Table Of Content

Operation and Modeling of The M O S Transistor Operation and Modeling of The M O S Transistor Second Edition Yannis Tsividis Columbia University New York Oxford OXFORD UNIVERSITY PRESS PREFACE This book provides a unified treatment of the many phenomena encountered in the operation of modern MOS transistors, and shows how such phenomena can be modeled analytically. The book is mainly written for use in a senior or first-year graduate course. It is felt that electrical engineering students have much to gain from a course devated to the subject. The MOS transistor is the dominant VLSI device. A course devoted to it is, of course, invaluable to those planning a career in device physics and modeling. For such people, the standard courses on semiconductor devices usually cover too many different devices to do justice to any one af them, and do not present the intricacies and tradeoffs involved in a detailed modeling effort. The value of a course devoted to the MOS transistor is also extremely high for those who want to use the device to design state-of-the-art circuits. Integrated circuit designers have the op- portunity to suit devices to circuit needs, and they can do this most intelligently if they really understand the workings of the devices. One can, of course, design systems by using predesigned circuit building blocks as black boxes, if truly high performance is not important. But when state-of-the-art performance is a must, one has to consider device details. In addition, a deep knowledge of device operation and modeling is needed for understanding the computer simlilator models a designer is working with, and for identifying their limitations. Many circuit designers in the industry spend end- less hours trying to interpret strange circuit simulation results, not realizing that these are largely due to modeling inadequacies. Without adequate device understanding, valuable time and effort is bound to be wasted on overdesign, brute-force approaches, and design iterations. This author believes that no IC designer's education is complete without detailed exposure to MOS transistor operation and modeling. In the dozen years that have passed since the publication of the first edition of this book there have been significant advances in the understanding and modeling of the MOS transistor. In addition, the requiren~entsf or modeling this device on the pari of the circuit design community are now much more demanding. For example, the push for low-voltage and micropower operation has made necessary careful modeling of the device below strong inversion, and the push for ever smaller dimensions has viii PREFACE revealed phenomena previously ignored. In addition, the advent of "mixed-signal" circuits, which combine analog and digital functions on the same chip, makes it necessary to use models that are good enough for analog work. These developments have pointed to the need for a major revision of this book, However, the basic philosophy of the first edition has been retained. Several aspects of this philosophy are summarized below. The book starts with basic concepts. Readers should be able to follow even if they had no prior exposure to the device. The discussions of these concepts are often from a perspective different from the one usually taken, thus making them inter- esting reading even to those with prior exposure. Every effort has been made to give the subject a careful treatment. The reader may at times get the feeling that the author is "splitting hairs." The author would rather be accused of this than fudge. The MOS transistor is a device so complex that, once one has decided to fudge, things grow out of hand very quickly, and one ends up with a hodgepodge of careless derivations, conflicting models, and a lot of patchwork. This has been avoided at all costs. Also, the reason for hair-splitting at some points can only be appreciated further along in the book, where those "too fine" details can be seen to make a lot of difference. The use of one name for several quantities, common in some of the literature, is carefully avoided. For example, at least four distinct quantities encountered in MOS transistor work are de- scribed by using the single name "threshold voltage." Although the reader is amply warned of this practice, the practice itself is avoided in this book. The emphasis is on principles. At the same time. to illustrate these principles, rele- vant models are extensively derived and discussed. Thus, physics and modeling are discussed in paralie1 throughout the book. Analytical results are derived in a logical manner after carefully stating the as- sumptions made. Empirical modeling is avoided as much as possible. However, there are phenomena for which the only analytical results available are empirical or serniempirical. Such results are presented for completeness after pointing out the necessary hand-waving behind them. This book is not a survey. In fact, a conscious effort has been made to avoid making it one. A well-connected set of topics has been chosen, and most of these are discussed in significant detail. Nevertheless. for completeness certain other topics are mentioned, albeit briefly. In such cases. some representative results are shown without proof, so that the reader can know what to expect if he or she consults the references provided. A great deal of emphasis is placed on providing intuition for the various phenomena discussed. It is rather hopeless to attempt working with a device as complex as the MOS transistor relying only on analytical relations. The emphasis on intuition has made lengthy discussions necessary. The pace is unhurried. The author believes that this actually makes it possible to study the material faster. Thus, whereas the treatment of a given topic may be [ong in terms of number of pages, it actually should take less time to comprehend it, because of both the detailed derivations and the intuitive discussions. At times, the reader