In the eight years since this book was first published, CMOS technology has steadily moved to occupy a central position in modern electronic system design. Whether digital systems are high speed, high density, low power, or low cost, CMOS technology finds ubiquitous use in the majority of leadingedge commercial applications. CMOS processes have shrunk, and more automated design tools have become commonplace, leading to far more complex chips operating at much higher speeds than a decade ago. While the basic theory of CMOS design remains unchanged, the emphasis and approach to design have changed. With smaller processes and higher speeds comes an increased emphasis on clocking and power distribution, while with complex chip designs and short time-to-market constraints, less emphasis is now placed on die size and the physical details of chip design. The requirement for higher-quality CMOS chips has also increased the need for good approaches to testing.
This edition was updated with these. changes in mind. All chapters have undergone extensive revision, and a new chapter on testing replaces one on symbolic layout. Sections on emerging technologies such as BiCMOS, logic synthesis, and parallel scan testing have been added. The overall emphasis has been to include as much as possible of the engineering (and to some extent, the economic) side of CMOS-system design. The artwork has been completely redone and many new figures have been added. All figures were captured on a CMOS VLSI design system. Thus, where possible, diagrams were checked via simulation or net comparison. The tendency has been to include figures where possible ("a picture is worth a thousand words") to trigger thereader's thinking.
As a text, this book provides students with the necessary background to complete CMOS designs and assess which particular design style to use on a given design, from Field Programmable Gate Arrays to full custom design. For the practicing designer, the book provides an extensive source of reference material that covers contemporary CMOS logic, circuit, design, and processing technology.
In common with the first edition, the text is divided into three main sections. The first deals with basic CMOS logic and circuit design and CMOS processing technology. This includes design issues such as speed, power dissipation, and clocking and subsystem design. The second section deals with design approaches and testing. The final section describes three examples of CMOS module/chip designs to provide working examples of the material presented in the first two parts of the book.
In the eighties, designers struggled with tools, circuit techniques, and technology to build CMOS digital systems that could frequently be mastered by one person. The. design issues, for example, related to whether a simulation for a circuit could be done and, if so, how accurately. Or perhaps the success of a project depended on a muter or a design-rule checker that could deal with large databases. Today, the technology has moved to a point where, to a first order, the technology always works. Failures in design relate to incomplete specifications, inadequate testing, poor communication between designers in a team, or other issues that are somewhat removed from the detailed engineering that still has to take place. That engineering is supported by well-developed design tools. A significant task to be mastered in today's world (once the basics have been learnt) is to take a specification, turn it into a design, enter the design into a CAD system, test it, have it manufactured, and then be able to ship the. product.
Increasingly, CMOS VLSI design is being seen as an ideal medium in which to teach the general digital (and analog) system design principles required in such a design process by introducing such issues as structured design and testing. Coupled with education-based Field Programmable Logic Array tools and prototyping kits, courses can be crafted around the basic principles of CMOS design, such as logic design and delay estimation, and coupled with more advanced topics such as simulation, timing analysis, placement and routing, and testing. With reprogrammable hardware, the concept-to-reality delay is reduced to minutes, and the education dynamics of almost-real-time feedback can only help in the education of tomorrow's system designers. The principles used in these laboratory systems are then applicable, with suitable modifications and information, to real-world products, whether such products employ gate-array, standard-cell, or full-custom CMOS design techniques. Burlington, Mass.
