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Gallium nitride (GaN) is now widely used in commercial white Light Emitting Diodes (LEDs) thanks to the emergence of high-brightness GaN blue LEDs in 1990s. In addition to its application in solid-state lighting, GaN has been also vowed as a strong contender for next-generation high power and frequency devices due to its high critical electric field (3.3 MV/cm) and high mobility of the 2-dimensional electron gas (2DEG) at the aluminum gallium nitride (AlGaN)/GaN interface. Lateral AlGaN/GaN high-electron-mobility-transistors (HEMTs) have been available as commercial off-the-shelf devices since 2005. However, with the demand for even higher power at reduced chip area and cost and with better thermal management at high currents, vertical device architectures have emerged as the chosen structure to meet these demands. But vertical devices that can hold high power require thick and high quality GaN layers. Recent developments of bulk GaN substrate growth technologies allowed vertical GaN device with thick drift layer to be more feasible. However, GaN substrate technology is challenged with cost, reliability and uniformity issues even at the currently commercially available 2” (diameter) substrates. Therefore, GaN vertical power devices on cheap substrates without compromising the GaN material quality remains to be of great interest. Si substrates with their fab-scale integrated circuit technology can propel the development of commercial vertical high power GaN devices. The biggest challenge for realizing thick GaN layers on Si to hold high voltage in the vertical direction is the large thermal and lattice mismatch between GaN and Si that leads to cracking of the GaN layers beyond only a few micrometers.In major part of this dissertation, we will focus on the epitaxy techniques of thick crack-free GaN layers on Si by selective area growth (SAG) and the fabrication of vertical GaN switches. The epitaxy technique developed in this work resulted in crack-free thick GaN layers on Si that are of high quality with low dislocation densities and low background doping in order to sustain high breakdown voltages. The developed processes hold the potential to significantly advance the fundamental electronic materials research in power devices and their efficient system level integration.