A simple structure with high breakdown voltage and a low leakage current of a vertical GaN p–n diode on a GaN free-standing substrate is demonstrated. We describe a vertical p–n diode with a simple edge termination that has a drift layer etched deeply and vertically. A device simulation revealed that the electric field was more relaxed at the device edge and applied uniformly in the entire device with increasing etching depth. We fabricated the simulated structure and succeeded in reducing the leakage current and improving the breakdown voltage. With this structure, a stable avalanche breakdown can be observed.

We fabricated p−n diodes under different growth pressures on free-standing GaN substrates of the same quality and observed a noteworthy difference in the reverse leakage current. A large reverse leakage current was generated by nanopipes, which were formed from screw dislocations in the homoepitaxial layer. There were two types of screw dislocation observed in this study. The first type already existed in the substrate and the other was newly generated in the epilayer by the coalescence of edge and mixed dislocations. An increase in the growth pressure suppressed the transformation of screw dislocations into nanopipes, which led to a reduction in the reverse leakage current. To reduce the leakage current further, it is necessary to apply growth conditions that do not transform screw dislocation into nanopipes and to use a free-standing substrate without threading dislocations, that become nanopipes.

A vertical p–n diode with a simple edge termination structure on a GaN free-standing substrate is demonstrated. The edge of this device is terminated simply by etching a drift layer deeply and vertically. A device simulation revealed that the electric field at the device edge was more relaxed and uniformly applied by etching the mesa deeper than the depletion region. The fabricated device showed low leakage current and avalanche capability, and its breakdown characteristics could be reproduced many times. By emission microscopy observation, we found that there was no leakage current at the side wall of the device and that avalanche breakdown occurred throughout the inside of the device. This indicates that the electric field crowding at the side wall of the device was completely suppressed and a uniform electric field distribution was obtained by this structure.