**2.4 Generation of microwave signals in microwave and mm wave**

RF Signals can be excited in vacuum-tube based devices and in solid-state oscillators. Solid state modules are produced on semiconductors such as silicon, SiGe, gallium arsenide, GaN and include bipolar junction transistors (BJTs), field-effect transistors (FETs), Gunn diodes, and IMPATT diodes. RF variations of BJTs include the heterojunction bipolar transistor (HBT), and RF variants of FETs include the MESFET, the HEMT, and LDMOS transistor. RF waves can be generated and processed using MIC circuits and MMICs. Traditionally these modules are produced on gallium arsenide wafers. However, silicon germanium and heavy-dope silicon are recently used to produce high power modules. Vacuum tube high power modules operate on the ballistic motion of electrons in a vacuum under the influence of controlling magnetic or electric fields. These devices work in the density modulated mode, rather than the current modulated mode. They work on clumps of electrons flying ballistically through them. These devices include traveling wave tube, klystron, magnetron, and gyrotron.
