In this work, two avenues for potentially improving the performance of future electronic and optoelectronic devices have been explored. Backside contact technology for high-performance III-V solar cells and the exploration of plasma-wave effects in GaN have been developed and evaluated. For the backside contact solar cell technology, we analyzed the performance of an advanced novel design for III-V multi-junction solar cells —the backside contact III-V triple-junction solar cell. For the study of plasma-wave effects, we have focused on observing and studying plasma-wave effects in III-N plasmonic devices at room temperature. The fabrication of both backside contact solar cells and III-N plasma-wave devices is discussed. Backside contact technology has been demonstrated successfully in silicon solar cells, but has not been explored carefully or demonstrated in III-V multi-junction solar cells so far. In this work, a numerical model was developed to evaluate and optimize both the electrical and thermal properties of backside-contact triple-junction solar cells. The optimization of the epitaxial structure resulted in a 14% (relative) efficiency improvement and the backside-contact technology results in another 5% (relative) efficiency enhancement. For the fabrication and ultimately the demonstration of backside-contact triple-junction solar cells, the critical process step — via-hole fabrication — is demonstrated. The developed full-wafer via-hole fabrication process, which is compatible with the process flow for backside-contact solar cells, resulted in uniform and smooth etch morphologies and near vertical sidewall profiles. For the study of plasma-wave effects, in this work we designed and fabricated GaN-based devices for observation of plasma-wave effects at room temperature. While plasma-wave effects have been reported previously at cryogenic temperatures, this study seeks to characterize the potential of these effects for room temperature operation. The devices explored here use grating-gate structures to enhance the plasma-wave signatures in the device response. The device geometries were optimized and nanoscale fabrication techniques were developed to fabricate the devices. On-wafer electrical testing showed clear signatures of plasma-wave effects at room temperature. These signatures were found to agree very well with analytical models of plasma-wave propagation. The observation of electrically-significant plasma-wave effects in GaN opens the possibility of future devices that exploit this physics for enhanced functionality.