23 December 2015

IQE, Notre Dame and Cornell claim near-ideal GaN p-n diodes

Epiwafer foundry and substrate maker IQE plc of Cardiff, Wales, UK has collaborated with research groups at Cornell University and the University of Notre Dame in the USA to create gallium nitride (GaN) power diodes capable of serving as the building blocks for future GaN power switches, paving the way to enabling a wide range of applications from electronic products to electricity distribution infrastructure (Zongyang Hu et al, 'Near unity ideality factor and SRH lifetime in GaN-on-GaN p-n diodes with avalanche breakdown', Applied Physics Letters 107 (2015) 243501).

Power semiconductor devices are a critical part of the energy infrastructure for electronics that control or convert electrical energy. Incumbent silicon-based technologies are rapidly approaching severe performance and cost limitations, which has focused much attention on materials such as GaN as a key material for next-generation electronics products.

GaN has many desirable features but is notorious for its inherent high concentrations of defects (so that GaN-based devices often operate at a fraction of what GaN is truly capable of) and their associated impact on reliability. Researchers worldwide are therefore working to find ways to make GaN materials reliable for use within future electronics. To overcome these issues the teams concentrated on developing GaN-based devices with record low concentrations of defects in order 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," says Dr Zongyang Hu, a postdoc working in the research group of professor Grace Huili Xing within Cornell University's School of Electrical and Computer Engineering.

The team examined p-n junctions, which have direct applications in solar cells, light-emitting diodes, rectifiers and power transistors. "For our work, high-voltage p-n junction diodes are used to probe the material properties of GaN," Hu says.

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 (an sensitive indicator of the bulk defects, interface and surface defects, and resistance of the device).

"One parameter we used to effectively describe the defect level in a material is the Shockley-Read-Hall (SRH) recombination lifetime," Hu notes. SRH lifetime is the averaged time it takes injected electrons and holes in the p-n junction to move around before recombining at defects. "The lower the defect level, the longer the SRH lifetime," Hu says. "It's also interesting to note that, for GaN, a longer SRH lifetime results in a brighter light emission produced by the diode."

The work led by Xing is reckoned to be 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.

"Our results are an important step toward understanding the intrinsic properties and the true potential of GaN, and these achievements are only possible in high-quality GaN device structures (an effort led by IQE engineers) prepared on high-quality GaN bulk substrates and with precisely tuned fabrication technologies (an effort led by Dr Kazuki Nomoto, a research associate at Cornell University)," comments Hu.

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

One big surprise for the team came in the form of unexpectedly low differential on-resistance of the GaN diode. "It's as if the body of the entire p-n diode is transparent to the current flow without resistance," Hu says. "We believe this is due to high-level injection of minority carriers and their long lifetime, and are exploring it further."

The team's work is part of the US Department of Energy's (DOE) Advanced Research Projects Agency-Energy (ARPA-E) 'SWITCHES' program, monitored by Dr Timothy Heidel. "Leading one of these projects, we at Cornell, in collaboration with our industrial partners IQE, Qorvo, and UTRC, have established an integrated plan to develop three-terminal GaN power transistors, package them, and insert them into circuits and products," says professor Huili Xing.

The team says that, beyond the DOE ARPA-E project, it is open to collaboration with any researchers or companies interested in helping to drive GaN power electronics to fruition.

Tags: IQE GaN p-n diodes

Visit: http://scitation.aip.org/content/aip/journal/apl/107/24/10.1063/1.4937436

Visit: www.iqep.com

Visit: http://engineering.nd.edu

Visit: http://grace.engineering.cornell.edu