Ozgur Aktas

Vertical Power p-n Diodes Based on Bulk GaN

There is a great interest in wide-bandgap semiconductor devices and most recently in monolithic GaN structures for power electronics applications. In this paper, vertical p-n diodes fabricated on pseudobulk low defect density (104-106 cm-2) GaN substrates are discussed. Homoepitaxial low-pressure metal organic chemical vapor deposition growth of GaN on its native substrate and being able to control and balance the n-type Si doping with background C impurity has allowed the realization of vertical device architectures with drift layer thicknesses of 6 to 40 μm and net carrier electron concentrations of 4 × 1015 to 2.5 × 1016 cm-3. This parameter range is suitable for applications requiring breakdown voltages (BVs) of 600 V-4 kV with a proper edge termination strategy. Measured devices demonstrate near power device figure of merit, that is, differential specific on-resistance (Rsp) of 2 mΩcm2 for a BV of 2.6 kV and 2.95 mΩcm2 for a 3.7-kV device, respectively. The improvement in the substrate quality over the last few years has resulted in the fabrication of diodes with areas as large as 16 mm2, with BVs exceeding 700 V and pulsed (100 μs) currents of 400 A. The structures fabricated are utilized to study in detail the temperature dependency of I-V characteristics, impact ionization and avalanche characteristics, and extract (estimate) modeling parameters such as electron mobility in the GaN c-direction (vertical) and hole minority carrier lifetimes. Some insight into device reliability is also provided.

Vertical GaN Power Diodes With a Bilayer Edge Termination

Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density (104 - 105 cm-2) GaN substrate, and has a 15-μm-thick n-type drift layer with a free carrier concentration of 5 × 1015 cm-3. The ET structure is formed by N implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p) layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. Simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages.

Optical studies of epitaxial GaN-based materials

A variety of spectroscopic techniques has been used to study the optical properties of epitaxial GaN based materials grown by metalorganic chemical vapor deposition and molecular beam epitaxy. The emphasis was on the issues vital to device applications such as stimulated emission and laser action, as well as carrier relaxation dynamics. Sharp exciton structures were observed by optical absorption measurements above 300 K, providing direct evidence of the formation of excitons in GaN at temperatures higher than room temperature. Using a picosecond streak camera, the time decay of free and bound exciton emissions was studied. By optical pumping, stimulated emission and lasing were investigated over a wide temperature range up to 420 K. In addition, the optical nonlinearity of GaN was studied using wave mixing techniques.

Imaging the Impact of Proton Irradiation on Edge Terminations in Vertical GaN PIN Diodes

Devices based on GaN have shown great promise for high power electronics, including their potential use as radiation tolerant components. An important step to realizing high power diodes is the design and implementation of an edge termination to mitigate field crowding, which can lead to premature breakdown. However, little is known about the effects of radiation on edge termination functionality. We experimentally examine the effects of proton irradiation on multiple field ring edge terminations in high power vertical GaN PIN diodes using in operando imaging with electron beam induced current (EBIC). We find that exposure to proton irradiation influences field spreading in the edge termination as well as carrier transport near the anode. By using depth-dependent EBIC measurements of hole diffusion length in homoepitaxial n-GaN we demonstrate that the carrier transport effect is due to a reduction in hole diffusion length following proton irradiation.