Vertical gallium nitride schottky diodes for power switching applications.

GaN schottky diodes;power switching;field termination structure;surface passivation

Sowmya Kolli

University:University of Louisville

Department:Electrical and Computer Engineering

关键词: GaN schottky diodes;    power switching;    field termination structure;    surface passivation;   

Others  :  https://ir.library.louisville.edu/cgi/viewcontent.cgi?article=4013&context=etd

美国|英语

来源: The Universite of Louisville's Institutional Repository

【 摘 要 】

Gallium nitride (GaN) has enormous potential for use in devices operating at high power, frequency and temperature. Its wide band gap, high critical electric field and favourable carrier properties lead to lower switching losses and conduction losses in power electronic devices. However, most GaN rectifiers reported to date exhibit an ON-resistance (Ron) versus breakdown voltage much below theoretical predictions. Heteroepitaxial growth of GaN on substrates such as SiC, Si, and sapphire suffer from a high density of threading dislocations defects due to the mismatch in lattice constants and thermal expansion coefficients. Vertical devices, in which a bulk GaN substrate is used, have much lower defect densities. However, field crowding at the periphery of the rectifying contact remains a problem and results in avalanche break down at much lower voltages than the theoretical maximum. This work will describe the design, simulation and fabrication of a novel wraparound field plate termination structure for high voltage Schottky diodes. Simulations show that the wrap around structure has an improved electric field distribution leading to higher breakdown voltages than conventional diode designs. The fabrication process was first developed using low-cost commercially grown HVPE GaN on sapphire substrates. This is the first work in the field of GaN based devices at the University of Louisville, so all fabrication processes, including ICP/RIE based dry etch, ohmic metal contact deposition and dielectric deposition steps, were developed and optimized. Current-voltage (I-V) measurements were used to extract on-resistance and break down voltage and these results were compared to simulation. Experimentally found breakdown values differed from the theoretical predictions. Device failure analysis based on I-V characterization showed the presence of additional current conduction paths along the SiNx and the defective HVPE films. To prevent these leakage currents a less defective MOCVD film grown on Ammono bulk GaN was used to fabricate the wrap-around diode. Planar GaN diodes, and diodes with standard field plate and our novel wraparound field plate were built and tested. Interestingly, planar diodes showed higher performance compared to standard field plate and wraparound field plate designs, contradicting to simulation results. Also, the diodes with a standard and a wraparound field plate structures showed higher leakage currents in both forward and reverse bias. To trace out the source of leakage currents, device failure analysis based on I-V measurements were carried out after each fabrication step of the diode. In this process, initially planar diodes were tested with a Schottky