GaN as a wide-bandgap semiconductor material (~3.4eV) with high critical electrical field (~3MV/cm) and direct bandgap, is a promising candidate for next-generation power devices and optoelectronics. In this thesis, the development of III-nitride bipolar devices is discussed. The studies and research mainly involve the design, fabrication process and characterization of vertical GaN p-i-n (PIN) rectifiers, GaN-based UV avalanche photodiodes (APDs) and UV laser diodes (LDs) based on the AlGaN/GaN non-planar growth (NPG) method. Vertical GaN PIN rectifiers with nitrogen-implanted floating guard rings (FGRs) are designed and fabricated. A minimum low specific-on resistance (RONA) of 0.23 mΩ-cm2 was achieved with high reverse-bias breakdown voltage up to -1360V on the fabricated PIN rectifiers. High ION/IOFF ratio was obtained >1012 at ±4 V, owning to the low reverse leakage and high forward-biased injection current. High optical avalanche gain and low reverse-bias leakage GaN APDs were developed by implementation of a ~1.8° shallow bevel mesa, achieving an ultra-low leakage current density less than 10-9 A/cm2 up to a reverse bias voltage of -70V and high avalanche multiplication gains > 107 under DUV illumination. The NPG on patterned (0001) GaN substrates was developed for growth of crack-free thick relatively high Al alloy composition AlGaN layers by suppression of tensile strain from AlGaN layer on GaN. The first UV pulsed-mode diode laser at ~370 nm based on AlGaN/GaN NPG on GaN substrates was demonstrated with a 300 K lasing wavelength of 370 nm and the minimal threshold current density achieved of 8.8 kA/cm2.