principle of a novel microchannel avalanche photodiode with the short recovery time of parameters are considered in this Letter. A distinctive feature of the new device is that at the operating potential on it, the n ⁺ regions (pixels) deeply submerged into the epitaxial layer with p-type conductivity are completely depleted; thus, the possibility of accumulation of multiplied charge carriers in them is carried out. This enables attaining the recovery time of pixels in the device of ~50 ns at photocurrent amplification factor equal to 250.     

Photon avalanche fluorescence and lasers

This paper presents an analysis of the three-level model for avalanche upconversion in the steady state case. The population of the upper level is studied with respect to the ratio between nonresonant and resonant absorptions. We deduce from this variation a limit for avalanche behaviour nearly independent of the actual compound considered. This limit corresponds to a very weak absorption from the ground state which can be estimated for multiphonon assisted absorption. Several examples from the literature are discussed on this basis and we show that a large mismatch between pump photon and electronic transition energies is necessary to observe avalanche upconversion. Application of the photon avalanche to upconversion pumped lasers is also investigated in terms of oscillation threshold and stimulated emission. Avalanche pumping suffers from two drawbacks. First, it results in a lower upconverted emission intensity than that obtained by resonant pumping, especially near avalanche threshold. In the second place, the output power of these lasers is seriously limited because of the weak absorption from the ground state.     

The other transistor: early history of the metal-oxidesemiconductor field-effect transistor

The silicon metal-oxide-semiconductor field-effect transistor (MOSFET or MOS transistor) did not become significant commercially until two decades after the 1948 announcement of the invention of the transistor by Bell Laboratories. The underlying concept of the MOSFET-modulation of conductivity in a semiconductor triode structure by a transverse electric field-first appeared in a 1928 patent application. It was confirmed experimentally in 1948. However early devices were not practical due to surface problems. Although these were solved at Bell Laboratories in 1958, Bell remained committed to earlier transistor technology. Development of the `other transistor' was first pursued elsewhere. It was finally the needs of computers and the opportunities created by integrated circuits that made the silicon MOSFET the basic element of late 20th-century digital electronics     

Particle simulation of snow avalanche motion

The continuum model of a snow avalanche is abandoned, and instead an avalanche is modeled as a collection of $\text{～10}{}^3$ particles that move randomly and independently subject to gravity and resistive forces which have a random fluctuation computed by Monte-Carlo simulation. The model includes entrainment at the avalanche front and the possibility of varying resistive parameters with speed and slope position. Particle statistics computed for an avalanche event in Norway, April 1982, provide a reasonable simulation of recorded speeds and debris distribution.     

A spiking neuron circuit based on a carbon nanotube transistor

A spiking neuron circuit based on a carbon nanotube (CNT) transistor is presented in this paper. The spiking neuron circuit has a crossbar architecture in which the transistor gates are connected to its row electrodes and the transistor sources are connected to its column electrodes. An electrochemical cell is incorporated in the gate of the transistor by sandwiching a hydrogen-doped poly(ethylene glycol)methyl ether (PEG) electrolyte between the CNT channel and the top gate electrode. An input spike applied to the gate triggers a dynamic drift of the hydrogen ions in the PEG electrolyte, resulting in a post-synaptic current (PSC) through the CNT channel. Spikes input into the rows trigger PSCs through multiple CNT transistors, and PSCs cumulate in the columns and integrate into a 'soma' circuit to trigger output spikes based on an integrate-and-fire mechanism. The spiking neuron circuit can potentially emulate biological neuron networks and their intelligent functions.     

A single electron transistor based on Si/SiGe wires

We report on the electrical transport of wire-based devices fabricated on modulation-doped Si/SiGe two-dimensional electron gas. Two different geometries were investigated: straight and bended wires. In both cases, we found typical features of Coulomb blockade regime. Nevertheless, single electron transistor effects were found only on wires with bends. Reported data suggest that the insertion of bends is a suitable tool for obtaining single electron transistor behavior in Si/SiGe wires.     

A thin film metal base transistor structure with amorphous silicon

We describe a metal base transistor structure using amorphous silicon prepared from a silane glow discharge. We give some of the operating characteristics and evidence for true injection. The highest measured injection ratio was 8%. We discuss the reasons for the low injection ratios and suggest some improvements.