Keeping an electron in its place

Low-Dimensional Semiconductors
November 8, 1996

If you thought that "foresight" was a recent product of the wealth-creating climate in UK research funding, think again. In Low-Dimensional Semiconductors, Michael Kelly ably demonstrates that he was exercising scientific foresight years before the term was coined. We should not find this surprising since Kelly's career, which has spanned the gap between the pragmatic demands of industry and the more curiosity-driven environment in academia, has given him a unique insight into the research and development process.

A recurrent theme of the book is the interdependence of technology, science and devices and this is strongly reflected in the rather unorthodox structure of its 22 chapters. Kelly argues convincingly that the science of low-dimensional semiconductors owes its existence to technological breakthroughs in multilayer growth and microfabrication. Indeed, he chooses to begin his book with a description of the way in which these techniques can be used, for example, to confine electrons to small regions of space where quantum mechanical effects can dominate their behaviour.

The next section is devoted to the novel science arising from the ways in which the electronic, optical or other properties can be tailored in such structures. This begins with a discussion of the relevant lengths or timescales associated with the dimensionality of a given property and progresses to a more in-depth analysis of electronic transport in low-dimensional systems, nonequilibrium phenomena and tunnelling.

The heart of the science section deals in detail with the electronic and optical properties of a range of quantum-confined structures as well as the nonaveraging (mesoscopic) phenomena associated with very small devices. Up to this point most examples have been drawn from the GaAs/AlGaAs materials system and the section finishes with two timely chapters on band structure engineering in strained layers and silicon-based heterojunctions.

The section on commercial devices is divided into chapters which cover the spectrum from field-effect and bipolar transistors to semiconductor lasers and solar cells. This is where Kelly's breadth of experience really comes into its own, and he is able to give critical insights into the factors which determine whether a novel device structure will ever make it into the marketplace. For example, there is little point in decreasing the gate length of a field-effect transistor to improve its high-frequency performance if the switching speed is principally controlled by considerations associated with a parasitic series resistance at one of its contacts. In fact a device can find a market niche for a variety of reasons, and need not be faster or more powerful than existing products. Such factors include lower noise figures, broader bandwidths, weaker temperature dependencies, or the ability to perform complex logic functions. Kelly cites a wide range of examples of low-dimensional devices which have made it in the marketplace and it is unsettling that, despite a decade of research on quantum wires and quantum dots, they are exclusively based on semiconductor multilayer structures. This trend seems set to continue for the next decade with the additional development of metallic and ferromagnetic multilayer devices.

In the final two chapters of the book Kelly does some crystal-ball gazing up to the millennium and beyond. He outlines a number of recent developments in the areas of superconducting and organic polymer electronics as well as bioelectronics which seem certain to find niche applications if not broader market penetration.

Throughout the book Kelly presents us with a very personal view of his field and, remarkably, the most of the examples are drawn from his own work or from that of people with whom he has been closely connected. He seems to be equally at ease describing the experimental and theoretical aspects of the material, lending coherency to the subject matter. Chapters are fairly self-contained and confine themselves to a discussion of the key points while the excellent list of references provided at the end of each chapter guides the reader rapidly to more detailed treatments if they should be required.

Kelly has succeeded splendidly in his stated ambition that the book should provide a good introduction to the field for post-graduate students from backgrounds as diverse as physics, electronic engineering and materials science. To my knowledge this is the first text that can genuinely claim to have covered low-dimensional structures from all of these different angles, and it is assured of a place on the shelf in my own research laboratory.

Simon Bending is reader in physics, University of Bath.

Low-Dimensional Semiconductors: Materials, Physics, Technology, Devices

Author - Michael Kelly
ISBN - 0 19 851781 5 and 851780 7
Publisher - Oxford University Press
Price - £65.00 and £32.00
Pages - 546

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