This thesis work features the exploration of the capabilities and limitations of devices based on MBE (Molecular Beam Epitaxy) grown IIInitride materials under three principal applications: photovoltaics, high electron mobility transistors (HEMTs), and terahertz emitters. InGaN solar cells covering the deep UV to the red ranges of the solar spectrum were fabricated. The increase in indium composition in the alloys to absorb longer wavelengths resulted in considerable leakage and series resistance caused by a combination of surface electron accumulation and electrically active line defects. A combination of thermal annealing, improved ohmic contact metal selection, electrochemical anodization, and grain coarsening resulted in improved performance. Higher sheet densities and sheet current with minimal leakage was obtained in an AlGaN/GaN HEMT by optimizing the design to include a GaN cap as a tunneling barrier and an AlN interbarrier to increase 2DEG confinement. Furthermore, a novel technique using a 325 nm, surface sensitive laser to perform micro-Raman thermal mapping on HEMTs was developed. The technique utilized higher order A1(LO) phonons for higher thermal sensitivity. Lastly, progress was made toward GaN THz emitter development by achieving low series resistance operation through improved metal contact selection and ultra high Ge donor doping via MBE.