Of the III-Nitride family the AlN/GaN heterojunction has demonstrated the largest combined polarization charge and energy band osets available in the system. Engineering the polarization fields through varying the AlN thickness leads to two-dimensional electron gas densities (2DEGs) that may be tailored between 0.5 - 5 x 1013 cm2. Furthermore, the ultra-thin (< 5 nm) barrier and excellent transport properties of this all binary heterostructure make it well suited for high electron mobility transistor applications where high frequency and high currentare required. This work encompasses various design aspects of GaN-based High Electron Mobility Transistors (HEMTs) which ultimately result in the realization of several generations that utilize the AlN/GaN heterostructure.HEMTs fabricated from high-mobility, low sheet resistance heterostructures have achieved drain current densities up to 2.3 A/mm and transconductance of 480 mS/mm, which set new benchmarks for GaN-based HEMTs. Ultra-thin pre-metallization etching has been employed for the rst time to reduce ohmic contact resistance for AlN/GaN HEMTs and has enabled small signal frequency performance in excess of 100 GHz. Moll’s method for delay time extraction has been utilized to extract an effective electron velocity in the intrinsic region of the AlN/GaN HEMT and was found to be ~ 1.2 x 107 cm/s.By leveraging the allowable thickness window of the AlN barrier along with the high density 2DEGs that result, several novel HEMT devices have been designed and realized. High Al-content AlxGa1-xN back barriers have been employed for improved 2DEG confinement in several new variations of the ultra-thin AlN/GaN HEMT. A dual, parallel-channel AlN/GaN-based HEMT structure is designed and realized for the first time as an epitaxial approach to mitigating DC-RF frequency dispersion. These structures emphasize the facilitation of new device designs that are made possible through the particular qualities the AlN/GaN heterostructure possesses.