Modern communication systems, both microwave and optical, are beginning to rely heavily on the high frequency circuit performance capabilities provided by III-V semiconductor materials, of which the most mature is Gallium Arsenide (GaAs). The applications of GaAs can be divided into digital and analogue.The analogue applications can be further subdivided into microwave circuits, and sampled data circuits based on switched capacitor circuit techniques. Both the high quality switches and the high gain, fast settling operational amplifiers required in these systems can be realised using the GaAs Metal Semiconductor FET, or MESFET.