Gallium Nitride (GaN) based on its superior material properties is emerging as a strong candidate and a material of choice for next-generation power semiconductor device technologies. The research presented in this thesis focusses on Polarisation Super Junction (PSJ) which is a promising technology for developing high voltage GaN power electronic devices. The research enabled the critical groundwork for scaling-up from Proof of Concept device test structures to large area high voltage PSJ GaN Transistors, Schottky Barrier Diodes, and PN Diodes with high power handling capability. Large area devices were fabricated on Sapphire and 6H-Silicon Carbide substrates and key determinants of the performance of scaled-up devices were identified through extensive electrical characterisation and numerical simulations. Implementation of an intrinsic diode within a high voltage GaN transistor and clamping capability during inductive switching was also demonstrated. GaN-on-Silicon is emerging as the preferred route of technology development because of a cost-effective and mature Silicon (Si) platform. Along with development of discrete devices, there is also focus on monolithic integration in this platform. Performance assessment of monolithically integrated circuits (ICs) requires evaluation of crosstalk amongst the otherwise isolated devices. This phenomenon was comprehensively investigated and it was demonstrated that high voltage GaN-on-Si platform suffered from crosstalk issues, coupled through the Si substrate and that grounding the substrate substantially mitigated the crosstalk effects under static conditions. On the same platform, it was also observed that there was an increase in the on-state resistance (RON) of a GaN-on-Si transistor when configured as a High-Side switch of a half-bridge circuit, which is a common circuit topology for power conversion applications. Crosstalk effects and increase in RON, are two major issues that will have to be addressed for developing high voltage monolithic GaN-on-Si solutions. These issues were identified and reported for the first time through this research.