Photoelectrochemical (PEC) wet etching is an attractive wet etch approach for III-Nitride materials. Compared to dry etch techniques normally applied in prevalent GaN device fabrications, PEC wet etching can provide low damage, selective etching and understanding of material defects. This dissertation work has carried out an in-depth exploration of the dependence of PEC etching on both process variables and materials composition. In particular, a detailed study of bandgap-selective etching is carried out. To focus these studies, we describe the fabrication of a novel vertical electronic device, the  CAVET (C&barbelow;urrent A&barbelow;perture V&barbelow;ertical E&barbelow;lectron T&barbelow;ransistor). The key feature of this device is a current aperture that restricts the flow of current in a 2DEG to a direction perpendicular to the surface. By keeping current away from surface states, this device geometry provides low DC-RF dispersion compared to a more conventional AlGaN/GaN HEMT. The composition and dimensions of the aperture cannot otherwise interfere with the operation of the device, thus the fabrication process will involve highly selective etching of a very thin sacrificial layer. We utilized PEC bandgap selective etching of a 60nm InGaN sacrificial layer, and great effort was employed to optimize the etch process to obtain smooth, controllable lateral undercut etching. We describe two generations of device fabrication and the accompanying modifications in selective etch process that were required.