13. Waveform Shaping Structures and Transmission Lines on CMOS

Most of the CMOS mixed-signal circuits, including digital, analog, and RF applications, have layout sizes much smaller than the wavelength of design interest (or equivalently the operating frequency is much lower than the speed of light divided by the layout size), and are hence treated as lumped circuit elements or scattering blocks in conventional microwave circuits. These lumped circuits do not need to consider the coupling between the electric and magnetic fields governed by the Maxwell equations, but only the electrostatic Poisson equation with the displacement current would be sufficient. On the other hand, this chapter will introduce how to design distributive circuit structures in logic CMOS processes. Distributive circuits means the physical size of the component under consideration is comparable to the wavelength of interest, the electrostatic picture is insufficient, and the electromagnetic wave propagation needs to be considered for the module characteristics. As wave propagation is part of the design considerations, we will investigate the waveform shaping characteristics for these distributive modules, whether the shaping is a desirable feature or an unwanted distortion. As there are many excellent texts on the CMOS RF circuit modules such as on-chip inductors and resonators, we will focus only on distributive structures such as on-chip waveguides and transmission lines. We will then illustrate the design and characteristics of both semidiscrete and lumped-element transmission lines, together with varactor loading to make functionalities available in nonlinear transmission lines (NLTL). We will finally investigate the layout dependence of line folding and floating-metal isolation structures in practical waveguide and transmission line structures.