Lateral GaN HEMT structures have been well studied and developed for high power applications. However, for very high breakdown voltages, a vertical device design is more favorable owing to better field distribution and higher power density. Current Aperture Vertical Electron Transistors (CAVETs) benefit from the high electron mobility of the 2-dimensional electron gas (2-DEG) in the lateral direction and better field distribution in the vertical direction. Current flows laterally in the 2-DEG at the AlGaN/GaN heterojunction and is directed vertically down to the drain through an aperture. A current blocking layer (CBL) is used to define the low resistance aperture. The focus of this thesis is developing CAVETS with Mg-doped conductive p-GaN layers as the current blocking layer. Current blocking is realized by the use of a reverse biased p-n junction as opposed to an insulating layer. The p-GaN is regrown either by Ammonia MBE or MOCVD and in order to ensure that the p-GaN layer remains active, the channel regrowth is done by Ammonia-MBE. The current blocking capacity of the p-n junction has been verified in an n/p/n structure and the highest breakdown voltage recorded was 870 V. The peak electric field has been estimated to be 3.1 MV/cm, which is close to the GaN critical field. With the current blocking capacity of the active p GaN layer verified, selective area regrowth was used to define the apertures and CAVETs were fabricated. Promising transistor performance was demonstrated with a high saturation current of 10.9 kA cm-2 and low ON-resistance of 0.4 mohm cm 2.