c- tively, for the device with 35 nm gate length and 20 um gate width. The linear gain of the two-stage amplifier is more than 10.5 dB in the 220-320 GHz frequency band, and the small signal gain is 13.5 dB at 330 GHz. In another study, the device with 20 nm gate length was reported, and the fT was 660 GHz for the mHEMT with 2 X 10 um gate width. GaAs mHEMT has been able to work into the H-band and enter the low-end of the THz region. It has become an important component of the sub-mmWave applications with a cost advantage [26]. The mmWave RF components are expected to be deployed in large scale and high density and to meet the multi-band and high-frequency requirements. This would underline the importance of mmWave filter technologies. The acoustic filter technologies continue to evolve to meet the 736 Chapter 5 challenges of the global transition to 5G networks. SAW 32 and bulk acoustic wave (BAW)3 filter technologies are widely used in today's mobile device filtering functions. Under the existing framework, SAW filters are widely used in 2G, 3G, and 4G wireless technologies, which has the advantages of design flexibility, good frequency selection characteristics, and small size; and can utilize the same production process as the integrated circuits. The frequency characteristic of the SAW filter is closely related to the pitch of the inter-digital transducers electrode, thus the higher the frequency, the smaller the distance required between the electro- des. The SAW filter quality drops rapidly when the frequency exceeds 2.5 GHz. Therefore, the SAW filter is mostly used in below 2.5 GHz applications. The BAW filters are gaining traction beyond 2.5 GHz. In contrast to SAW filters, they are insensitive to temperature changes, with low insertion loss and high OOB rejection, making them a good candidate for 4G/5G wireless systems' RF filtering applications. However, most BAW filters are used below 6 GHz, although they can reach to a maximum frequency of 20 GHz. The mmWave micro-electro-mechanical systems (MEMS) f