Semiconductor diodes and vacuum tube sources are explained in further subsections, which use electronic techniques to generate the terahertz. 12.3.1.1 Semiconductor Diodes Diodes like resonant tunnelling diodes (RTD), Gunn diodes, and IMPATT diodes can be used to convert the electronic functionality of the lower frequencies to high fre- quencies such as the terahertz band. The first terahertz diodes, the whisker contacted devices, are very useful because they are feasible and provide less shunt capacitance. The drawbacks are difficulty in assembly. They have allowed many ground breaking scientific measurements in some applications such as radio astronomy. These semi- conductor diodes are based on the negative differential resistance principle. However, these sources have their own advantages and disadvantages. But due to the prac- tical limitations, their performance is degraded dramatically while shifting to higher frequencies. The power from the semiconductor diode is reduced while shifting to higher frequency, because the series resistance increases due to the small size of the diode. Frequency multipliers can also be used to generate terahertz. These are solid- state electronic sources. However, the resultant power of the frequency multiplier is decreased at higher frequencies with limited bandwidth. The doublers and triplers have been implemented in series as the higher order multipliers are inefficient. For example, in series arrangements, the GaAs Schottky diodes can be used as chains of RF/MW sources. Terahertz Antenna in Future Healthcare Informatics 12.3.1.2 Vacuum Tubes One of the traditional terahertz generation techniques is through microwave tubes which emit free electrons. Some of the THz tube sources are klystron, gyrotron, and travelling wave tube (TWT). In these sources, the electron beam is interacted with the EM waves to generate the THz wave. The source consists of electron gun, regenera- tive oscillator, depressed collector, and magnetic solenoid which helps in confining the electron beam in