Design, fabrication, and characterization of gallium nitride high power rectifiers

The edge termination design, device modeling, fabrication and characterization of gallium nitride (GaN) high power diode rectifiers are reported in this dissertation. The important parameter sets of GaN materials and physical models are first reviewed and applied to the standard drift-diffusion device simulator MEDICI(TM). Theoretical calculations of GaN high power rectifiers have been made based on the breakdown voltage, the on-state resistance, the forward voltage drop, and reverse leakage currents. The breakdown analysis of the various edge terminations has been performed with impact ionization model. The field termination study shows that the use of an optimized SiO2 field plate edge termination can increase the reverse breakdown voltage of bulk GaN rectifiers by up to a factor of two compared to unterminated devices. The dielectric materials, thickness and ramp angle all influence the resulting breakdown voltage of the rectifier by determining where the maximum field strength occurs in the device structure. The key aspect in designing the field plate edge termination is to shift the region of the high field region away from the periphery of the rectifying contact. The junction termination study shows that the JTE produces the highest blocking voltages for vertical bulk GaN rectifiers, although the VB values are highly sensitive to the charge in the JTE layer. Guard-rings, field plates and planar junction were also examined in increasing VB over the value in unterminated rectifiers. Various bulk GaN p-i-n junction and Schottky rectifiers have been simulated as a function of temperature and analyzed in terms of their forward turn-on voltages and on-state resistances. GaN Schottky diodes with vertical and lateral geometries were fabricated on both conventional sapphire and free-standing GaN wafers. The typical on-state resistance of GaN Schottky diodes with lateral geometries was ∼2--3 mO · cm2, with reverse breakdown voltages at 25°C of 140--240V. Bulk GaN Schottky diodes with simple metal overlap edge termination show fast switching times, low on-state resistances and a low negative temperature coefficient of breakdown voltage. Pt Schottky rectifier arrays were fabricated on 200 mum thick, free-standing GaN layers even with the reduced dislocation density in these layers (∼105 cm-2) relative to conventional GaN on sapphire (>108 cm-2). We show that by interconnecting the output of many smaller rectifiers, we can achieve high total forward output current (161 A at 7.12 V), low forward turn-on voltage of ∼3 V. The potential for use of GaN bulk rectifiers in high power distribution and conversion remains high because of the rapid advances in material quality and processing technology that can be borrowed from the laser and light-emitting diode technology.