Nanometer CMOS

Juin J. Liou, Frank Schwierz, Hei Wong

• Publisher: Jenny Stanford Publishing
• Year: 2010
• Topic: Technology & Engineering
• eBook ISBN:
  9781000045246
• Subtopic: Biology
• Index: Technology & Engineering

This book presents the material necessary for understanding the physics, operation, design, and performance of modern MOSFETs with nanometer dimensions. It offers a brief introduction to the field and a thorough overview of MOSFET physics, detailing the relevant basics. The authors apply presented models to calculate and demonstrate transistor characteristics, and they include required input data (e.g., dimensions, doping) enabling readers to repeat the calculations and compare their results. The book introduces conventional and novel advanced MOSFET concepts, such as multiple-gate structures or alternative channel materials. Other topics covered include high-k dielectrics and mobility enhancement techniques, MOSFETs for RF (radio frequency) applications, MOSFET fabrication technology.

Chapter One

The Evolution of Silicon Electronics

1.1 INTRODUCTION

This chapter provides an overview on major developments in electronics that took place during the past decades and finally led to the current status of semiconductor electronics. With “current status” we mean the stage of development at the time of finishing this book, i.e., in early 2008. Since semiconductor electronics is a very dynamic field, information on the state-of-the-art performance of transistors and circuits quickly obsolesce and should only be considered as a momentary snapshot of a vibrant evolution. On the other hand, there are also general and long-lasting trends in semiconductor electronics, which we will discuss in the following.

Today, the Si MOSFET (Silicon Metal-Oxide-Semiconductor Field-Effect Transistor) is the backbone of semiconductor electronics. MOSFET scaling, i.e., the continuous decrease of the transistor dimensions, has been the driver of electronics for several decades. This is one of the general trends mentioned above and it brought many years of healthy growth to the semiconductor industry. Although the end of MOSFET scaling has been conjured up quite frequently and scaling limits have been predicted several times, so far engineers could find solutions to overcome any predicted limit. Since the early 2000s, however, the situation has changed in the sense that meanwhile there is a consensus in the MOSFET community that the scaling limit is looming at the horizon not too far from now. In other words, one of the general, long-lasting, and most desirable trends in semiconductor electronics will come to an end and a paradigm shift will be needed. While this is an unpleasant situation for chipmakers, it is good news for visionary researchers.

In this book, we will not discuss in detail the options for a paradigm shift in semiconductor electronics or deal in detail with alternative device options as possible successors of the MOSFET. Instead, we will discuss the operation, physics, and performance of MOSFETs with nanometer dimensions close to the scaling limit. In the remainder of this chapter, some of the major milestones of the development of semiconductor electronics will be reviewed. We will deal with questions like

• Why have vacuum tubes been replaced by transistors after World War II?

• Why was the invention of the IC (integrated circuit) that important?

• What is the motivation to build smaller and smaller transistors?

1.2 THE EARLY DAYS OF SEMICONDUCTOR ELECTRONICS

The operation of electronic circuits relies on devices with the ability to either amplify signals or to switch between the on-state and the off-state. Amplification is important for analog applications while switching between on and off constitutes the basis of digital logic, e.g., in computers.

In the first half of the last century, vacuum tubes have been used as amplifying devices in radios and as switching devices in the few computers available at that time. One of the most famous vacuum tube-based computers was the ENIAC (Electronic Numerical Integrator and Computer) built in 1945. It was the first digital electronic computer worldwide and has been used for defense purposes. The ENIAC cost approximately $750,000, contained (among other elements) about 17,500 vacuum tubes, consumed 174 kW electrical power, occupied a space of 1800 square feet, and showed an average error-free running period of only a few hours, see, e.g., Refs. 1, 2, 3. These few numbers already reveal the most severe drawbacks of vacuum tubes. Tubes are bulky, expensive, unreliable, and consume large amounts of power. Therefore engineers soon started to search for options to replace vacuum tubes by solid-state devices.

