The work is significant because many researchers around the globe are working to find ways to make GaN materials reliable for use within future electronics. Due to the presence of defects with high concentrations in typical GaN materials today, GaN-based devices often operate at a fraction of full capability.

The work led by Xing at Cornell University is the first report of GaN p-n diodes with near-ideal performance in all aspects simultaneously: a unity ideality factor, avalanche breakdown voltage, and about a two-fold improvement in device figure-of-merits over previous records.

A state-of-the-art design of GaN p-n junction diodes that has resulted in near-unity ideality factor, avalanche breakdown capability, and record-breaking power performance. Insets show a GaN p-n diode fabricated on a high-quality bulk GaN substrate and light emission from the junction under forward bias. CREDIT: Zongyang Hu. Click to enlarge.

Power semiconductor devices are a critical part of the energy infrastructure—all electronics rely on them to control or convert electrical energy. Silicon-based semiconductors are rapidly approaching their performance limits within electronics, so materials such as GaN are being explored as potential replacements that may render silicon switches obsolete.

But along with having many desirable features as a material, GaN is notorious for its defects and reliability issues. The team zeroed in on devices based on GaN with record-low defect concentrations to probe GaN’s ultimate performance limits for power electronics.

Our engineering goal is to develop inexpensive, reliable, high-efficiency switches to condition electricity—from where it’s generated to where it’s consumed within electric power systems—to replace generations-old, bulky, and inefficient technologies. GaN-based power devices are enabling technologies to achieve this goal.

—Zongyang Hu, lead author

The team examined semiconductor p-n junctions, made by joining p-type (free holes) and n-type (free electrons) semiconductor materials, which have direct applications in solar cells, light-emitting diodes (LEDs), rectifiers in circuits, and numerous variations in more complex devices such as power transistors. They used the high-voltage p-n junction diodes to probe the material properties of GaN.

To describe how much the device’s current-voltage characteristics deviate from the ideal case in a defect-free semiconductor system, the team uses a “diode ideality factor.” This is an extremely sensitive indicator of the bulk defects, interface and surface defects, and resistance of the device, Hu said.

Defects exist within all materials, but at varying levels. The team used the Shockley-Read-Hall (SRH) recombination lifetime parameter to describe effectively the defect level.

SRH lifetime is the averaged time it takes injected electrons and holes in the junction to move around before recombining at defects. The lower the defect level, the longer the SRH lifetime. For GaN, a longer SRH lifetime results in a brighter light emission produced by the diode, Hu added.

The team’s work is part of the USDepartment of Energy’s (DOE) Advanced Research Projects Agency-Energy (ARPA-E) “SWITCHES” program, monitored by Dr. Timothy Heidel.

Beyond the DOE ARPA-E project, the team is open to collaboration with any researchers or companies interested in helping drive GaN power electronics to its fruition.

Resources

• Hu, Zongyang and Nomoto, Kazuki and Song, Bo and Zhu, Mingda and Qi, Meng and Pan, Ming and Gao, Xiang and Protasenko, Vladimir and Jena, Debdeep and Xing, Huili Grace, Applied Physics Letters, 107, 243501 (2015) “Near unity ideality factor and Shockley-Read-Hall lifetime in GaN-on-GaN p-n diodes with avalanche breakdown” doi: 10.1063/1.4937436