may get the feeling of dc'ja vu, since some points are repeated more than once to make sure they are not missed, especially if the reading of the topics is done out of sequence. In generai, the book is written in the style in which the author would Iike to have any new subject presented to him. He would very much llke to see the new subject "beaten to death:' presented with several points of view to increase perspective and with a significant amount of repetition. The author has been in the past grateful for treatments of this type and never felt offended by this style. If the reader happens to be "faster" than the author in this respect, he or she can easily skip some of the discussions. Almost all chapters in this book, and almost all sections within each chapter, have been extensively revised. Many sections have been rewritten, and new ones have been added. One chapter is entirely new. A list of chapters follows, along with an ex- planation of their features and of what is new in them in this edition. Chapter I: Semiconductors, Junctions, and MOSFET Overview All preliminary materid necessary for the understanding of MOS structures is given here. This material is important to the newcomer, but part of it should also make interest- ing reading for those with some previous exposure to basics. This includes the material on contact potentials, which is used to advantage in the following chapter. The chapter concludes with an overview of the MOS transistor. This section is new to this edition. It provides a framework for the rest of the book, and makes it clear why particular details of the two- and three-terminai MOS structures are studied in the following two chapters. Chapter 2: The Two-Terminal MOS Structure Here the reader will find a treatment of the MOS structure with gate and substrate terminals only. Concepts not directly related to the presence of the source and drain in the MOS transistor are treated here. The regions of weak, moderate, and strong inversion are all introduced in this chapter. Potentials are used throughout rather than energy bands. This is not only common in current literature but also helps provide rigorous straightforward derivations. Consider, for example, the well-known term , @, appearing in the expression for the flat-band voltage. In energy band treatments it is often not clear where in the MOS structure this potential actually resides. In this book, it is made evident that #MS is nothing but a contact potential, and the places where it resides are made obvious. Also, its presence in the flat-band voltage expression is rigorously justified through Kirchhoff's voltage law. The material on weak and moderate inversion has been improved and expanded in this edition. Chapter 3: The Three-Teminal MOS Structure Here one more terminal is added to the structure of Chap. 2, to connect the inversion layer to the external world. MOS transistor concepts that are not directly related to X PREFACE current flow are presented in this chapter. This includes the important "substrate ef- fect," which is amply treated. The section on limits of regions of inversion has been streamlined and shortened. On the other hand, an entirely new section, "A 'V,, Con- trol' Point of View," has been added, to lay the foundation for the discussion of certain recent models covered in Chap. 4. Chapter 4: The Four-Terminal MOS Transistor The four-terminal MOS transistor is obtained in this chapter by adding one terminal to the structure of Chap. 3. This device is now very easy to understand, on the basis of the concepts already presented for the two- and three-terminal structures. This is the central chapter in the book. Several models are presented in detail. The first of them is the complete charge sheet model, including drift and diffusion currents, valid in all regions of operation. Thanks to a simplified derivation, this material is brief but thorough. This is followed by new material on simplified charge sheet models, including both symmetric and source-referenced versions. The above models form the basis for deriving several popular strong- and weak-inversion models, which are covered in detail. A considerable part of this material is new to this edition, and has been included to reflect recent trends. Some of this material can be shpped without loss of continuity, and this is indicated at the appro- priate points. The various models are extensively related and/or compared to each other, and the way they can all be derived from one master model (the complete charge sheet model) is pointed out. A new section on interpolation models has been added. Sections on effective mobility (expanded), temperature effects, etc., are also included. The tradeoffs between accuracy and simplicity are pointed out throughout the chapter. Chapter 5: MOS Transistors with Ion-I mplanted Channeis This chapter was Chap. 6 in the first edition. It now precedes the chapter on small dimension effects. This change was made because all modern devices, small or large, have ion-implanted channels; also, having been exposed to this material, the reader can understand better certain small-dimension effects discussed in the following chapter. Nevertheless, Chaps. 5 and 6 have been revised in such a way that they can be covered in either order, in order to accommodate the need of instructors who pre- fer the original order. This chapter. arguably the most tersely written one in the first edition, has been extensively revised. it has now been written so that specific sections correspond more closely to actual devices (enhancement nMOS, depletion nMOS. surface- or buried- channel PMOS), and a much smoother development is given. The revision also makes it possible. if desired, for an instructor to cover only the parts which discuss the effects of ion implantation on threshold voltage, and to skip the detailed development of other aspects of I-V characteristics. In certain settings, this may be necessary because of time limitations.