GaN quasi-vertical p–i–n diode with polarized bevel termination by sidewall passivation

The bevel edge termination technique has been widely applied in Si-based power devices to enhance the off-state properties, which has been also been adopted in the design of vertical GaN power devices. However, the intrinsic polarization properties of GaN, which can generate polarization charges at the bevel surface, have often been neglected because of screening by donor-like surface states. In this work, GaN p–i–n diodes with bevel termination structure were fabricated on a sapphire substrate, whose avalanche breakdown voltage was found to be independent of the bevel angles, and passivation on the bevel surface was found to be a critical process that ensures repeatable and uniform avalanche breakdown just below the anode area rather than at its edge. Considering the polarization effect and as indicated by simulation, the negative polarization charges on the bevel surface with sufficient passivation are compensated by mobile holes instead of fixed surface states at equilibrium. Under reverse bias, because of partial depletion of holes, the effective net negative fixed charges further cause the bevel surface to become negatively charged. Thus, it is the negatively charged bevel surface, achieved through passivation, that extends the depleted region near the bevel surface and suppresses the electric field peak under reverse bias, unlike in the non-passivated case, in which electric field crowding occurs. The work provides an alternative design approach for the termination structure of polar GaN vertical devices.