N. H. E. W.
Chapter 1: Introduction to CMOS Circuits
A Brief History Over the past decade, Complementary Metal Oxide Silicon (CMOS for short) technology has played an increasingly important role in the global integrated circuit industry. Not that CMOS technology is that new. In fact, the basic principle behind the MOS field-effect transistor was proposed by J. Lilienfeld as early as 1925, and a similar structure closely resembling a modern MOS transistor was proposed by O. Heil in 1935. Problems with materials foiled these early attempts. Experiments with early field-effect transistors led to the invention of the bipolar transistor. The success of the latter device led to a decline of interest in the MOS transistor. MOS devices remained an oddity until the invention of the silicon planar process around 1960. Although the first MOS calculator was introduced in 1965, material and quality-control problems dogged the expansion of the MOS device into a variety of commercial uses until about 1967.1 Even then, single-polarity p-type transistors were favored until the emergence of the nMOS-silicongate technology in about 1971. The use of both polarity devices on the same substrate was invented by at least two people in the early 1960s. P. K. Weimer, of RCA, filed a patent (U.S. 3,191,061) on May 31st, 1962, issued on May 22, 1965, that featured the elements of modern CMOS flip-flops, demonstrating possible implementations in thin-film-transistor technology. Frank Wanlass, of Fairchild Semiconductor Research and Development, filed a patent on June 18th, 1963, (U.S. Patent 3,356,858), granted on December 5th, 1967,2 that covered the CMOS concept and three circuits, the inverter, NOR gate, and NAND gate implemented as MOS devices. Wanlass had to build his own nMOS transistors because only pMOS devices were available. The initial circuits were developed using discrete MOS transistors and demonstrated what was for many years the hallmark of CMOS-low power dissipation. The first inverter dissipated nanowatts of power compared with milliwatts for pMOS or the then popular bipolar gates. The lowpower attribute led CMOS to be initially used for very low power applications, such as watches. Since the processing technology required in the fabrication of CMOS circuits was more complex and the required silicon area was significantly larger than that for single polarity transistors, CMOS was applied sparingly to general system designs. As nMOS production processes became more complicated, the additional complexity of the basic CMOS process decreased in importance. Additionally, as the technology improved to support very large chip sizes, system designers were faced with power consumption problems. For this, and for other reasons that will become evident during the course of this book, CMOS technology has increased in level of importance to the point where it now clearly holds center stage as the dominant VLSI technology.
The purpose of this book is to provide designers of hardware or software systems with an understanding of CMOS technology, circuit design, layout, and system design sufficient to feel confident with the technology. The text deals with the technology from a digital systems level down to the layout level of detail, thereby providing a view of the technology for both the system level ASIC designer and the full custom designer.
Book Summary
This book is divided into three main sections. Chapters 1-5 provide a circuit view of the CMOS IC design. In the first chapter, a simplified view of CMOS technology will be taken and some basic forms of logic and memory will be introduced. The aim is to provide an unencumbered picture of the technology without delving into unnecessary detail. A small chip project is used to illustrate the steps in modern CMOS design. Chapter 2 deals at greater depth with the operation of the MOS transistor and the DC operation of the CMOS inverter and a few other basic circuits of interest. It also introduces the junction diode and bipolar transistor. A summary of CMOS processing technology is presented in Chapter 3. The basic processes in current use are described along with some interesting process enhancements. Some representative geometric design rules are also presented in this chapter. Chapter 4 treats the important subject of performance estimation and characterization of circuit operation. This covers circuit speed and power dissipation. A section summarizing some first-order scaling effects is also included. A summary of basic CMOS circuit forms is provided in Chapter 5. Various clocking schemes are discussed, with emphasis on good engineering practice.
The second section of this book comprises Chapters 6-8. These chapters present a subsystem view of CMOS design. Chapter 6 focuses on a range of current design methods, identifying where appropriate the issues peculiar to CMOS. Testing and test techniques are discussed in Chapter 7. Chapter 8 is a rather hefty chapter on subsystem design, using for illustration the circuits discussed in Chapter 5. A discussion of a variety of datapath operators opens the chapter. RAMS, ROMs, and the implementation of control logic are then covered.
The book's final section is contained in Chapter 9. It consists of several examples of CMOS VLSI designs that combine many of the design approaches covered in the preceding chapters, and demonstrate some of the practical tradeoffs in the design of actual chips.
The remainder of the current chapter provides a basic introduction to CMOS switches, logic gates, memory elements, and the various abstractions that are used to design integrated systems.
MOS Transistors
Silicon, a semiconductor, forms the basic starting material for a large class of integrated circuits. An MOS (Metal-Oxide-Silicon) structure is created by superimposing several layers of conducting, insulating, and transistorforming materials to create a sandwich-like structure. These structures are created by a series of chemical processing steps involving oxidation of the silicon, diffusion of impurities into the silicon to give it certain conduction characteristics, and deposition and etching of aluminum on the silicon to provide interconnection in the same way that a printed wiring board is constructed. This construction process is carried out on a single crystal of silicon, which is available in the form of thin, flat circular wafers around 15cm in diameter. CMOS technology provides two types of transistors (also called devices in this text), an n-type transistor (nMOS) and a p-type transistor (pMOS). These are fabricated in silicon by using either negatively diffused (doped) silicon that is rich in electrons (negatively charged) or positively doped silicon that is rich in holes (the dual of electrons, and positively charged)...
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