Initially, elemental semiconductors such as Si and Ge were used in devices such as the transistor which operated at low frequencies. Subsequent advances in materials science and technology made it possible to fabricate compound semiconductor transistors capable of operating at microwave frequencies and high speeds. This offered the possibility of using these devices in both analog and digital circuits. Furthermore, the wide choice of high-performance compound semiconductor materials also enabled the development of optoelectronic devices such as lasers and light-emitting diodes. The communications industry continues to grow and diversify, necessitating the design of circuits that will satisfy the requirements of mobile telephones, which are becoming increasingly sophisticated, as well as other areas of application such as optical communications. Hence the importance of compound semiconductor devices in all aspects of communications, especially above 1 GHz.