The transition to energy efficient smart grid and wireless communication with improved capacity require the development and optimization of next generation semiconductor technologies and electronic device components. Indium nitride (InN), gallium nitride (GaN) and aluminum nitride (AlN) compounds and their alloys are direct bandgap semiconductors with bandgap energies ranging from 0.7 to 6.0 eV, facilitating their utilization in optoelectronics and photonics. The GaN-based blue light-emitting diodes (LEDs) have enabled efficient and energy saving lighting, for which the Nobel Prize in Physics 2014 was awarded. GaN and AlN have high critical electric fields, high saturation carrier velocities and high thermal conductivities, which make them promising candidates for replacing silicon (Si) in next-generation power devices. The polarization-induced two-dimensional electron gas (2DEG), formed at the interface of AlGaN and GaN has enabled GaN-based high electron mobility transistors (HEMTs). These devices are suitable for high-power (HP) switching, power amplification and high-frequency (HF) applications in the millimeter-wave range up to THz frequencies. As such, HEMTs are suitable for next-generation 5G and 6G communication systems, radars, satellites, and a plethora of other related applications.

Despite the immense efforts in the field, several material related issues still hinder the full exploitation of the unique properties of GaN-based semiconductors in HF and HP electronic applications. These limitations and challenges are related among others to: i) poor efficiency of p-type doping in GaN, ii) lack of linearity in AlGaN/GaN HEMTs used in low-noise RF amplifiers and, iii) MOCVD growth related difficulties in achieving ultra-thin and high Alcontent AlGaN barrier layers with compositionally sharp Al profiles in AlGaN/GaN HEMTs for HF applications.

In this PhD thesis, we address the abovementioned issues by exploiting the hot-wall MOCVD combined with extensive material characterization. Main results can be grouped as follows:

i) state-of-art p-GaN with room-temperature free-hole concentrations in the low 10¹⁸ cm^-3 range and mobilities of ~10 cm²/Vs has been developed via in-situ doping. A comprehensive understanding of the growth process and its limiting factors, as related to magnesium (Mg), hydrogen (H) and carbon (C) incorporation in GaN is established. Further improvement of p-type doping in as-grown GaN:Mg is achieved by using GaN/AlN/4H-SiC templates and/or by modifying the gas environment in the growth reactor through the introduction of high amounts of hydrogen (H₂) in the process. Using advanced scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) we have established an improved comprehensive model of the pyramidal inversion domain defects (PIDs) in relation to the ambient matrix. First experimental evidence that Mg is present at all interfaces between PID and matrix allows for more accurate evaluation of Mg segregated at the PID, necessary for understanding the main limiting factor for p-type conductivity in GaN against alternative compensating donor or passivation sources.

ii) Compositionally graded AlGaN channel layers in AlGaN/(Al)GaN HEMTs with various types of compositional grading have been developed, and graded channel devices were compared with conventional AlGaN/GaN HEMT indicating improved linearity. The first large signal measurements in Europe of a graded channel AlGaN/GaN HEMT has been carried out demonstrating improved linearity figure of merit IM₃ by 10 dB compared to conventional Fe-doped GaN buffer devices. These results are showing state-of-the-art performance and pave the way for novel highly linear GaN receivers.

iii) Ultrathin (sub-10nm) and high Al-content (>50%) AlGaN barrier GaN HEMT structures have been developed with 2DEG carrier densities ~1.1×10¹³ cm^-2 and mobilities ~1700 cm²/Vs. Advanced characterization with atomic precision involving STEM and energy dispersive X-ray spectroscopy (EDS), has allowed experimental determination of the Al profiles and has revealed deviations from the nominally intended structures. Such deviations are found also in different source materials including commercial HEMT epistructures grown by MOCVD. The implications of the Al-profile deviations are critically analyzed in terms of 2DEG properties and device fabrication and performance. The capabilities and the limitations of MOCVD processes, related to growth of compositionally sharp and ultrathin high-Al-content AlGaN layers on GaN have been evaluated and their prospects in HF have been assessed.