S. Bandyopadhyay

Supercomputing with spin-polarized single electrons in a quantum coupled architecture

We describe a novel quantum technology for possible ultra-fast, ultra-dense and ultra-low-power supercomputing. The technology utilizes single electrons as binary logic devices in which the spin of the electron encodes the bit information. Both two-dimensional cellular automata and random wired logic can be realized by laying out on a wafer specific geometric patterns of quantum dots each hosting a single electron. Various types of logic gates, combinational circuits for arithmetic logic units, and sequential circuits for memory have been designed. The technology has many advantages such as (1) the absence of physical interconnects between devices (inter-device interaction is provided by quantum mechanical spin-spin coupling between single electrons in adjacent quantum dots), (2) ultra-fast switching times of approximately 1 picosecond for individual devices, (3) extremely high bit density approaching 10 terabits cm-2, (4) non-volatile memory, (5) robustness and possible room-temperature operation with very high noise margin and reliability, (6) a very low power delay product ( approximately 10-20 J) for switching between logic levels, and (7) a very small power dissipation of a few tens of nanowatts per switching event. In spite of the above advantages, the technology also has some serious drawbacks in that the fan-out of individual logic devices may be small, wiring crossover is very problematic and the devices themselves have no inherent gain so that isolation between input and output is virtually non-existent. These are problems that plague all similar quantum technologies although they are seldom recognized as such. We will discuss these problems, and where possible, offer plausible solutions. In spite of these drawbacks, however, there are still enough attractive features of this technology to merit serious research. In this paper, we will describe how the spin-polarized single-electron logic devices work, along with the associated circuits and architecture. Finally, we will propose a new fabrication technique for realizing these chips which we believe is much more compatible with the demands of the technology than conventional nanofabrication methods.

Numerical calculation of hybrid magneto‐electric states in an electron waveguide

We have performed a numerical calculation of the energy dispersion relation of hybrid magneto‐electric states (both propagating and evanescent) in an electron waveguide subjected to a magnetic field. Our results are considerably different from those obtained through the Bohr–Sommerfield quantization condition. We have also calculated the density of the magneto‐electric states as a function of energy and the velocity versus energy relationships. Finally, we show how the wavefunctions of these states evolve with increasing magnetic field from particle in a box states to edge states. These results are useful in the analysis of numerous recent magnetotransport experiments performed in electron waveguides.

Analysis of the device performance of quantum interference transistors utilizing ultrasmall semiconductor T structures

We present a theoretical study of a recently proposed class of quantum interference transistors that utilize quantum interference effects in ultrasmall semiconductor T structures. Our analysis reveals that the attractive features of these transistors are the very low power‐delay product and multifunctionality; whereas the major drawbacks are extreme sensitivity of the device characteristics to slight structural variations, low gain, and low extrinsic switching speed in digital circuits caused by a large resistance‐capacitance (RC) time constant arising from an inherently low current‐carrying capability. The low switching speed of the transistors can however be improved dramatically by switching the device optically rather than electronically, using virtual charge polarization caused by optical excitation. This mode of switching (which is possible because of the small value of the threshold voltage) eliminates the RC time constant limitation on the switching time and results in an ultrafast optoelectronic ...

Semiconductor quantum devices

The operation of devices with small enough dimensions for electrons to exhibit wavelike behavior is explained. Two types of device are examined: vertical quantum devices, which include the resonant tunneling structure and the single-electron transistor, and lateral, which include the quantum interference transistor and the spin precession device. The advantages and drawbacks of the two types are identified.<<ETX>>

Coupling and crosstalk between high speed interconnects in ultralarge scale integrated circuits

The advent of sophisticated lithographic techniques has made it possible to fabricate densely packed ultra-large-scale-integrated (ULSI) circuits. In these chips, interconnect lines are so narrow and spaced in such close proximity that signal from one line could easily get coupled to another, causing interference and crosstalk. A general theory to model coupling between optical interconnects (waveguides) and quantum-mechanical coupling between narrow and very closely spaced silicide interconnects embedded in dielectrics (SiO/sub 2/) is presented. >