Already long before the ENIAC was built and the vacuum tube went through its heyday, some of the basic ideas of the MOSFET have been developed. During the 1920s, J. E. Lilienfeld described in a series of patents^4,5,6 how in a device nowadays known as field-effect transistor (FET) the current through a semiconducting channel could be controlled by a perpendicular electric field. Shortly after, O. Heil proposed a device with a basic structure close to that of n-channel enhancement inversion-mode MOSFETs.⁷ However, it is not known (and rather unlikely) that Lilienfeld or Heil ever made a working, amplifying solid-state device.

After World War II, several research programs aimed at developing a solid-state device capable of replacing the vacuum tube have been established. The most successful one was the program originated under the lead of W. Shockley at Bell Laboratories in Murray Hill, New Jersey. Shockley first tried to develop FET devices similar to those described by Lilienfeld and Heil. These efforts had no success since the expected conductivity modulation at the semiconductor surface was more than 1000 times smaller than predicted by theory.⁸ J. Bardeen, a member of Shockley’s team, found that the reason for the failure of the field-effect were surface states which trapped free carriers and shielded the semiconductor underneath the surface from the gate field. The work of the Bell Labs team led, however, to the invention of the bipolar transistor in 1947. The first bipolar transistor, a Ge (Germanium) device, was fabricated on 16 December 1947 by J. Bardeen and W. H. Brattain and presented to Bell Labs executives on 23 December 1947. At that time, the device still did not possess an appropriate name. The name “transistor” has been devised by J. Pierce in May 1948 after a discussion with Brattain.⁹ In June 1948, in a press conference, Bell Labs announced the invention of the transistor. During the following years, the development and commercialization of the bipolar transistor has been pushed aggressively and successfully by Bell Labs. Shockley, Bardeen, and Brattain received the 1956 Nobel price in physics for the invention of the transistor.

It is barely known today that Bell Labs were not the only place where actual pioneering work on transistors was made and where early operating transistors have been fabricated. Independently from the activities at Bell Labs, in 1948 the two Germans H. Welker and H. Matare working with Westinghouse in France realized an amplifying semiconductor device very similar to the Bell Labs transistor and called it transistron.¹⁰ Although these transistrons had been fabricated and shipped in limited numbers, the French government and Westinghouse failed to capitalize the achievement of Matare and Welker on the transistron, while the transistor had first priority at Bell Labs.

The bipolar transistor soon became the dominating semiconductor device and step-by-step the importance of Si bipolar transistor increased while the share of Ge transistors declined. In spite of the success of the bipolar transistor, engineers continued working on field-effect devices. In 1955, I. M. Ross described an n-channel enhancement MOSFET with n-type source and drain regions, p-type bulk, and a gate control electrode separated from the channel by an insulator.¹¹ The name MOS (metal-oxide-semiconductor) was coined by J. Moll in a conference paper on the metal-oxide-semiconductor capacitor¹² which is the heart of the MOSFET. In 1960, Khang and Attala were the first to realize an operating enhancement Si MOSFET.¹³ This device employed a silicon dioxide (SiO₂) gate insulator and marked a major milestone in the development of MOSFETs. Finally, in 1963, Wanlass and Sah introduced the combination of n-channel and p-channel MOSFETs called complementary MOS (CMOS), which now dominates digital logic circuits and comprises the basis of modern semiconductor electronics.¹⁴

The basic structure of enhancement MOSFETs is shown in Fig. 1.1. MOSFETs with either n-channel (nMOSFET) or p-channel (pMOSFET) can be realized. The nMOSFET shown on the left consists of a p-type Si substrate frequently called bulk. At the surface, the highly n-type doped (n⁺) source and drain regions are located. Between source and drain, the surface is covered by an insulator with a thickness t_ox and on top of th...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1. The Evolution of Silicon Electronics
• 2. MOSFET Theory
• 3. Nanoscale MOSFETs
• 4. MOSFETs for RF Applications
• 5. Overview of Nanometer CMOS Technology
• 6. Outlook
• Appendix A Frequently Used Symbols
• Appendix B Physical Constants and Unit Conversions
• Appendix C Important Properties of Si and SiO2
• Appendix D Carrier Concentrations, Energy, and Potential
• Appendix E Frequently Used Abbreviations
• Index