This challenge has been the lack of a viable GaN selective area doping or selective area epitaxial regrowth process that yields material of sufficiently high quality to enable a defect-free p-n junction on patterned GaN surfaces.

Success in this area will allow further development of a revolutionary and powerful class of vertical GaN power electronic devices suitable for 1200V to 10kV broad range of applications (consumer electronics, power supplies, solar inverters, wind power, automotive, motor drives, ship propulsion, rail, and the grid).

Background. It has been estimated that as much as 80% of electricity could pass through power electronics between generation and consumption by 2030; 30% of electrical energy passed through power electronics converters in 2005. Technical advances in power electronics promise enormous energy efficiency gains throughout the United States economy.

Achieving high power conversion efficiency in these systems requires low-loss power semiconductor switches. The current incumbent power semiconductor switch technology is silicon-based Metal Oxide Semiconductor Field Effect Transistors (MOSFET), Insulated Gate Bipolar Transistors (IGBT) and thyristors.

Silicon power semiconductor devices have several important limitations, including high losses; low switching frequency; and poort high-temperature performance.

Wide-bandgap (WBG) semiconductors offer new opportunities for higher efficiency power semiconductor devices by circumventing the fundamental physical limits associated with silicon.

Substantial investment and technical progress has been made on WBG-based power switches over the past decade. ARPA-E has invested in WBG-based power devices (SiC, GaN, & Diamond) with the Agile Delivery of Electrical Power Technologies (ADEPT) program initiated in 2010; the Solar Agile Delivery of Electrical Power Technology (SOLAR ADEPT) program initiated in 2011; and the Strategies for Wide Bandgap, Inexpensive Transistors for Controlling High- Efficiency Systems (SWITCHES) program initiated in 2013.

To date, the majority of GaN power device development has been directed toward lateral architectures, such as high-electron mobility transistors (HEMTs), fabricated in thin layers of GaN grown on foreign substrates (including Si). Such lateral devices suffer from well-known issues such as current-collapse; dynamic on-resistance; inability to support avalanche breakdown; and inefficient thermal management.

Many of these shortcomings arise from defects originating in the very large lattice and coefficient of thermal expansion (CTE) mismatch between GaN and the non-native substrate. If instead one could fabricate vertical architectures on lattice and CTE matched bulk GaN substrates, it might be possible to realize the material-limited potential of GaN including true avalanche-limited breakdown, increased number of die on a wafer, and more efficient thermal management leading to large device currents (> 100A) without resorting to device parallelization.

Recently, bulk GaN substrates have become more widely available, a breakthrough that is enabling vertical architectures such as GaN planar as grown p-n diodes with breakdown voltages up to 5 kV, on-state currents approaching 400A, and avalanche capability.

However, the full potential of vertical architectures also requires the development of selective area doping, a breakthrough which would enable high performance vertical GaN transistors as well as merged p-n/Schottky (MPS) low turn-on-voltage diodes, and junction termination extension structures (p-type GaN rings surrounding the device perimeter) for edge termination of vertical GaN devices.

Selective area p-type doping has proved elusive in GaN, however, because the most obvious approach, laterally patterned ion implantation and activation or selective area diffusion of p-type dopants (e.g. Mg, Be, Zn) has not produced p-type regions or satisfactory (i.e., equivalent to as-grown) p-n junctions.

In addition, selective area etch and regrowth approaches have not resulted in sufficient electrical performance to be useful in power electronic applications. Junction leakage currents have been large; breakdown voltages much lower than expected; and avalanche breakdown ruggedness has not been convincingly demonstrated. The microscopic mechanistic understanding for the poor electrical performance to date is incomplete or non-existent, says ARPA-E.

The primary goal of the program is to demonstrate or provide a pathway based on fundamental science to fabricating high quality randomly placed, reliable, contactable, and generally useable GaN p-n junctions using selective area doping of GaN.