Device performance of mesoscopic T-structure quantum interference transistors

Abstract We present an analysis of the device performance of quantum interference transistors that utilize electron interference in ultrasmall semiconductor T-structures. The major drawback of these devices is found to be the extreme sensitivity of the device characteristics to structural variations that seriously impede their implementation in integrated circuits. However, the most significant advantage is the low threshold voltage for switching that makes it possible to switch these devices optically. We propose using these devices as ultrafast optoelectronic switches in which the switching action is realized optically, rather than electronically, so that the switching speed is not linited by the RC time constant of the switching circuit or that of the interconnects.

Quantum confined Lorentz effect in a quantum wire

We have studied band‐to‐band optical magnetoabsorption in a semiconductor quantum wire subjected to a transverse magnetic field. The magnetic field induces a blueshift in the absorption peaks and makes the linewidth narrower. Furthermore, it quenches photoluminescence and absorption in much the same way as an electric field in the quantum confined Franz–Keldysh effect (QCFKE). We call this the quantum confined Lorentz effect (QCLE), since it is the Lorentz force skewing the electron and hole wavefunctions in the quantum wire that causes the quenching. The QCLE has an advantage over the QCFKE in that it may be observed even in quantum wires with relatively leaky barriers. The other important difference is that while the QCFKE is accompanied by a redshift in the absorption or photoluminescence peak, the QCLE is accompanied by a blueshift.

The role of evanescent states in quantum transport through disordered mesoscopic structures

Abstract The influence of evanescent states on phase-coherent electron transport through disordered mesoscopic structures is examined. These states are found to play a critical role in transport, especially if the structures are heavily disordered and the scattering potentials are attractive, rather than repulsive. We present results from our study of the effect of evanescent states on Anderson localization and conductance fluctuations in disordered semiconductor nanostructures.

Double quantum wire Aharonov-Bohm interferometers for possible LN2 temperature operation

Abstract In this paper, we discuss the design of semiconductor electrostatic and magnetostatic Aharonov-Bohm interferometers that could operate at liquid nitrogen temperature. We find that for elevated temperature operation, one dimensional structures constructed from quantum wires are invariably the only choice, especially when transport is diffusive instead of ballistic. We have proposed such a structure which can be fabricated by present day technology. It may exhibit large conductance modulation in an electric field at 77 K and is an ideal configuration for “Quantum Interference Transistors” (QUITS) based on the electrostatic Aharonov-Bohm effect.

Nanomagnetic and Spintronic Devices for Energy-Efficient Memory and Computing: Atulasimha/Nanomagnetic

Nanomagnetic and spintronic computing devices are strong contenders for future replacements of CMOS. This is an important and rapidly evolving area with the semiconductor industry investing significantly in the study of nanomagnetic phenomena and in developing strategies to pinpoint and regulate nanomagnetic reliably with a high degree of energy efficiency. This timely book explores the recent and on-going research into nanomagnetic-based technology. Key features: Detailed background material and comprehensive descriptions of the current state-of-the-art research on each topic. Focuses on direct applications to devices that have potential to replace CMOS devices for computing applications such as memory, logic and higher order information processing. Discusses spin-based devices where the spin degree of freedom of charge carriers are exploited for device operation and ultimately information processing. Describes magnet switching methodologies to minimize energy dissipation. Comprehensive bibliographies included for each chapter enabling readers to conduct further research in this field. Written by internationally recognized experts, this book provides an overview of a rapidly burgeoning field for electronic device engineers, field-based applied physicists, material scientists and nanotechnologists. Furthermore, its clear and concise form equips readers with the basic understanding required to comprehend the present stage of development and to be able to contribute to future development. Nanomagnetic and Spintronic Devices for Energy-Efficient Memory and Computing is also an indispensable resource for students and researchers interested in computer hardware, device physics and circuits design.