e material should be a low loss dielectric material, such as fiberglass-reinforced-epoxy-laminate sheet. Gallium arsenide (GaAs) having high carrier mobility can be used as the substrate material in the monolithic micro- wave integrated circuit (MMIC). But in THz PCA, semiconductor materials like silicon, GaAs, InGaAs and low temperature grown-GaAs (LT-GaAs) can be used because of their characteristics, such as high electron mobility, supporting high bias voltage, ultra-short carrier lifetime, and high intrinsic resistivity. Metals such as gold, silver, aluminium, and the like, can be used for biasing pads as well as antenna electrodes. The advantages of PCAs are that it is simple, compact, capable of gener- ating moderate THz intensity without pumping by a high-power laser source and that it operates at room temperature. 12.6 IMPORTANCE OF PCA In the previous section, the advantages and drawbacks of various THz sources have been discussed. Among these sources, the photoconductive antenna shows better per- formance in different aspects of THz generation [Ferguson & Zhang (2002)]. It can be easily integrated in fiber optics laser systems. Hence, it can be operated remotely in outdoor conditions. Therefore, in many applications, such as, medical diagnostics, agro applications, defense and security applications, manufacturing industries and many more, these PCAs are highly acceptable. 12.7 CLASSIFICATIONS OF PCA Based on the antenna gap size, the terahertz PCAs are generally classified into small gap antennas, semi-large gap antennas, and large gap antennas. There are different types of antenna structures. some of these are dipole, bowtie, interdigitated structures, nano-antennas, split ring resonators, and SO forth. These different antenna structures can be optimized to provide better performance in THz radiation. The performance of PCA depends on the optical source, geometrical parameters, and antenna impedance matching. The geometry considerations for the antenna electrode is more important as the photocarriers a