R. Akis

Quantum effects in MOSFETs: use of an effective potential in 3D Monte Carlo simulation of ultra-short channel devices

We incorporate an effective potential in a three-dimensional MOSFET simulation, in which the transport is handled by an ensemble Monte Carlo approach. We find that the threshold voltage is shifted and the carrier density is moved away from the interface, both effects given by quantization provided within the channel. However, the mean velocity of the carriers is not affected significantly by the introduction of this effective potential, and is only reduced by about 10%.

Phonon-assisted ballistic to diffusive crossover in silicon nanowire transistors

As transistors get smaller, the simulations require full quantum-mechanical treatments. Most such approaches have treated the transport as ballistic, ignoring the scattering that is known to occur in such devices. We present the results of a three-dimensional, self-consistent quantum simulation of a silicon nanowire transistor. In these simulations we have included phonon scattering through a real-space self-energy assuming weak interactions. In these silicon nanowire transistors, the ballistic to diffusive crossover occurs at much smaller distances than previously anticipated.

The Upper Limit of the Cutoff Frequency in Ultrashort Gate-Length InGaAs/InAlAs HEMTs: A New Definition of Effective Gate Length

Ultrashort gate-length pseudomorphic high-electron mobility transistors have been modeled using a full-band cellular Monte Carlo simulator. The RF response and the cutoff frequency fT have been obtained for physical gate lengths ranging from 10 to 50 nm. These results, in turn, have been used in a transit-time analysis to determine the effective gate length in each case. By interpolation, one can make an estimate of the absolute upper limit for fT, which we find to be 2.9 THz in the device studied. Importantly, the effective gate lengths are considerably shorter than the depletion lengths. Thus, in general, any estimate of fT based on the latter quantity is likely too small by a quite significant amount.

The Effective Potential in Device Modeling: The Good, the Bad and the Ugly

We discuss the use of the effective potential to incorporate quantum effects in device models. While threshold shifts and charge set-back are handled well, tunneling is not well handled by this approach, or by any other local potential approach.

Open quantum dots—probing the quantum to classical transition

Quantum dots provide a natural system in which to study both quantum and classical features of transport. As a closed testbed, they provide a natural system with a very rich set of eigenstates. When coupled to the environment through a pair of quantum point contacts, each of which passes several modes, the original quantum environment evolves into a set of decoherent and coherent states, which classically would compose a mixed phase space. The manner of this breakup is governed strongly by Zureks decoherence theory, and the remaining coherent states possess all the properties of his pointer states. These states are naturally studied via traditional magnetotransport at low temperatures. More recently, we have used scanning gate (conductance) microscopy to probe the nature of the coherent states, and have shown that families of states exist through the spectrum in a manner consistent with quantum Darwinism. In this review, we discuss the nature of the various states, how they are formed, and the signatures that appear in magnetotransport and general conductance studies.

Aspect Ratio Impact on RF and DC Performance of State-of-the-Art Short-Channel GaN and InGaAs HEMTs

We report a comparison between state-of-the-art GaN and InGaAs HEMTs in terms of the minimum aspect ratio required to limit short-channel effects. DC and RF simulations were carried out through our full-band cellular Monte Carlo simulator, which includes the full details of the band structure and the phonon spectra. Our results indicate that the minimum aspect ratio for GaN devices is 15 for negligible short-channel effects and 10 for reduced short-channel effects. On the other hand, InGaAs devices perform well for lower aspect ratio values such as 7.5 and 4-5 for negligible and reduced effects, respectively. The origin of this difference between GaN and InGaAs HEMTs is believed to be related to the different dielectric constants of the two materials and the corresponding difference in the electric field distributions related to short-channel effects.

Correspondence between quantum and classical motion: comparing Bohmian mechanics with a smoothed effective potential approach

Abstract We simulate Bohm and classical trajectories through a constricted quantum wire. Results are obtained with and without self-consistency as well as with an “effective” potential, calculated by smoothing the self-consistent potential. Including this effective potential generates classical trajectories with behavior remarkably similar to that of the actual quantum system.

Interference and interactions in open quantum dots

In this report, we review the results of our joint experimental and theoretical studies of electron-interference, and interaction, phenomena in open electron cavities known as quantum dots. The transport through these structures is shown to be heavily influenced by the remnants of their discrete density of states, elements of which remain resolved in spite of the strong coupling that exists between the cavity and its reservoirs. The experimental signatures of this density of states are discussed at length in this report, and are shown to be related to characteristic wavefunction scarring, involving a small number of classical orbits. A semiclassical analysis of this behaviour shows it to be related to the effect of dynamical tunnelling, in which electrons are injected into the dot tunnel through classically forbidden regions of phase space, to access isolated regular orbits. The dynamical tunnelling gives rise to the formation of long-lived quasi-bound states in the open dots, and the many-body implications associated with electron charging at these resonances are also explored in this report.

Simulation of Ultrasubmicrometer-Gate

Pseudomorphic delta-doped ultrasubmicrometer-gate high-electron mobility transistors have been modeled using a full-band cellular Monte Carlo simulator. Reasonable agreement between experimental and numerical results is obtained for a 70-nm gate length. We discuss the scaling of this device to shorter gate lengths and the role played by various dimensions in the structure. Devices with 20-nm gate lengths should produce fTs above 1.5 THz without difficulty. This paper demonstrates the power of particle-based simulation tools in capturing the relevant physics responsible for device operation and key to performance optimization.

\hbox{In}_{0.52} \hbox{Al}_{0.48}\hbox{As/In}_{0.75}\hbox{Ga}_{0.25}\hbox{As/In}_{0.52}\hbox{Al}_{0.48}\hbox{As/InP}

In recent years, quantum computing and information theory has received a great deal of attention as a means of drastically improving the computational speed and resources traditionally associated with current binary implementations. We present an electrically tunable semiconductor quantum waveguide implementation of an inverter gate in a GaAs/AlGaAs heterostructure in which the output of the waveguide structure may be selected via the application of an appropriate magnetic field or electrical bias. The resulting behavior observed by our implementation shows a great deal of promise for an eventual semiconductor realization of this basic qubit structure.