Ultra-wide bandgap (UWBG) semiconductors are of interest for next-generation radio frequency (RF) and high-voltage power electronic devices. Owing to its ~4.8 eV bandgap (Eg) and estimated critical field strength of ~8 MV/cm, β-Ga2O3 is an UWBG material exhibiting a theoretical Johnson’s figure of merit and Baliga’s figure of merit exceeding that of current state-of-the-art devices based on SiC (Eg ~3.2 eV) and GaN (Eg ~3.4 eV). Melt-grown single-crystal β-Ga2O3 substrates are commercially available with diameters that are rapidly scaling up. Shallow ‘hydrogenic’ donors exist (Si, Ge, and Sn) ~36 meV below the conduction band minimum, allowing for controllable n-type conductivity over 5 orders of magnitude. However, acoustic phonon modes with low group velocities, arising from the large formula unit and low symmetry of β-Ga2O3 (space group C2/m), lead to fundamental challenges such as low thermal conductivity. Compared to other semiconductors, the intrinsic electron mobility of β-Ga2O3 is especially limited due to strong electron-polar-optical-phonon (POP) interactions. Moreover, the flat valence band, comprised of weakly overlapping oxygen 2p orbitals, renders direct p-type doping extremely challenging. β-Ga2O3 thin films have been grown by many epitaxial techniques such as molecular beam epitaxy, pulsed laser deposition, mist-chemical vapor deposition, halide vapor phase epitaxy, and metalorganic chemical vapor deposition (MOCVD). MOCVD, used in this work, is the industry-preferred technique for the growth of compound semiconductors. To date, β-Ga2O3 films grown by MOCVD exhibit the highest electron mobilities, approaching the POP limit. A major challenge for lateral β-Ga2O3 transistors is off-state leakage due to the presence of a parasitic conducting channel, which has been attributed to silicon contamination at the substrate-epitaxial film interface. In this dissertation, we definitively demonstrate that a significant source of the contamination is ambient air exposure. A 30-minute hydrofluoric acid etch is effective for mitigating the contamination; however, the samples must be loaded under vacuum quickly to prevent reaccumulation. Minimizing air exposure at critical device interfaces motivated our work to develop an in situ MOCVD etch technique. We investigate the use of HCl-based etching using tert-butyl chloride (TBCl), systematically elucidating the etch mechanism. At susceptor temperatures below ∼800 °C, the etch rate is likely limited by the desorption of etch products. We further demonstrate that the etch is anisotropic, which can be used for etching finFETs with nearly vertical sidewalls. To maximize the mobility of β-Ga2O3 while maintaining high sheet charge, ionized impurities can be spatially separated from the channel in a β-(Al,Ga)2O3/β-Ga2O3 heterostructure. Typically, trimethylaluminum (TMAl) is used as an Al precursor; however, growth conditions must be tuned (a combination of increasing the susceptor temperature and the O/Ga ratio) to prevent carbon incorporation. Instead, using triethylaluminium (TEAl), which pyrolyzes via β-hydrogen elimination, enables a lower growth temperature essential for preventing donor compensation and β-(Al,Ga)2O3 film cracking. We implement δ-doping into a heterostructure where we observe Shubnikov-de Haas oscillations for the first time in an MOCVD-grown sample, indicating high crystalline quality and homogeneity. All the building blocks presented in this thesis will pave the way for higher performance β-Ga2O3 based RF and power electronic devices.