Edge Termination Design of Vertical GaN High Voltage P-N Diodes and Current Leakage Study of GaN P-N Diodes

III-nitride semiconductors like AlN, GaN, InN and BN are used in a range of optical and power devices due to their different bandgaps and superior optical and electrical properties. Gallium nitride (GaN) is a wide bandgap semiconductor with huge potential in power electronics devices due to its large direct bandgap (3.4eV), high critical electrical field, high electron saturation velocity, large electron mobility, and high thermal conductivity compared with other common materials like Si, GaAs and SiC. Based on this promise, vertical GaN diodes have been demonstrated that are promising for high-voltage power applications. However, the edges of the device can lead to compromised performance due to electric field crowding that leads to premature breakdown at the device perimeter. Therefore, in order to improve the performance of high power GaN devices, it is essential to study the role of edge effects in these devices.

In this thesis, we have evaluated the edge termination design of vertical GaN p-n diodes through numerical TCAD simulations. The effectiveness of beveled edges, field plates, and field plates incorporating a step are evaluated for their ability to suppress edge breakdown and increase the avalanche breakdown voltage of the device. The stepped field plate design was the most effective edge termination design for improving the breakdown voltage.

In addition, vertical GaN p-n diodes with junction termination extension (JTE) are also studied through simulation. JTE designs were explored to maximize the breakdown performance of vertical GaN p-n junction diodes. For a diode structure with 1.5 µm drift layer and drift layer doping of 4x10¹⁶ cm^-3, breakdown voltages of up to 372V was obtained with the optimum JTE design. Based on this design, devices were fabricated and tested experimentally using material grown under different conditions. A breakdown voltage of ~344V was achieved in the sample with highest growth temperature. The current leakage difference between each sample was also investigated.