10.2 Monolithic Microwave Circuit Design

As may be appreciated from the rest of this book, GaAs is an excellent material for field effect transistors and Schottky diodes. Because it can also be a low loss dielectric in its semi-insulating form it has become the basic material for microwave integrated circuits where both active and passive elements are combined on the same chip. The passive elements take the form of either distributed or lumped elements. The two most popular transmission lines used on GaAs are microstrip and coplanar waveguide. Of these two, microstrip has become the most exploited since the advantage put forward for coplanar waveguide, that of accessible ground planes on the top surface of the chip is only useful for simple circuits. With the advent of via technology microstrip is much more flexible. Lumped elements produce more circuit design flexibility, too, provided that the equivalent circuits of these components (i.e. the component parasitics and loss) can be accurately modeled. In many cases these models are most easily predicted by the use of transmission line theory, either single or coupled. Lumped components are particularly useful at frequencies up to J-band. However, in certain cases (for example, where the chip needs to be thin for thermal dissipation reasons) the loss factor of these lumped components can be low (i.e. 20 or so). Much of the early work on GaAs monolithic circuits used lumped elements to gain their advantages in broad-band circuits.