Aluminum Nitride with a direct ultra-wide bandgap (UWBG) of 6.1 eV and a very highDebye temperature has traditionally been considered as a piezoelectric insulator. AlN, as an electronics platform, can lead to a revolution in power and RF electronics where it has a much higher breakdown field than GaN (15 MV/cm) and its ability to operate at higher temperatures. Recently, the AlN/GaN/AlN high electron mobility transistors (HEMTs) on the AlN platform have demonstrated output powers of ~2 W/mm at operating frequencies up to 94 GHz. Building on these transistors, this dissertation aims to push the capabilities of AlN HEMTs to new limits through innovative heterostructure design, polarization engineering, and theoretical transport studies. The dissertation begins with the exploration of UWBG material AlGaN as a potential electron channel on the AlN platform. With a systematic study of the properties of metal-polar AlN/AlGaN/AlN heterostructure, we examine 2d electron gases (2DEGs) obtained by polarization-doping in some of the widest bandgap semiconductors to date. Next, the device applications of these AlN/AlGaN/AlN 2DEGs, specifically in AlN HEMTs for mm-wave power amplifiers (PAs) are presented. The effect of increasing Al composition on the transistor characteristics of AlGaN channel quantum well HEMTs is shown. Finally, the RF operation of UWBG channel AlN/Al0.25Ga0.75N/AlN HEMTs with a record high fT/fmax of 67/166 GHz is demonstrated. Next, the discussion shifts to the bulk single crystal N-polar AlN platform, and its advantages over the Al-polar AlN platform are examined. For the first time, the molecular beam homoepitaxy of high-quality N-polar AlN on the N-face of bulk AlN substrates is demonstrated. Building on this, various epitaxial structures hosting polarization-induced electron gases on the N-polar AlN platform are presented. Structures presented range from the undoped N-polar GaN/AlN 2DEGs, N-polar AlGaN/AlN 2DEGs, and N-polar graded AlGaN/AlN 3d electron gases. Lastly, the discussion is moved to theoretical aspects of carrier transport where a novel analytical formulation for calculating ballistic transport properties of fermions and bosons in arbitrary dimensions and for generic dispersions is presented. This model is then applied to polar semiconductors to obtain the intrinsic limit of optical phonon scattering limited electronic currents.