Chapter 14

Capacitors, Switches and the Occasional Passive Resistor

14.1 Introduction

Many analog integrated circuits are constructed using operational Op Amps, OTAs, capacitors, switches and resistors. We have dealt with Op Amps and OTAs in the previous chapters, and it now remains to examine the design of the other building blocks. This chapter presents the various integrated circuit versions of capacitors and switches [24] and discusses the non-ideal effects in these components in relation to their use in analog and mixed-mode signal processing systems. Furthermore, resistors are sometimes also required as on-chip components. These can be realized as active devices as explained in Chapter 11; alternatively some situations call for the high degree of linearity associated with passive components. The possible passive MOS resistor structures are also reviewed.

14.2 MOS Capacitors

14.2.1 Capacitor Structures

An important building block of analog integrated circuits is the MOS capacitor. The most commonly used dielectric is SiO2 which is a very stable insulator with εox ≅ 3.9 and a high breakdown electric field of about 8 × 106 V/cm (although Si3N4 is also used). The choice of electrodes for the capacitor varies according to the available technology that is used for the fabrication of the entire integrated circuit. This leads to the following types:

1. Metal (or polysilicon) over diffusion structure. This is shown in Figure 14.1(a) and is formed by growing a thin SiO2 layer over ...

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