Chapter 1: Cmos Circuits - a Brief Introduction

CHAPTER 1: CMOS CIRCUITS - A BRIEF INTRODUCTION Over the past two decades, Complementary Metal Oxide Silicon (CMOS) technology has played a very important role in the global integrated circuit industry. Although the basic principle of the MOS field-effect transistor was explained by J. Lilienfeld in 1925, commercial success of MOS devices could be ensured only during the 1960s with the invention of the silicon planar process. Nevertheless, the nMOS devices, fabricated by the nMOS-silicon-gate technology, came to be used in the early 1970s, prior to which only single-polarity p-type transistors were in use. At the same time, P.K. Weimer and F. Wanlass demonstrated the possibility of using both polarity devices on the same substrate. With the implementation of the CMOS inverter, NOR gate and NAND gate, initially using discrete transistors however, the CMOS concept took root, demonstrating the low power dissipation characteristics. Initially, requirement of more complex processing technology and larger silicon area compared to the single polarity transistors led to limited application of CMOS transistors to general system designs. However, as CMOS technology rapidly improved to support large chip sizes, and the issue of power consumption became more and more critical, CMOS technology has firmly established itself as the dominant VLSI technology. This first chapter introduces the reader to CMOS logic design and design representations.

1.1 MOS Transistors and Switches

Silicon is predominantly used in the fabrication of semiconductor devices and microcircuits. A MOS (Metal-Oxide-Silicon) transistor structure is built by stacking several layers of conducting, insulating and semiconductor materials. This structure is produced involving a series of chemical processing steps such as oxidation of silicon, diffusion of impurities into silicon following etching of silicon oxide from selected locations, and deposition and etching of aluminum on silicon to provide connections with the external environment of the transistor. The fabrication process is carried out on a single crystal of silicon available as thin circular wafers of diameter about 10 cm. CMOS technology makes way for two kinds of transistors, namely nMOS (n-channel) transistor and pMOS (p-channel) transistor built by using negatively diffused silicon (rich in electrons) and p ositively doped silicon (rich in holes) respectively. Some of the distinct layers resulting after the fabrication of a MOS transistor happen to be diffusion, polysilicon and metal (aluminum), separated by insulating layers.

Figure 1.1 depicts the physical structures and circuit symbols of an n-channel and a p-channel transistor. The structure of the n-channel transistor is made of a p-type silicon substrate accommodating two diffused islands of n-type silicon. Selected areas of the p-substrate are altered by a chemical process into n-type silicon. On top of the area separating the n-type islands lies a thin insulating layer of silicon dioxide (SiO2) above which there is a conducting layer (usually made of polycrystalline silicon) called the gate .

Fig 1.1 : Physical structure and schematic representation of MOS transistors A p-channel transistor, on the other hand, is made of an n-substrate separating two diffused p-type islands. Like an n- transistor, it too has a gate electrode. Apart from the gate electrode, an nMOS transistor has two more terminals known as the source and the drain which connect the two n-diffused regions (p-diffused regions in a pMOS transistor) with the external environment of the device. The gate acts as a control input, regulating the current flow between the source and the drain. Although the source and the drain are physically equivalent, the name source is reserved for the terminal by