Passive devices have gained some attention in this codesign situation. The increasing need for rf devices in products such as cellular phones and other mobile devices will make codesign of passives along with the chip and package a necessity. RF modules typically are made as multichip modules for well-known reasons. Signal processing, memory and mixing functions need to be together in one module, and it is impractical if not impossible to make a single-chip solution. Mixing functions are best made on GaAs, while signal processing functions and memory are best made on silicon.
Passives are the dominant presence in an rf module.
A typical cellular phone may have only a few chips and dozens of passives. Since signal traces are essentially transmission lines, changing a layout within an rf module can lead to a complete change in the values and arrangement of the passives. Part of the reason there are so many passives is the use of standard value components. Reducing the number and size of these passives is the critical path for reducing the size of rf modules, and that requires custom design of the passives themselves. In globally optimizing an rf module, the passives and the substrate are the most important considerations.